Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

13 files changed, 3379 insertions, 0 deletions

diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
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+==============
+BPF Design Q&A
+==============
+
+BPF extensibility and applicability to networking, tracing, security
+in the linux kernel and several user space implementations of BPF
+virtual machine led to a number of misunderstanding on what BPF actually is.
+This short QA is an attempt to address that and outline a direction
+of where BPF is heading long term.
+
+.. contents::
+    :local:
+    :depth: 3
+
+Questions and Answers
+=====================
+
+Q: Is BPF a generic instruction set similar to x64 and arm64?
+-------------------------------------------------------------
+A: NO.
+
+Q: Is BPF a generic virtual machine ?
+-------------------------------------
+A: NO.
+
+BPF is generic instruction set *with* C calling convention.
+-----------------------------------------------------------
+
+Q: Why C calling convention was chosen?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A: Because BPF programs are designed to run in the linux kernel
+which is written in C, hence BPF defines instruction set compatible
+with two most used architectures x64 and arm64 (and takes into
+consideration important quirks of other architectures) and
+defines calling convention that is compatible with C calling
+convention of the linux kernel on those architectures.
+
+Q: Can multiple return values be supported in the future?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: NO. BPF allows only register R0 to be used as return value.
+
+Q: Can more than 5 function arguments be supported in the future?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: NO. BPF calling convention only allows registers R1-R5 to be used
+as arguments. BPF is not a standalone instruction set.
+(unlike x64 ISA that allows msft, cdecl and other conventions)
+
+Q: Can BPF programs access instruction pointer or return address?
+-----------------------------------------------------------------
+A: NO.
+
+Q: Can BPF programs access stack pointer ?
+------------------------------------------
+A: NO.
+
+Only frame pointer (register R10) is accessible.
+From compiler point of view it's necessary to have stack pointer.
+For example, LLVM defines register R11 as stack pointer in its
+BPF backend, but it makes sure that generated code never uses it.
+
+Q: Does C-calling convention diminishes possible use cases?
+-----------------------------------------------------------
+A: YES.
+
+BPF design forces addition of major functionality in the form
+of kernel helper functions and kernel objects like BPF maps with
+seamless interoperability between them. It lets kernel call into
+BPF programs and programs call kernel helpers with zero overhead,
+as all of them were native C code. That is particularly the case
+for JITed BPF programs that are indistinguishable from
+native kernel C code.
+
+Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
+------------------------------------------------------------------------
+A: Soft yes.
+
+At least for now, until BPF core has support for
+bpf-to-bpf calls, indirect calls, loops, global variables,
+jump tables, read-only sections, and all other normal constructs
+that C code can produce.
+
+Q: Can loops be supported in a safe way?
+----------------------------------------
+A: It's not clear yet.
+
+BPF developers are trying to find a way to
+support bounded loops.
+
+Q: What are the verifier limits?
+--------------------------------
+A: The only limit known to the user space is BPF_MAXINSNS (4096).
+It's the maximum number of instructions that the unprivileged bpf
+program can have. The verifier has various internal limits.
+Like the maximum number of instructions that can be explored during
+program analysis. Currently, that limit is set to 1 million.
+Which essentially means that the largest program can consist
+of 1 million NOP instructions. There is a limit to the maximum number
+of subsequent branches, a limit to the number of nested bpf-to-bpf
+calls, a limit to the number of the verifier states per instruction,
+a limit to the number of maps used by the program.
+All these limits can be hit with a sufficiently complex program.
+There are also non-numerical limits that can cause the program
+to be rejected. The verifier used to recognize only pointer + constant
+expressions. Now it can recognize pointer + bounded_register.
+bpf_lookup_map_elem(key) had a requirement that 'key' must be
+a pointer to the stack. Now, 'key' can be a pointer to map value.
+The verifier is steadily getting 'smarter'. The limits are
+being removed. The only way to know that the program is going to
+be accepted by the verifier is to try to load it.
+The bpf development process guarantees that the future kernel
+versions will accept all bpf programs that were accepted by
+the earlier versions.
+
+
+Instruction level questions
+---------------------------
+
+Q: LD_ABS and LD_IND instructions vs C code
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Q: How come LD_ABS and LD_IND instruction are present in BPF whereas
+C code cannot express them and has to use builtin intrinsics?
+
+A: This is artifact of compatibility with classic BPF. Modern
+networking code in BPF performs better without them.
+See 'direct packet access'.
+
+Q: BPF instructions mapping not one-to-one to native CPU
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Q: It seems not all BPF instructions are one-to-one to native CPU.
+For example why BPF_JNE and other compare and jumps are not cpu-like?
+
+A: This was necessary to avoid introducing flags into ISA which are
+impossible to make generic and efficient across CPU architectures.
+
+Q: Why BPF_DIV instruction doesn't map to x64 div?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because if we picked one-to-one relationship to x64 it would have made
+it more complicated to support on arm64 and other archs. Also it
+needs div-by-zero runtime check.
+
+Q: Why there is no BPF_SDIV for signed divide operation?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because it would be rarely used. llvm errors in such case and
+prints a suggestion to use unsigned divide instead.
+
+Q: Why BPF has implicit prologue and epilogue?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because architectures like sparc have register windows and in general
+there are enough subtle differences between architectures, so naive
+store return address into stack won't work. Another reason is BPF has
+to be safe from division by zero (and legacy exception path
+of LD_ABS insn). Those instructions need to invoke epilogue and
+return implicitly.
+
+Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because classic BPF didn't have them and BPF authors felt that compiler
+workaround would be acceptable. Turned out that programs lose performance
+due to lack of these compare instructions and they were added.
+These two instructions is a perfect example what kind of new BPF
+instructions are acceptable and can be added in the future.
+These two already had equivalent instructions in native CPUs.
+New instructions that don't have one-to-one mapping to HW instructions
+will not be accepted.
+
+Q: BPF 32-bit subregister requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF
+registers which makes BPF inefficient virtual machine for 32-bit
+CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
+be added to BPF in the future?
+
+A: NO.
+
+But some optimizations on zero-ing the upper 32 bits for BPF registers are
+available, and can be leveraged to improve the performance of JITed BPF
+programs for 32-bit architectures.
+
+Starting with version 7, LLVM is able to generate instructions that operate
+on 32-bit subregisters, provided the option -mattr=+alu32 is passed for
+compiling a program. Furthermore, the verifier can now mark the
+instructions for which zero-ing the upper bits of the destination register
+is required, and insert an explicit zero-extension (zext) instruction
+(a mov32 variant). This means that for architectures without zext hardware
+support, the JIT back-ends do not need to clear the upper bits for
+subregisters written by alu32 instructions or narrow loads. Instead, the
+back-ends simply need to support code generation for that mov32 variant,
+and to overwrite bpf_jit_needs_zext() to make it return "true" (in order to
+enable zext insertion in the verifier).
+
+Note that it is possible for a JIT back-end to have partial hardware
+support for zext. In that case, if verifier zext insertion is enabled,
+it could lead to the insertion of unnecessary zext instructions. Such
+instructions could be removed by creating a simple peephole inside the JIT
+back-end: if one instruction has hardware support for zext and if the next
+instruction is an explicit zext, then the latter can be skipped when doing
+the code generation.
+
+Q: Does BPF have a stable ABI?
+------------------------------
+A: YES. BPF instructions, arguments to BPF programs, set of helper
+functions and their arguments, recognized return codes are all part
+of ABI. However there is one specific exception to tracing programs
+which are using helpers like bpf_probe_read() to walk kernel internal
+data structures and compile with kernel internal headers. Both of these
+kernel internals are subject to change and can break with newer kernels
+such that the program needs to be adapted accordingly.
+
+Q: How much stack space a BPF program uses?
+-------------------------------------------
+A: Currently all program types are limited to 512 bytes of stack
+space, but the verifier computes the actual amount of stack used
+and both interpreter and most JITed code consume necessary amount.
+
+Q: Can BPF be offloaded to HW?
+------------------------------
+A: YES. BPF HW offload is supported by NFP driver.
+
+Q: Does classic BPF interpreter still exist?
+--------------------------------------------
+A: NO. Classic BPF programs are converted into extend BPF instructions.
+
+Q: Can BPF call arbitrary kernel functions?
+-------------------------------------------
+A: NO. BPF programs can only call a set of helper functions which
+is defined for every program type.
+
+Q: Can BPF overwrite arbitrary kernel memory?
+---------------------------------------------
+A: NO.
+
+Tracing bpf programs can *read* arbitrary memory with bpf_probe_read()
+and bpf_probe_read_str() helpers. Networking programs cannot read
+arbitrary memory, since they don't have access to these helpers.
+Programs can never read or write arbitrary memory directly.
+
+Q: Can BPF overwrite arbitrary user memory?
+-------------------------------------------
+A: Sort-of.
+
+Tracing BPF programs can overwrite the user memory
+of the current task with bpf_probe_write_user(). Every time such
+program is loaded the kernel will print warning message, so
+this helper is only useful for experiments and prototypes.
+Tracing BPF programs are root only.
+
+Q: New functionality via kernel modules?
+----------------------------------------
+Q: Can BPF functionality such as new program or map types, new
+helpers, etc be added out of kernel module code?
+
+A: NO.
diff --git a/Documentation/bpf/bpf_devel_QA.rst b/Documentation/bpf/bpf_devel_QA.rst
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index 000000000..5b613d2a5
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@@ -0,0 +1,653 @@
+=================================
+HOWTO interact with BPF subsystem
+=================================
+
+This document provides information for the BPF subsystem about various
+workflows related to reporting bugs, submitting patches, and queueing
+patches for stable kernels.
+
+For general information about submitting patches, please refer to
+`Documentation/process/`_. This document only describes additional specifics
+related to BPF.
+
+.. contents::
+    :local:
+    :depth: 2
+
+Reporting bugs
+==============
+
+Q: How do I report bugs for BPF kernel code?
+--------------------------------------------
+A: Since all BPF kernel development as well as bpftool and iproute2 BPF
+loader development happens through the bpf kernel mailing list,
+please report any found issues around BPF to the following mailing
+list:
+
+ bpf@vger.kernel.org
+
+This may also include issues related to XDP, BPF tracing, etc.
+
+Given netdev has a high volume of traffic, please also add the BPF
+maintainers to Cc (from kernel MAINTAINERS_ file):
+
+* Alexei Starovoitov <ast@kernel.org>
+* Daniel Borkmann <daniel@iogearbox.net>
+
+In case a buggy commit has already been identified, make sure to keep
+the actual commit authors in Cc as well for the report. They can
+typically be identified through the kernel's git tree.
+
+**Please do NOT report BPF issues to bugzilla.kernel.org since it
+is a guarantee that the reported issue will be overlooked.**
+
+Submitting patches
+==================
+
+Q: To which mailing list do I need to submit my BPF patches?
+------------------------------------------------------------
+A: Please submit your BPF patches to the bpf kernel mailing list:
+
+ bpf@vger.kernel.org
+
+In case your patch has changes in various different subsystems (e.g.
+networking, tracing, security, etc), make sure to Cc the related kernel mailing
+lists and maintainers from there as well, so they are able to review
+the changes and provide their Acked-by's to the patches.
+
+Q: Where can I find patches currently under discussion for BPF subsystem?
+-------------------------------------------------------------------------
+A: All patches that are Cc'ed to netdev are queued for review under netdev
+patchwork project:
+
+  https://patchwork.kernel.org/project/netdevbpf/list/
+
+Those patches which target BPF, are assigned to a 'bpf' delegate for
+further processing from BPF maintainers. The current queue with
+patches under review can be found at:
+
+  https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173
+
+Once the patches have been reviewed by the BPF community as a whole
+and approved by the BPF maintainers, their status in patchwork will be
+changed to 'Accepted' and the submitter will be notified by mail. This
+means that the patches look good from a BPF perspective and have been
+applied to one of the two BPF kernel trees.
+
+In case feedback from the community requires a respin of the patches,
+their status in patchwork will be set to 'Changes Requested', and purged
+from the current review queue. Likewise for cases where patches would
+get rejected or are not applicable to the BPF trees (but assigned to
+the 'bpf' delegate).
+
+Q: How do the changes make their way into Linux?
+------------------------------------------------
+A: There are two BPF kernel trees (git repositories). Once patches have
+been accepted by the BPF maintainers, they will be applied to one
+of the two BPF trees:
+
+ * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
+ * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
+
+The bpf tree itself is for fixes only, whereas bpf-next for features,
+cleanups or other kind of improvements ("next-like" content). This is
+analogous to net and net-next trees for networking. Both bpf and
+bpf-next will only have a master branch in order to simplify against
+which branch patches should get rebased to.
+
+Accumulated BPF patches in the bpf tree will regularly get pulled
+into the net kernel tree. Likewise, accumulated BPF patches accepted
+into the bpf-next tree will make their way into net-next tree. net and
+net-next are both run by David S. Miller. From there, they will go
+into the kernel mainline tree run by Linus Torvalds. To read up on the
+process of net and net-next being merged into the mainline tree, see
+the :ref:`netdev-FAQ`
+
+
+
+Occasionally, to prevent merge conflicts, we might send pull requests
+to other trees (e.g. tracing) with a small subset of the patches, but
+net and net-next are always the main trees targeted for integration.
+
+The pull requests will contain a high-level summary of the accumulated
+patches and can be searched on netdev kernel mailing list through the
+following subject lines (``yyyy-mm-dd`` is the date of the pull
+request)::
+
+  pull-request: bpf yyyy-mm-dd
+  pull-request: bpf-next yyyy-mm-dd
+
+Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
+---------------------------------------------------------------------------------
+
+A: The process is the very same as described in the :ref:`netdev-FAQ`,
+so please read up on it. The subject line must indicate whether the
+patch is a fix or rather "next-like" content in order to let the
+maintainers know whether it is targeted at bpf or bpf-next.
+
+For fixes eventually landing in bpf -> net tree, the subject must
+look like::
+
+  git format-patch --subject-prefix='PATCH bpf' start..finish
+
+For features/improvements/etc that should eventually land in
+bpf-next -> net-next, the subject must look like::
+
+  git format-patch --subject-prefix='PATCH bpf-next' start..finish
+
+If unsure whether the patch or patch series should go into bpf
+or net directly, or bpf-next or net-next directly, it is not a
+problem either if the subject line says net or net-next as target.
+It is eventually up to the maintainers to do the delegation of
+the patches.
+
+If it is clear that patches should go into bpf or bpf-next tree,
+please make sure to rebase the patches against those trees in
+order to reduce potential conflicts.
+
+In case the patch or patch series has to be reworked and sent out
+again in a second or later revision, it is also required to add a
+version number (``v2``, ``v3``, ...) into the subject prefix::
+
+  git format-patch --subject-prefix='PATCH bpf-next v2' start..finish
+
+When changes have been requested to the patch series, always send the
+whole patch series again with the feedback incorporated (never send
+individual diffs on top of the old series).
+
+Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
+-----------------------------------------------------------------------
+A: It means that the patch looks good for mainline inclusion from
+a BPF point of view.
+
+Be aware that this is not a final verdict that the patch will
+automatically get accepted into net or net-next trees eventually:
+
+On the bpf kernel mailing list reviews can come in at any point
+in time. If discussions around a patch conclude that they cannot
+get included as-is, we will either apply a follow-up fix or drop
+them from the trees entirely. Therefore, we also reserve to rebase
+the trees when deemed necessary. After all, the purpose of the tree
+is to:
+
+i) accumulate and stage BPF patches for integration into trees
+   like net and net-next, and
+
+ii) run extensive BPF test suite and
+    workloads on the patches before they make their way any further.
+
+Once the BPF pull request was accepted by David S. Miller, then
+the patches end up in net or net-next tree, respectively, and
+make their way from there further into mainline. Again, see the
+:ref:`netdev-FAQ` for additional information e.g. on how often they are
+merged to mainline.
+
+Q: How long do I need to wait for feedback on my BPF patches?
+-------------------------------------------------------------
+A: We try to keep the latency low. The usual time to feedback will
+be around 2 or 3 business days. It may vary depending on the
+complexity of changes and current patch load.
+
+Q: How often do you send pull requests to major kernel trees like net or net-next?
+----------------------------------------------------------------------------------
+
+A: Pull requests will be sent out rather often in order to not
+accumulate too many patches in bpf or bpf-next.
+
+As a rule of thumb, expect pull requests for each tree regularly
+at the end of the week. In some cases pull requests could additionally
+come also in the middle of the week depending on the current patch
+load or urgency.
+
+Q: Are patches applied to bpf-next when the merge window is open?
+-----------------------------------------------------------------
+A: For the time when the merge window is open, bpf-next will not be
+processed. This is roughly analogous to net-next patch processing,
+so feel free to read up on the :ref:`netdev-FAQ` about further details.
+
+During those two weeks of merge window, we might ask you to resend
+your patch series once bpf-next is open again. Once Linus released
+a ``v*-rc1`` after the merge window, we continue processing of bpf-next.
+
+For non-subscribers to kernel mailing lists, there is also a status
+page run by David S. Miller on net-next that provides guidance:
+
+  http://vger.kernel.org/~davem/net-next.html
+
+Q: Verifier changes and test cases
+----------------------------------
+Q: I made a BPF verifier change, do I need to add test cases for
+BPF kernel selftests_?
+
+A: If the patch has changes to the behavior of the verifier, then yes,
+it is absolutely necessary to add test cases to the BPF kernel
+selftests_ suite. If they are not present and we think they are
+needed, then we might ask for them before accepting any changes.
+
+In particular, test_verifier.c is tracking a high number of BPF test
+cases, including a lot of corner cases that LLVM BPF back end may
+generate out of the restricted C code. Thus, adding test cases is
+absolutely crucial to make sure future changes do not accidentally
+affect prior use-cases. Thus, treat those test cases as: verifier
+behavior that is not tracked in test_verifier.c could potentially
+be subject to change.
+
+Q: samples/bpf preference vs selftests?
+---------------------------------------
+Q: When should I add code to `samples/bpf/`_ and when to BPF kernel
+selftests_ ?
+
+A: In general, we prefer additions to BPF kernel selftests_ rather than
+`samples/bpf/`_. The rationale is very simple: kernel selftests are
+regularly run by various bots to test for kernel regressions.
+
+The more test cases we add to BPF selftests, the better the coverage
+and the less likely it is that those could accidentally break. It is
+not that BPF kernel selftests cannot demo how a specific feature can
+be used.
+
+That said, `samples/bpf/`_ may be a good place for people to get started,
+so it might be advisable that simple demos of features could go into
+`samples/bpf/`_, but advanced functional and corner-case testing rather
+into kernel selftests.
+
+If your sample looks like a test case, then go for BPF kernel selftests
+instead!
+
+Q: When should I add code to the bpftool?
+-----------------------------------------
+A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
+a central user space tool for debugging and introspection of BPF programs
+and maps that are active in the kernel. If UAPI changes related to BPF
+enable for dumping additional information of programs or maps, then
+bpftool should be extended as well to support dumping them.
+
+Q: When should I add code to iproute2's BPF loader?
+---------------------------------------------------
+A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
+the convention is that those control-path related changes are added to
+iproute2's BPF loader as well from user space side. This is not only
+useful to have UAPI changes properly designed to be usable, but also
+to make those changes available to a wider user base of major
+downstream distributions.
+
+Q: Do you accept patches as well for iproute2's BPF loader?
+-----------------------------------------------------------
+A: Patches for the iproute2's BPF loader have to be sent to:
+
+  netdev@vger.kernel.org
+
+While those patches are not processed by the BPF kernel maintainers,
+please keep them in Cc as well, so they can be reviewed.
+
+The official git repository for iproute2 is run by Stephen Hemminger
+and can be found at:
+
+  https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
+
+The patches need to have a subject prefix of '``[PATCH iproute2
+master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
+'``net-next``' describes the target branch where the patch should be
+applied to. Meaning, if kernel changes went into the net-next kernel
+tree, then the related iproute2 changes need to go into the iproute2
+net-next branch, otherwise they can be targeted at master branch. The
+iproute2 net-next branch will get merged into the master branch after
+the current iproute2 version from master has been released.
+
+Like BPF, the patches end up in patchwork under the netdev project and
+are delegated to 'shemminger' for further processing:
+
+  http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
+
+Q: What is the minimum requirement before I submit my BPF patches?
+------------------------------------------------------------------
+A: When submitting patches, always take the time and properly test your
+patches *prior* to submission. Never rush them! If maintainers find
+that your patches have not been properly tested, it is a good way to
+get them grumpy. Testing patch submissions is a hard requirement!
+
+Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
+The same applies to fixes that target bpf-next, where the affected
+commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
+crucial in order to identify follow-up commits and tremendously helps
+for people having to do backporting, so it is a must have!
+
+We also don't accept patches with an empty commit message. Take your
+time and properly write up a high quality commit message, it is
+essential!
+
+Think about it this way: other developers looking at your code a month
+from now need to understand *why* a certain change has been done that
+way, and whether there have been flaws in the analysis or assumptions
+that the original author did. Thus providing a proper rationale and
+describing the use-case for the changes is a must.
+
+Patch submissions with >1 patch must have a cover letter which includes
+a high level description of the series. This high level summary will
+then be placed into the merge commit by the BPF maintainers such that
+it is also accessible from the git log for future reference.
+
+Q: Features changing BPF JIT and/or LLVM
+----------------------------------------
+Q: What do I need to consider when adding a new instruction or feature
+that would require BPF JIT and/or LLVM integration as well?
+
+A: We try hard to keep all BPF JITs up to date such that the same user
+experience can be guaranteed when running BPF programs on different
+architectures without having the program punt to the less efficient
+interpreter in case the in-kernel BPF JIT is enabled.
+
+If you are unable to implement or test the required JIT changes for
+certain architectures, please work together with the related BPF JIT
+developers in order to get the feature implemented in a timely manner.
+Please refer to the git log (``arch/*/net/``) to locate the necessary
+people for helping out.
+
+Also always make sure to add BPF test cases (e.g. test_bpf.c and
+test_verifier.c) for new instructions, so that they can receive
+broad test coverage and help run-time testing the various BPF JITs.
+
+In case of new BPF instructions, once the changes have been accepted
+into the Linux kernel, please implement support into LLVM's BPF back
+end. See LLVM_ section below for further information.
+
+Stable submission
+=================
+
+Q: I need a specific BPF commit in stable kernels. What should I do?
+--------------------------------------------------------------------
+A: In case you need a specific fix in stable kernels, first check whether
+the commit has already been applied in the related ``linux-*.y`` branches:
+
+  https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
+
+If not the case, then drop an email to the BPF maintainers with the
+netdev kernel mailing list in Cc and ask for the fix to be queued up:
+
+  netdev@vger.kernel.org
+
+The process in general is the same as on netdev itself, see also the
+:ref:`netdev-FAQ`.
+
+Q: Do you also backport to kernels not currently maintained as stable?
+----------------------------------------------------------------------
+A: No. If you need a specific BPF commit in kernels that are currently not
+maintained by the stable maintainers, then you are on your own.
+
+The current stable and longterm stable kernels are all listed here:
+
+  https://www.kernel.org/
+
+Q: The BPF patch I am about to submit needs to go to stable as well
+-------------------------------------------------------------------
+What should I do?
+
+A: The same rules apply as with netdev patch submissions in general, see
+the :ref:`netdev-FAQ`.
+
+Never add "``Cc: stable@vger.kernel.org``" to the patch description, but
+ask the BPF maintainers to queue the patches instead. This can be done
+with a note, for example, under the ``---`` part of the patch which does
+not go into the git log. Alternatively, this can be done as a simple
+request by mail instead.
+
+Q: Queue stable patches
+-----------------------
+Q: Where do I find currently queued BPF patches that will be submitted
+to stable?
+
+A: Once patches that fix critical bugs got applied into the bpf tree, they
+are queued up for stable submission under:
+
+  http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
+
+They will be on hold there at minimum until the related commit made its
+way into the mainline kernel tree.
+
+After having been under broader exposure, the queued patches will be
+submitted by the BPF maintainers to the stable maintainers.
+
+Testing patches
+===============
+
+Q: How to run BPF selftests
+---------------------------
+A: After you have booted into the newly compiled kernel, navigate to
+the BPF selftests_ suite in order to test BPF functionality (current
+working directory points to the root of the cloned git tree)::
+
+  $ cd tools/testing/selftests/bpf/
+  $ make
+
+To run the verifier tests::
+
+  $ sudo ./test_verifier
+
+The verifier tests print out all the current checks being
+performed. The summary at the end of running all tests will dump
+information of test successes and failures::
+
+  Summary: 418 PASSED, 0 FAILED
+
+In order to run through all BPF selftests, the following command is
+needed::
+
+  $ sudo make run_tests
+
+See the kernels selftest `Documentation/dev-tools/kselftest.rst`_
+document for further documentation.
+
+To maximize the number of tests passing, the .config of the kernel
+under test should match the config file fragment in
+tools/testing/selftests/bpf as closely as possible.
+
+Finally to ensure support for latest BPF Type Format features -
+discussed in `Documentation/bpf/btf.rst`_ - pahole version 1.16
+is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
+pahole is delivered in the dwarves package or can be built
+from source at
+
+https://github.com/acmel/dwarves
+
+Some distros have pahole version 1.16 packaged already, e.g.
+Fedora, Gentoo.
+
+Q: Which BPF kernel selftests version should I run my kernel against?
+---------------------------------------------------------------------
+A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
+from that kernel ``xyz`` as well. Do not expect that the BPF selftest
+from the latest mainline tree will pass all the time.
+
+In particular, test_bpf.c and test_verifier.c have a large number of
+test cases and are constantly updated with new BPF test sequences, or
+existing ones are adapted to verifier changes e.g. due to verifier
+becoming smarter and being able to better track certain things.
+
+LLVM
+====
+
+Q: Where do I find LLVM with BPF support?
+-----------------------------------------
+A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
+
+All major distributions these days ship LLVM with BPF back end enabled,
+so for the majority of use-cases it is not required to compile LLVM by
+hand anymore, just install the distribution provided package.
+
+LLVM's static compiler lists the supported targets through
+``llc --version``, make sure BPF targets are listed. Example::
+
+     $ llc --version
+     LLVM (http://llvm.org/):
+       LLVM version 10.0.0
+       Optimized build.
+       Default target: x86_64-unknown-linux-gnu
+       Host CPU: skylake
+
+       Registered Targets:
+         aarch64    - AArch64 (little endian)
+         bpf        - BPF (host endian)
+         bpfeb      - BPF (big endian)
+         bpfel      - BPF (little endian)
+         x86        - 32-bit X86: Pentium-Pro and above
+         x86-64     - 64-bit X86: EM64T and AMD64
+
+For developers in order to utilize the latest features added to LLVM's
+BPF back end, it is advisable to run the latest LLVM releases. Support
+for new BPF kernel features such as additions to the BPF instruction
+set are often developed together.
+
+All LLVM releases can be found at: http://releases.llvm.org/
+
+Q: Got it, so how do I build LLVM manually anyway?
+--------------------------------------------------
+A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have
+that set up, proceed with building the latest LLVM and clang version
+from the git repositories::
+
+     $ git clone https://github.com/llvm/llvm-project.git
+     $ mkdir -p llvm-project/llvm/build/install
+     $ cd llvm-project/llvm/build
+     $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
+                -DLLVM_ENABLE_PROJECTS="clang"    \
+                -DBUILD_SHARED_LIBS=OFF           \
+                -DCMAKE_BUILD_TYPE=Release        \
+                -DLLVM_BUILD_RUNTIME=OFF
+     $ ninja
+
+The built binaries can then be found in the build/bin/ directory, where
+you can point the PATH variable to.
+
+Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
+will find a full list of targets within the llvm-project/llvm/lib/Target
+directory.
+
+Q: Reporting LLVM BPF issues
+----------------------------
+Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
+generation back end or about LLVM generated code that the verifier
+refuses to accept?
+
+A: Yes, please do!
+
+LLVM's BPF back end is a key piece of the whole BPF
+infrastructure and it ties deeply into verification of programs from the
+kernel side. Therefore, any issues on either side need to be investigated
+and fixed whenever necessary.
+
+Therefore, please make sure to bring them up at netdev kernel mailing
+list and Cc BPF maintainers for LLVM and kernel bits:
+
+* Yonghong Song <yhs@fb.com>
+* Alexei Starovoitov <ast@kernel.org>
+* Daniel Borkmann <daniel@iogearbox.net>
+
+LLVM also has an issue tracker where BPF related bugs can be found:
+
+  https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
+
+However, it is better to reach out through mailing lists with having
+maintainers in Cc.
+
+Q: New BPF instruction for kernel and LLVM
+------------------------------------------
+Q: I have added a new BPF instruction to the kernel, how can I integrate
+it into LLVM?
+
+A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
+the selection of BPF instruction set extensions. By default the
+``generic`` processor target is used, which is the base instruction set
+(v1) of BPF.
+
+LLVM has an option to select ``-mcpu=probe`` where it will probe the host
+kernel for supported BPF instruction set extensions and selects the
+optimal set automatically.
+
+For cross-compilation, a specific version can be select manually as well ::
+
+     $ llc -march bpf -mcpu=help
+     Available CPUs for this target:
+
+       generic - Select the generic processor.
+       probe   - Select the probe processor.
+       v1      - Select the v1 processor.
+       v2      - Select the v2 processor.
+     [...]
+
+Newly added BPF instructions to the Linux kernel need to follow the same
+scheme, bump the instruction set version and implement probing for the
+extensions such that ``-mcpu=probe`` users can benefit from the
+optimization transparently when upgrading their kernels.
+
+If you are unable to implement support for the newly added BPF instruction
+please reach out to BPF developers for help.
+
+By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
+test coverage.
+
+Q: clang flag for target bpf?
+-----------------------------
+Q: In some cases clang flag ``-target bpf`` is used but in other cases the
+default clang target, which matches the underlying architecture, is used.
+What is the difference and when I should use which?
+
+A: Although LLVM IR generation and optimization try to stay architecture
+independent, ``-target <arch>`` still has some impact on generated code:
+
+- BPF program may recursively include header file(s) with file scope
+  inline assembly codes. The default target can handle this well,
+  while ``bpf`` target may fail if bpf backend assembler does not
+  understand these assembly codes, which is true in most cases.
+
+- When compiled without ``-g``, additional elf sections, e.g.,
+  .eh_frame and .rela.eh_frame, may be present in the object file
+  with default target, but not with ``bpf`` target.
+
+- The default target may turn a C switch statement into a switch table
+  lookup and jump operation. Since the switch table is placed
+  in the global readonly section, the bpf program will fail to load.
+  The bpf target does not support switch table optimization.
+  The clang option ``-fno-jump-tables`` can be used to disable
+  switch table generation.
+
+- For clang ``-target bpf``, it is guaranteed that pointer or long /
+  unsigned long types will always have a width of 64 bit, no matter
+  whether underlying clang binary or default target (or kernel) is
+  32 bit. However, when native clang target is used, then it will
+  compile these types based on the underlying architecture's conventions,
+  meaning in case of 32 bit architecture, pointer or long / unsigned
+  long types e.g. in BPF context structure will have width of 32 bit
+  while the BPF LLVM back end still operates in 64 bit. The native
+  target is mostly needed in tracing for the case of walking ``pt_regs``
+  or other kernel structures where CPU's register width matters.
+  Otherwise, ``clang -target bpf`` is generally recommended.
+
+You should use default target when:
+
+- Your program includes a header file, e.g., ptrace.h, which eventually
+  pulls in some header files containing file scope host assembly codes.
+
+- You can add ``-fno-jump-tables`` to work around the switch table issue.
+
+Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
+when:
+
+- Your program uses data structures with pointer or long / unsigned long
+  types that interface with BPF helpers or context data structures. Access
+  into these structures is verified by the BPF verifier and may result
+  in verification failures if the native architecture is not aligned with
+  the BPF architecture, e.g. 64-bit. An example of this is
+  BPF_PROG_TYPE_SK_MSG require ``-target bpf``
+
+
+.. Links
+.. _Documentation/process/: https://www.kernel.org/doc/html/latest/process/
+.. _MAINTAINERS: ../../MAINTAINERS
+.. _netdev-FAQ: ../networking/netdev-FAQ.rst
+.. _samples/bpf/: ../../samples/bpf/
+.. _selftests: ../../tools/testing/selftests/bpf/
+.. _Documentation/dev-tools/kselftest.rst:
+   https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
+.. _Documentation/bpf/btf.rst: btf.rst
+
+Happy BPF hacking!
diff --git a/Documentation/bpf/bpf_lsm.rst b/Documentation/bpf/bpf_lsm.rst
new file mode 100644
index 000000000..1c0a75a51
--- /dev/null
+++ b/Documentation/bpf/bpf_lsm.rst
@@ -0,0 +1,142 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2020 Google LLC.
+
+================
+LSM BPF Programs
+================
+
+These BPF programs allow runtime instrumentation of the LSM hooks by privileged
+users to implement system-wide MAC (Mandatory Access Control) and Audit
+policies using eBPF.
+
+Structure
+---------
+
+The example shows an eBPF program that can be attached to the ``file_mprotect``
+LSM hook:
+
+.. c:function:: int file_mprotect(struct vm_area_struct *vma, unsigned long reqprot, unsigned long prot);
+
+Other LSM hooks which can be instrumented can be found in
+``include/linux/lsm_hooks.h``.
+
+eBPF programs that use :doc:`/bpf/btf` do not need to include kernel headers
+for accessing information from the attached eBPF program's context. They can
+simply declare the structures in the eBPF program and only specify the fields
+that need to be accessed.
+
+.. code-block:: c
+
+	struct mm_struct {
+		unsigned long start_brk, brk, start_stack;
+	} __attribute__((preserve_access_index));
+
+	struct vm_area_struct {
+		unsigned long start_brk, brk, start_stack;
+		unsigned long vm_start, vm_end;
+		struct mm_struct *vm_mm;
+	} __attribute__((preserve_access_index));
+
+
+.. note:: The order of the fields is irrelevant.
+
+This can be further simplified (if one has access to the BTF information at
+build time) by generating the ``vmlinux.h`` with:
+
+.. code-block:: console
+
+	# bpftool btf dump file <path-to-btf-vmlinux> format c > vmlinux.h
+
+.. note:: ``path-to-btf-vmlinux`` can be ``/sys/kernel/btf/vmlinux`` if the
+	  build environment matches the environment the BPF programs are
+	  deployed in.
+
+The ``vmlinux.h`` can then simply be included in the BPF programs without
+requiring the definition of the types.
+
+The eBPF programs can be declared using the``BPF_PROG``
+macros defined in `tools/lib/bpf/bpf_tracing.h`_. In this
+example:
+
+	* ``"lsm/file_mprotect"`` indicates the LSM hook that the program must
+	  be attached to
+	* ``mprotect_audit`` is the name of the eBPF program
+
+.. code-block:: c
+
+	SEC("lsm/file_mprotect")
+	int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
+		     unsigned long reqprot, unsigned long prot, int ret)
+	{
+		/* ret is the return value from the previous BPF program
+		 * or 0 if it's the first hook.
+		 */
+		if (ret != 0)
+			return ret;
+
+		int is_heap;
+
+		is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
+			   vma->vm_end <= vma->vm_mm->brk);
+
+		/* Return an -EPERM or write information to the perf events buffer
+		 * for auditing
+		 */
+		if (is_heap)
+			return -EPERM;
+	}
+
+The ``__attribute__((preserve_access_index))`` is a clang feature that allows
+the BPF verifier to update the offsets for the access at runtime using the
+:doc:`/bpf/btf` information. Since the BPF verifier is aware of the types, it
+also validates all the accesses made to the various types in the eBPF program.
+
+Loading
+-------
+
+eBPF programs can be loaded with the :manpage:`bpf(2)` syscall's
+``BPF_PROG_LOAD`` operation:
+
+.. code-block:: c
+
+	struct bpf_object *obj;
+
+	obj = bpf_object__open("./my_prog.o");
+	bpf_object__load(obj);
+
+This can be simplified by using a skeleton header generated by ``bpftool``:
+
+.. code-block:: console
+
+	# bpftool gen skeleton my_prog.o > my_prog.skel.h
+
+and the program can be loaded by including ``my_prog.skel.h`` and using
+the generated helper, ``my_prog__open_and_load``.
+
+Attachment to LSM Hooks
+-----------------------
+
+The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`
+syscall's ``BPF_RAW_TRACEPOINT_OPEN`` operation or more simply by
+using the libbpf helper ``bpf_program__attach_lsm``.
+
+The program can be detached from the LSM hook by *destroying* the ``link``
+link returned by ``bpf_program__attach_lsm`` using ``bpf_link__destroy``.
+
+One can also use the helpers generated in ``my_prog.skel.h`` i.e.
+``my_prog__attach`` for attachment and ``my_prog__destroy`` for cleaning up.
+
+Examples
+--------
+
+An example eBPF program can be found in
+`tools/testing/selftests/bpf/progs/lsm.c`_ and the corresponding
+userspace code in `tools/testing/selftests/bpf/prog_tests/test_lsm.c`_
+
+.. Links
+.. _tools/lib/bpf/bpf_tracing.h:
+   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/lib/bpf/bpf_tracing.h
+.. _tools/testing/selftests/bpf/progs/lsm.c:
+   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm.c
+.. _tools/testing/selftests/bpf/prog_tests/test_lsm.c:
+   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/test_lsm.c
diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
new file mode 100644
index 000000000..44dc789de
--- /dev/null
+++ b/Documentation/bpf/btf.rst
@@ -0,0 +1,970 @@
+=====================
+BPF Type Format (BTF)
+=====================
+
+1. Introduction
+***************
+
+BTF (BPF Type Format) is the metadata format which encodes the debug info
+related to BPF program/map. The name BTF was used initially to describe data
+types. The BTF was later extended to include function info for defined
+subroutines, and line info for source/line information.
+
+The debug info is used for map pretty print, function signature, etc. The
+function signature enables better bpf program/function kernel symbol. The line
+info helps generate source annotated translated byte code, jited code and
+verifier log.
+
+The BTF specification contains two parts,
+  * BTF kernel API
+  * BTF ELF file format
+
+The kernel API is the contract between user space and kernel. The kernel
+verifies the BTF info before using it. The ELF file format is a user space
+contract between ELF file and libbpf loader.
+
+The type and string sections are part of the BTF kernel API, describing the
+debug info (mostly types related) referenced by the bpf program. These two
+sections are discussed in details in :ref:`BTF_Type_String`.
+
+.. _BTF_Type_String:
+
+2. BTF Type and String Encoding
+*******************************
+
+The file ``include/uapi/linux/btf.h`` provides high-level definition of how
+types/strings are encoded.
+
+The beginning of data blob must be::
+
+    struct btf_header {
+        __u16   magic;
+        __u8    version;
+        __u8    flags;
+        __u32   hdr_len;
+
+        /* All offsets are in bytes relative to the end of this header */
+        __u32   type_off;       /* offset of type section       */
+        __u32   type_len;       /* length of type section       */
+        __u32   str_off;        /* offset of string section     */
+        __u32   str_len;        /* length of string section     */
+    };
+
+The magic is ``0xeB9F``, which has different encoding for big and little
+endian systems, and can be used to test whether BTF is generated for big- or
+little-endian target. The ``btf_header`` is designed to be extensible with
+``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
+generated.
+
+2.1 String Encoding
+===================
+
+The first string in the string section must be a null string. The rest of
+string table is a concatenation of other null-terminated strings.
+
+2.2 Type Encoding
+=================
+
+The type id ``0`` is reserved for ``void`` type. The type section is parsed
+sequentially and type id is assigned to each recognized type starting from id
+``1``. Currently, the following types are supported::
+
+    #define BTF_KIND_INT            1       /* Integer      */
+    #define BTF_KIND_PTR            2       /* Pointer      */
+    #define BTF_KIND_ARRAY          3       /* Array        */
+    #define BTF_KIND_STRUCT         4       /* Struct       */
+    #define BTF_KIND_UNION          5       /* Union        */
+    #define BTF_KIND_ENUM           6       /* Enumeration  */
+    #define BTF_KIND_FWD            7       /* Forward      */
+    #define BTF_KIND_TYPEDEF        8       /* Typedef      */
+    #define BTF_KIND_VOLATILE       9       /* Volatile     */
+    #define BTF_KIND_CONST          10      /* Const        */
+    #define BTF_KIND_RESTRICT       11      /* Restrict     */
+    #define BTF_KIND_FUNC           12      /* Function     */
+    #define BTF_KIND_FUNC_PROTO     13      /* Function Proto       */
+    #define BTF_KIND_VAR            14      /* Variable     */
+    #define BTF_KIND_DATASEC        15      /* Section      */
+
+Note that the type section encodes debug info, not just pure types.
+``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
+
+Each type contains the following common data::
+
+    struct btf_type {
+        __u32 name_off;
+        /* "info" bits arrangement
+         * bits  0-15: vlen (e.g. # of struct's members)
+         * bits 16-23: unused
+         * bits 24-27: kind (e.g. int, ptr, array...etc)
+         * bits 28-30: unused
+         * bit     31: kind_flag, currently used by
+         *             struct, union and fwd
+         */
+        __u32 info;
+        /* "size" is used by INT, ENUM, STRUCT and UNION.
+         * "size" tells the size of the type it is describing.
+         *
+         * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
+         * FUNC and FUNC_PROTO.
+         * "type" is a type_id referring to another type.
+         */
+        union {
+                __u32 size;
+                __u32 type;
+        };
+    };
+
+For certain kinds, the common data are followed by kind-specific data. The
+``name_off`` in ``struct btf_type`` specifies the offset in the string table.
+The following sections detail encoding of each kind.
+
+2.2.1 BTF_KIND_INT
+~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+ * ``name_off``: any valid offset
+ * ``info.kind_flag``: 0
+ * ``info.kind``: BTF_KIND_INT
+ * ``info.vlen``: 0
+ * ``size``: the size of the int type in bytes.
+
+``btf_type`` is followed by a ``u32`` with the following bits arrangement::
+
+  #define BTF_INT_ENCODING(VAL)   (((VAL) & 0x0f000000) >> 24)
+  #define BTF_INT_OFFSET(VAL)     (((VAL) & 0x00ff0000) >> 16)
+  #define BTF_INT_BITS(VAL)       ((VAL)  & 0x000000ff)
+
+The ``BTF_INT_ENCODING`` has the following attributes::
+
+  #define BTF_INT_SIGNED  (1 << 0)
+  #define BTF_INT_CHAR    (1 << 1)
+  #define BTF_INT_BOOL    (1 << 2)
+
+The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
+bool, for the int type. The char and bool encoding are mostly useful for
+pretty print. At most one encoding can be specified for the int type.
+
+The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
+type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
+The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
+for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
+
+The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
+for this int. For example, a bitfield struct member has:
+
+ * btf member bit offset 100 from the start of the structure,
+ * btf member pointing to an int type,
+ * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
+
+Then in the struct memory layout, this member will occupy ``4`` bits starting
+from bits ``100 + 2 = 102``.
+
+Alternatively, the bitfield struct member can be the following to access the
+same bits as the above:
+
+ * btf member bit offset 102,
+ * btf member pointing to an int type,
+ * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
+
+The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
+bitfield encoding. Currently, both llvm and pahole generate
+``BTF_INT_OFFSET() = 0`` for all int types.
+
+2.2.2 BTF_KIND_PTR
+~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_PTR
+  * ``info.vlen``: 0
+  * ``type``: the pointee type of the pointer
+
+No additional type data follow ``btf_type``.
+
+2.2.3 BTF_KIND_ARRAY
+~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_ARRAY
+  * ``info.vlen``: 0
+  * ``size/type``: 0, not used
+
+``btf_type`` is followed by one ``struct btf_array``::
+
+    struct btf_array {
+        __u32   type;
+        __u32   index_type;
+        __u32   nelems;
+    };
+
+The ``struct btf_array`` encoding:
+  * ``type``: the element type
+  * ``index_type``: the index type
+  * ``nelems``: the number of elements for this array (``0`` is also allowed).
+
+The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
+``u64``, ``unsigned __int128``). The original design of including
+``index_type`` follows DWARF, which has an ``index_type`` for its array type.
+Currently in BTF, beyond type verification, the ``index_type`` is not used.
+
+The ``struct btf_array`` allows chaining through element type to represent
+multidimensional arrays. For example, for ``int a[5][6]``, the following type
+information illustrates the chaining:
+
+  * [1]: int
+  * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
+  * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
+
+Currently, both pahole and llvm collapse multidimensional array into
+one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
+equal to ``30``. This is because the original use case is map pretty print
+where the whole array is dumped out so one-dimensional array is enough. As
+more BTF usage is explored, pahole and llvm can be changed to generate proper
+chained representation for multidimensional arrays.
+
+2.2.4 BTF_KIND_STRUCT
+~~~~~~~~~~~~~~~~~~~~~
+2.2.5 BTF_KIND_UNION
+~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0 or offset to a valid C identifier
+  * ``info.kind_flag``: 0 or 1
+  * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
+  * ``info.vlen``: the number of struct/union members
+  * ``info.size``: the size of the struct/union in bytes
+
+``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
+
+    struct btf_member {
+        __u32   name_off;
+        __u32   type;
+        __u32   offset;
+    };
+
+``struct btf_member`` encoding:
+  * ``name_off``: offset to a valid C identifier
+  * ``type``: the member type
+  * ``offset``: <see below>
+
+If the type info ``kind_flag`` is not set, the offset contains only bit offset
+of the member. Note that the base type of the bitfield can only be int or enum
+type. If the bitfield size is 32, the base type can be either int or enum
+type. If the bitfield size is not 32, the base type must be int, and int type
+``BTF_INT_BITS()`` encodes the bitfield size.
+
+If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
+bitfield size and bit offset. The bitfield size and bit offset are calculated
+as below.::
+
+  #define BTF_MEMBER_BITFIELD_SIZE(val)   ((val) >> 24)
+  #define BTF_MEMBER_BIT_OFFSET(val)      ((val) & 0xffffff)
+
+In this case, if the base type is an int type, it must be a regular int type:
+
+  * ``BTF_INT_OFFSET()`` must be 0.
+  * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
+
+The following kernel patch introduced ``kind_flag`` and explained why both
+modes exist:
+
+  https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3
+
+2.2.6 BTF_KIND_ENUM
+~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0 or offset to a valid C identifier
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_ENUM
+  * ``info.vlen``: number of enum values
+  * ``size``: 4
+
+``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
+
+    struct btf_enum {
+        __u32   name_off;
+        __s32   val;
+    };
+
+The ``btf_enum`` encoding:
+  * ``name_off``: offset to a valid C identifier
+  * ``val``: any value
+
+2.2.7 BTF_KIND_FWD
+~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: offset to a valid C identifier
+  * ``info.kind_flag``: 0 for struct, 1 for union
+  * ``info.kind``: BTF_KIND_FWD
+  * ``info.vlen``: 0
+  * ``type``: 0
+
+No additional type data follow ``btf_type``.
+
+2.2.8 BTF_KIND_TYPEDEF
+~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: offset to a valid C identifier
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_TYPEDEF
+  * ``info.vlen``: 0
+  * ``type``: the type which can be referred by name at ``name_off``
+
+No additional type data follow ``btf_type``.
+
+2.2.9 BTF_KIND_VOLATILE
+~~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_VOLATILE
+  * ``info.vlen``: 0
+  * ``type``: the type with ``volatile`` qualifier
+
+No additional type data follow ``btf_type``.
+
+2.2.10 BTF_KIND_CONST
+~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_CONST
+  * ``info.vlen``: 0
+  * ``type``: the type with ``const`` qualifier
+
+No additional type data follow ``btf_type``.
+
+2.2.11 BTF_KIND_RESTRICT
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_RESTRICT
+  * ``info.vlen``: 0
+  * ``type``: the type with ``restrict`` qualifier
+
+No additional type data follow ``btf_type``.
+
+2.2.12 BTF_KIND_FUNC
+~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: offset to a valid C identifier
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_FUNC
+  * ``info.vlen``: 0
+  * ``type``: a BTF_KIND_FUNC_PROTO type
+
+No additional type data follow ``btf_type``.
+
+A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
+signature is defined by ``type``. The subprogram is thus an instance of that
+type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
+:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
+(ABI).
+
+2.2.13 BTF_KIND_FUNC_PROTO
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_FUNC_PROTO
+  * ``info.vlen``: # of parameters
+  * ``type``: the return type
+
+``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
+
+    struct btf_param {
+        __u32   name_off;
+        __u32   type;
+    };
+
+If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
+``btf_param.name_off`` must point to a valid C identifier except for the
+possible last argument representing the variable argument. The btf_param.type
+refers to parameter type.
+
+If the function has variable arguments, the last parameter is encoded with
+``name_off = 0`` and ``type = 0``.
+
+2.2.14 BTF_KIND_VAR
+~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: offset to a valid C identifier
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_VAR
+  * ``info.vlen``: 0
+  * ``type``: the type of the variable
+
+``btf_type`` is followed by a single ``struct btf_variable`` with the
+following data::
+
+    struct btf_var {
+        __u32   linkage;
+    };
+
+``struct btf_var`` encoding:
+  * ``linkage``: currently only static variable 0, or globally allocated
+                 variable in ELF sections 1
+
+Not all type of global variables are supported by LLVM at this point.
+The following is currently available:
+
+  * static variables with or without section attributes
+  * global variables with section attributes
+
+The latter is for future extraction of map key/value type id's from a
+map definition.
+
+2.2.15 BTF_KIND_DATASEC
+~~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: offset to a valid name associated with a variable or
+                  one of .data/.bss/.rodata
+  * ``info.kind_flag``: 0
+  * ``info.kind``: BTF_KIND_DATASEC
+  * ``info.vlen``: # of variables
+  * ``size``: total section size in bytes (0 at compilation time, patched
+              to actual size by BPF loaders such as libbpf)
+
+``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
+
+    struct btf_var_secinfo {
+        __u32   type;
+        __u32   offset;
+        __u32   size;
+    };
+
+``struct btf_var_secinfo`` encoding:
+  * ``type``: the type of the BTF_KIND_VAR variable
+  * ``offset``: the in-section offset of the variable
+  * ``size``: the size of the variable in bytes
+
+3. BTF Kernel API
+*****************
+
+The following bpf syscall command involves BTF:
+   * BPF_BTF_LOAD: load a blob of BTF data into kernel
+   * BPF_MAP_CREATE: map creation with btf key and value type info.
+   * BPF_PROG_LOAD: prog load with btf function and line info.
+   * BPF_BTF_GET_FD_BY_ID: get a btf fd
+   * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
+     and other btf related info are returned.
+
+The workflow typically looks like:
+::
+
+  Application:
+      BPF_BTF_LOAD
+          |
+          v
+      BPF_MAP_CREATE and BPF_PROG_LOAD
+          |
+          V
+      ......
+
+  Introspection tool:
+      ......
+      BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
+          |
+          V
+      BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
+          |
+          V
+      BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
+          |                                     |
+          V                                     |
+      BPF_BTF_GET_FD_BY_ID (get btf_fd)         |
+          |                                     |
+          V                                     |
+      BPF_OBJ_GET_INFO_BY_FD (get btf)          |
+          |                                     |
+          V                                     V
+      pretty print types, dump func signatures and line info, etc.
+
+
+3.1 BPF_BTF_LOAD
+================
+
+Load a blob of BTF data into kernel. A blob of data, described in
+:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
+is returned to a userspace.
+
+3.2 BPF_MAP_CREATE
+==================
+
+A map can be created with ``btf_fd`` and specified key/value type id.::
+
+    __u32   btf_fd;         /* fd pointing to a BTF type data */
+    __u32   btf_key_type_id;        /* BTF type_id of the key */
+    __u32   btf_value_type_id;      /* BTF type_id of the value */
+
+In libbpf, the map can be defined with extra annotation like below:
+::
+
+    struct bpf_map_def SEC("maps") btf_map = {
+        .type = BPF_MAP_TYPE_ARRAY,
+        .key_size = sizeof(int),
+        .value_size = sizeof(struct ipv_counts),
+        .max_entries = 4,
+    };
+    BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
+
+Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and
+value types for the map. During ELF parsing, libbpf is able to extract
+key/value type_id's and assign them to BPF_MAP_CREATE attributes
+automatically.
+
+.. _BPF_Prog_Load:
+
+3.3 BPF_PROG_LOAD
+=================
+
+During prog_load, func_info and line_info can be passed to kernel with proper
+values for the following attributes:
+::
+
+    __u32           insn_cnt;
+    __aligned_u64   insns;
+    ......
+    __u32           prog_btf_fd;    /* fd pointing to BTF type data */
+    __u32           func_info_rec_size;     /* userspace bpf_func_info size */
+    __aligned_u64   func_info;      /* func info */
+    __u32           func_info_cnt;  /* number of bpf_func_info records */
+    __u32           line_info_rec_size;     /* userspace bpf_line_info size */
+    __aligned_u64   line_info;      /* line info */
+    __u32           line_info_cnt;  /* number of bpf_line_info records */
+
+The func_info and line_info are an array of below, respectively.::
+
+    struct bpf_func_info {
+        __u32   insn_off; /* [0, insn_cnt - 1] */
+        __u32   type_id;  /* pointing to a BTF_KIND_FUNC type */
+    };
+    struct bpf_line_info {
+        __u32   insn_off; /* [0, insn_cnt - 1] */
+        __u32   file_name_off; /* offset to string table for the filename */
+        __u32   line_off; /* offset to string table for the source line */
+        __u32   line_col; /* line number and column number */
+    };
+
+func_info_rec_size is the size of each func_info record, and
+line_info_rec_size is the size of each line_info record. Passing the record
+size to kernel make it possible to extend the record itself in the future.
+
+Below are requirements for func_info:
+  * func_info[0].insn_off must be 0.
+  * the func_info insn_off is in strictly increasing order and matches
+    bpf func boundaries.
+
+Below are requirements for line_info:
+  * the first insn in each func must have a line_info record pointing to it.
+  * the line_info insn_off is in strictly increasing order.
+
+For line_info, the line number and column number are defined as below:
+::
+
+    #define BPF_LINE_INFO_LINE_NUM(line_col)        ((line_col) >> 10)
+    #define BPF_LINE_INFO_LINE_COL(line_col)        ((line_col) & 0x3ff)
+
+3.4 BPF_{PROG,MAP}_GET_NEXT_ID
+==============================
+
+In kernel, every loaded program, map or btf has a unique id. The id won't
+change during the lifetime of a program, map, or btf.
+
+The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
+each command, to user space, for bpf program or maps, respectively, so an
+inspection tool can inspect all programs and maps.
+
+3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
+===============================
+
+An introspection tool cannot use id to get details about program or maps.
+A file descriptor needs to be obtained first for reference-counting purpose.
+
+3.6 BPF_OBJ_GET_INFO_BY_FD
+==========================
+
+Once a program/map fd is acquired, an introspection tool can get the detailed
+information from kernel about this fd, some of which are BTF-related. For
+example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
+``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
+bpf byte codes, and jited_line_info.
+
+3.7 BPF_BTF_GET_FD_BY_ID
+========================
+
+With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
+syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
+command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
+kernel with BPF_BTF_LOAD, can be retrieved.
+
+With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
+tool has full btf knowledge and is able to pretty print map key/values, dump
+func signatures and line info, along with byte/jit codes.
+
+4. ELF File Format Interface
+****************************
+
+4.1 .BTF section
+================
+
+The .BTF section contains type and string data. The format of this section is
+same as the one describe in :ref:`BTF_Type_String`.
+
+.. _BTF_Ext_Section:
+
+4.2 .BTF.ext section
+====================
+
+The .BTF.ext section encodes func_info and line_info which needs loader
+manipulation before loading into the kernel.
+
+The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
+and ``tools/lib/bpf/btf.c``.
+
+The current header of .BTF.ext section::
+
+    struct btf_ext_header {
+        __u16   magic;
+        __u8    version;
+        __u8    flags;
+        __u32   hdr_len;
+
+        /* All offsets are in bytes relative to the end of this header */
+        __u32   func_info_off;
+        __u32   func_info_len;
+        __u32   line_info_off;
+        __u32   line_info_len;
+    };
+
+It is very similar to .BTF section. Instead of type/string section, it
+contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details
+about func_info and line_info record format.
+
+The func_info is organized as below.::
+
+     func_info_rec_size
+     btf_ext_info_sec for section #1 /* func_info for section #1 */
+     btf_ext_info_sec for section #2 /* func_info for section #2 */
+     ...
+
+``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
+.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
+func_info for each specific ELF section.::
+
+     struct btf_ext_info_sec {
+        __u32   sec_name_off; /* offset to section name */
+        __u32   num_info;
+        /* Followed by num_info * record_size number of bytes */
+        __u8    data[0];
+     };
+
+Here, num_info must be greater than 0.
+
+The line_info is organized as below.::
+
+     line_info_rec_size
+     btf_ext_info_sec for section #1 /* line_info for section #1 */
+     btf_ext_info_sec for section #2 /* line_info for section #2 */
+     ...
+
+``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
+.BTF.ext is generated.
+
+The interpretation of ``bpf_func_info->insn_off`` and
+``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
+kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
+bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
+beginning of section (``btf_ext_info_sec->sec_name_off``).
+
+4.2 .BTF_ids section
+====================
+
+The .BTF_ids section encodes BTF ID values that are used within the kernel.
+
+This section is created during the kernel compilation with the help of
+macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
+use them to create lists and sets (sorted lists) of BTF ID values.
+
+The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
+with following syntax::
+
+  BTF_ID_LIST(list)
+  BTF_ID(type1, name1)
+  BTF_ID(type2, name2)
+
+resulting in following layout in .BTF_ids section::
+
+  __BTF_ID__type1__name1__1:
+  .zero 4
+  __BTF_ID__type2__name2__2:
+  .zero 4
+
+The ``u32 list[];`` variable is defined to access the list.
+
+The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
+want to define unused entry in BTF_ID_LIST, like::
+
+      BTF_ID_LIST(bpf_skb_output_btf_ids)
+      BTF_ID(struct, sk_buff)
+      BTF_ID_UNUSED
+      BTF_ID(struct, task_struct)
+
+The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
+and their count, with following syntax::
+
+  BTF_SET_START(set)
+  BTF_ID(type1, name1)
+  BTF_ID(type2, name2)
+  BTF_SET_END(set)
+
+resulting in following layout in .BTF_ids section::
+
+  __BTF_ID__set__set:
+  .zero 4
+  __BTF_ID__type1__name1__3:
+  .zero 4
+  __BTF_ID__type2__name2__4:
+  .zero 4
+
+The ``struct btf_id_set set;`` variable is defined to access the list.
+
+The ``typeX`` name can be one of following::
+
+   struct, union, typedef, func
+
+and is used as a filter when resolving the BTF ID value.
+
+All the BTF ID lists and sets are compiled in the .BTF_ids section and
+resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
+
+5. Using BTF
+************
+
+5.1 bpftool map pretty print
+============================
+
+With BTF, the map key/value can be printed based on fields rather than simply
+raw bytes. This is especially valuable for large structure or if your data
+structure has bitfields. For example, for the following map,::
+
+      enum A { A1, A2, A3, A4, A5 };
+      typedef enum A ___A;
+      struct tmp_t {
+           char a1:4;
+           int  a2:4;
+           int  :4;
+           __u32 a3:4;
+           int b;
+           ___A b1:4;
+           enum A b2:4;
+      };
+      struct bpf_map_def SEC("maps") tmpmap = {
+           .type = BPF_MAP_TYPE_ARRAY,
+           .key_size = sizeof(__u32),
+           .value_size = sizeof(struct tmp_t),
+           .max_entries = 1,
+      };
+      BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t);
+
+bpftool is able to pretty print like below:
+::
+
+      [{
+            "key": 0,
+            "value": {
+                "a1": 0x2,
+                "a2": 0x4,
+                "a3": 0x6,
+                "b": 7,
+                "b1": 0x8,
+                "b2": 0xa
+            }
+        }
+      ]
+
+5.2 bpftool prog dump
+=====================
+
+The following is an example showing how func_info and line_info can help prog
+dump with better kernel symbol names, function prototypes and line
+information.::
+
+    $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
+    [...]
+    int test_long_fname_2(struct dummy_tracepoint_args * arg):
+    bpf_prog_44a040bf25481309_test_long_fname_2:
+    ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
+       0:   push   %rbp
+       1:   mov    %rsp,%rbp
+       4:   sub    $0x30,%rsp
+       b:   sub    $0x28,%rbp
+       f:   mov    %rbx,0x0(%rbp)
+      13:   mov    %r13,0x8(%rbp)
+      17:   mov    %r14,0x10(%rbp)
+      1b:   mov    %r15,0x18(%rbp)
+      1f:   xor    %eax,%eax
+      21:   mov    %rax,0x20(%rbp)
+      25:   xor    %esi,%esi
+    ; int key = 0;
+      27:   mov    %esi,-0x4(%rbp)
+    ; if (!arg->sock)
+      2a:   mov    0x8(%rdi),%rdi
+    ; if (!arg->sock)
+      2e:   cmp    $0x0,%rdi
+      32:   je     0x0000000000000070
+      34:   mov    %rbp,%rsi
+    ; counts = bpf_map_lookup_elem(&btf_map, &key);
+    [...]
+
+5.3 Verifier Log
+================
+
+The following is an example of how line_info can help debugging verification
+failure.::
+
+       /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
+        * is modified as below.
+        */
+       data = (void *)(long)xdp->data;
+       data_end = (void *)(long)xdp->data_end;
+       /*
+       if (data + 4 > data_end)
+               return XDP_DROP;
+       */
+       *(u32 *)data = dst->dst;
+
+    $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
+        ; data = (void *)(long)xdp->data;
+        224: (79) r2 = *(u64 *)(r10 -112)
+        225: (61) r2 = *(u32 *)(r2 +0)
+        ; *(u32 *)data = dst->dst;
+        226: (63) *(u32 *)(r2 +0) = r1
+        invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
+        R2 offset is outside of the packet
+
+6. BTF Generation
+*****************
+
+You need latest pahole
+
+  https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
+
+or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
+support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
+
+      -bash-4.4$ cat t.c
+      struct t {
+        int a:2;
+        int b:3;
+        int c:2;
+      } g;
+      -bash-4.4$ gcc -c -O2 -g t.c
+      -bash-4.4$ pahole -JV t.o
+      File t.o:
+      [1] STRUCT t kind_flag=1 size=4 vlen=3
+              a type_id=2 bitfield_size=2 bits_offset=0
+              b type_id=2 bitfield_size=3 bits_offset=2
+              c type_id=2 bitfield_size=2 bits_offset=5
+      [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
+
+The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
+only. The assembly code (-S) is able to show the BTF encoding in assembly
+format.::
+
+    -bash-4.4$ cat t2.c
+    typedef int __int32;
+    struct t2 {
+      int a2;
+      int (*f2)(char q1, __int32 q2, ...);
+      int (*f3)();
+    } g2;
+    int main() { return 0; }
+    int test() { return 0; }
+    -bash-4.4$ clang -c -g -O2 -target bpf t2.c
+    -bash-4.4$ readelf -S t2.o
+      ......
+      [ 8] .BTF              PROGBITS         0000000000000000  00000247
+           000000000000016e  0000000000000000           0     0     1
+      [ 9] .BTF.ext          PROGBITS         0000000000000000  000003b5
+           0000000000000060  0000000000000000           0     0     1
+      [10] .rel.BTF.ext      REL              0000000000000000  000007e0
+           0000000000000040  0000000000000010          16     9     8
+      ......
+    -bash-4.4$ clang -S -g -O2 -target bpf t2.c
+    -bash-4.4$ cat t2.s
+      ......
+            .section        .BTF,"",@progbits
+            .short  60319                   # 0xeb9f
+            .byte   1
+            .byte   0
+            .long   24
+            .long   0
+            .long   220
+            .long   220
+            .long   122
+            .long   0                       # BTF_KIND_FUNC_PROTO(id = 1)
+            .long   218103808               # 0xd000000
+            .long   2
+            .long   83                      # BTF_KIND_INT(id = 2)
+            .long   16777216                # 0x1000000
+            .long   4
+            .long   16777248                # 0x1000020
+      ......
+            .byte   0                       # string offset=0
+            .ascii  ".text"                 # string offset=1
+            .byte   0
+            .ascii  "/home/yhs/tmp-pahole/t2.c" # string offset=7
+            .byte   0
+            .ascii  "int main() { return 0; }" # string offset=33
+            .byte   0
+            .ascii  "int test() { return 0; }" # string offset=58
+            .byte   0
+            .ascii  "int"                   # string offset=83
+      ......
+            .section        .BTF.ext,"",@progbits
+            .short  60319                   # 0xeb9f
+            .byte   1
+            .byte   0
+            .long   24
+            .long   0
+            .long   28
+            .long   28
+            .long   44
+            .long   8                       # FuncInfo
+            .long   1                       # FuncInfo section string offset=1
+            .long   2
+            .long   .Lfunc_begin0
+            .long   3
+            .long   .Lfunc_begin1
+            .long   5
+            .long   16                      # LineInfo
+            .long   1                       # LineInfo section string offset=1
+            .long   2
+            .long   .Ltmp0
+            .long   7
+            .long   33
+            .long   7182                    # Line 7 Col 14
+            .long   .Ltmp3
+            .long   7
+            .long   58
+            .long   8206                    # Line 8 Col 14
+
+7. Testing
+**********
+
+Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests.
diff --git a/Documentation/bpf/drgn.rst b/Documentation/bpf/drgn.rst
new file mode 100644
index 000000000..41f223c31
--- /dev/null
+++ b/Documentation/bpf/drgn.rst
@@ -0,0 +1,213 @@
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+==============
+BPF drgn tools
+==============
+
+drgn scripts is a convenient and easy to use mechanism to retrieve arbitrary
+kernel data structures. drgn is not relying on kernel UAPI to read the data.
+Instead it's reading directly from ``/proc/kcore`` or vmcore and pretty prints
+the data based on DWARF debug information from vmlinux.
+
+This document describes BPF related drgn tools.
+
+See `drgn/tools`_ for all tools available at the moment and `drgn/doc`_ for
+more details on drgn itself.
+
+bpf_inspect.py
+--------------
+
+Description
+===========
+
+`bpf_inspect.py`_ is a tool intended to inspect BPF programs and maps. It can
+iterate over all programs and maps in the system and print basic information
+about these objects, including id, type and name.
+
+The main use-case `bpf_inspect.py`_ covers is to show BPF programs of types
+``BPF_PROG_TYPE_EXT`` and ``BPF_PROG_TYPE_TRACING`` attached to other BPF
+programs via ``freplace``/``fentry``/``fexit`` mechanisms, since there is no
+user-space API to get this information.
+
+Getting started
+===============
+
+List BPF programs (full names are obtained from BTF)::
+
+    % sudo bpf_inspect.py prog
+        27: BPF_PROG_TYPE_TRACEPOINT         tracepoint__tcp__tcp_send_reset
+      4632: BPF_PROG_TYPE_CGROUP_SOCK_ADDR   tw_ipt_bind
+     49464: BPF_PROG_TYPE_RAW_TRACEPOINT     raw_tracepoint__sched_process_exit
+
+List BPF maps::
+
+      % sudo bpf_inspect.py map
+        2577: BPF_MAP_TYPE_HASH                tw_ipt_vips
+        4050: BPF_MAP_TYPE_STACK_TRACE         stack_traces
+        4069: BPF_MAP_TYPE_PERCPU_ARRAY        ned_dctcp_cntr
+
+Find BPF programs attached to BPF program ``test_pkt_access``::
+
+      % sudo bpf_inspect.py p | grep test_pkt_access
+         650: BPF_PROG_TYPE_SCHED_CLS          test_pkt_access
+         654: BPF_PROG_TYPE_TRACING            test_main                        linked:[650->25: BPF_TRAMP_FEXIT test_pkt_access->test_pkt_access()]
+         655: BPF_PROG_TYPE_TRACING            test_subprog1                    linked:[650->29: BPF_TRAMP_FEXIT test_pkt_access->test_pkt_access_subprog1()]
+         656: BPF_PROG_TYPE_TRACING            test_subprog2                    linked:[650->31: BPF_TRAMP_FEXIT test_pkt_access->test_pkt_access_subprog2()]
+         657: BPF_PROG_TYPE_TRACING            test_subprog3                    linked:[650->21: BPF_TRAMP_FEXIT test_pkt_access->test_pkt_access_subprog3()]
+         658: BPF_PROG_TYPE_EXT                new_get_skb_len                  linked:[650->16: BPF_TRAMP_REPLACE test_pkt_access->get_skb_len()]
+         659: BPF_PROG_TYPE_EXT                new_get_skb_ifindex              linked:[650->23: BPF_TRAMP_REPLACE test_pkt_access->get_skb_ifindex()]
+         660: BPF_PROG_TYPE_EXT                new_get_constant                 linked:[650->19: BPF_TRAMP_REPLACE test_pkt_access->get_constant()]
+
+It can be seen that there is a program ``test_pkt_access``, id 650 and there
+are multiple other tracing and ext programs attached to functions in
+``test_pkt_access``.
+
+For example the line::
+
+         658: BPF_PROG_TYPE_EXT                new_get_skb_len                  linked:[650->16: BPF_TRAMP_REPLACE test_pkt_access->get_skb_len()]
+
+, means that BPF program id 658, type ``BPF_PROG_TYPE_EXT``, name
+``new_get_skb_len`` replaces (``BPF_TRAMP_REPLACE``) function ``get_skb_len()``
+that has BTF id 16 in BPF program id 650, name ``test_pkt_access``.
+
+Getting help:
+
+.. code-block:: none
+
+    % sudo bpf_inspect.py
+    usage: bpf_inspect.py [-h] {prog,p,map,m} ...
+
+    drgn script to list BPF programs or maps and their properties
+    unavailable via kernel API.
+
+    See https://github.com/osandov/drgn/ for more details on drgn.
+
+    optional arguments:
+      -h, --help      show this help message and exit
+
+    subcommands:
+      {prog,p,map,m}
+        prog (p)      list BPF programs
+        map (m)       list BPF maps
+
+Customization
+=============
+
+The script is intended to be customized by developers to print relevant
+information about BPF programs, maps and other objects.
+
+For example, to print ``struct bpf_prog_aux`` for BPF program id 53077:
+
+.. code-block:: none
+
+    % git diff
+    diff --git a/tools/bpf_inspect.py b/tools/bpf_inspect.py
+    index 650e228..aea2357 100755
+    --- a/tools/bpf_inspect.py
+    +++ b/tools/bpf_inspect.py
+    @@ -112,7 +112,9 @@ def list_bpf_progs(args):
+             if linked:
+                 linked = f" linked:[{linked}]"
+
+    -        print(f"{id_:>6}: {type_:32} {name:32} {linked}")
+    +        if id_ == 53077:
+    +            print(f"{id_:>6}: {type_:32} {name:32}")
+    +            print(f"{bpf_prog.aux}")
+
+
+     def list_bpf_maps(args):
+
+It produces the output::
+
+    % sudo bpf_inspect.py p
+     53077: BPF_PROG_TYPE_XDP                tw_xdp_policer
+    *(struct bpf_prog_aux *)0xffff8893fad4b400 = {
+            .refcnt = (atomic64_t){
+                    .counter = (long)58,
+            },
+            .used_map_cnt = (u32)1,
+            .max_ctx_offset = (u32)8,
+            .max_pkt_offset = (u32)15,
+            .max_tp_access = (u32)0,
+            .stack_depth = (u32)8,
+            .id = (u32)53077,
+            .func_cnt = (u32)0,
+            .func_idx = (u32)0,
+            .attach_btf_id = (u32)0,
+            .linked_prog = (struct bpf_prog *)0x0,
+            .verifier_zext = (bool)0,
+            .offload_requested = (bool)0,
+            .attach_btf_trace = (bool)0,
+            .func_proto_unreliable = (bool)0,
+            .trampoline_prog_type = (enum bpf_tramp_prog_type)BPF_TRAMP_FENTRY,
+            .trampoline = (struct bpf_trampoline *)0x0,
+            .tramp_hlist = (struct hlist_node){
+                    .next = (struct hlist_node *)0x0,
+                    .pprev = (struct hlist_node **)0x0,
+            },
+            .attach_func_proto = (const struct btf_type *)0x0,
+            .attach_func_name = (const char *)0x0,
+            .func = (struct bpf_prog **)0x0,
+            .jit_data = (void *)0x0,
+            .poke_tab = (struct bpf_jit_poke_descriptor *)0x0,
+            .size_poke_tab = (u32)0,
+            .ksym_tnode = (struct latch_tree_node){
+                    .node = (struct rb_node [2]){
+                            {
+                                    .__rb_parent_color = (unsigned long)18446612956263126665,
+                                    .rb_right = (struct rb_node *)0x0,
+                                    .rb_left = (struct rb_node *)0xffff88a0be3d0088,
+                            },
+                            {
+                                    .__rb_parent_color = (unsigned long)18446612956263126689,
+                                    .rb_right = (struct rb_node *)0x0,
+                                    .rb_left = (struct rb_node *)0xffff88a0be3d00a0,
+                            },
+                    },
+            },
+            .ksym_lnode = (struct list_head){
+                    .next = (struct list_head *)0xffff88bf481830b8,
+                    .prev = (struct list_head *)0xffff888309f536b8,
+            },
+            .ops = (const struct bpf_prog_ops *)xdp_prog_ops+0x0 = 0xffffffff820fa350,
+            .used_maps = (struct bpf_map **)0xffff889ff795de98,
+            .prog = (struct bpf_prog *)0xffffc9000cf2d000,
+            .user = (struct user_struct *)root_user+0x0 = 0xffffffff82444820,
+            .load_time = (u64)2408348759285319,
+            .cgroup_storage = (struct bpf_map *[2]){},
+            .name = (char [16])"tw_xdp_policer",
+            .security = (void *)0xffff889ff795d548,
+            .offload = (struct bpf_prog_offload *)0x0,
+            .btf = (struct btf *)0xffff8890ce6d0580,
+            .func_info = (struct bpf_func_info *)0xffff889ff795d240,
+            .func_info_aux = (struct bpf_func_info_aux *)0xffff889ff795de20,
+            .linfo = (struct bpf_line_info *)0xffff888a707afc00,
+            .jited_linfo = (void **)0xffff8893fad48600,
+            .func_info_cnt = (u32)1,
+            .nr_linfo = (u32)37,
+            .linfo_idx = (u32)0,
+            .num_exentries = (u32)0,
+            .extable = (struct exception_table_entry *)0xffffffffa032d950,
+            .stats = (struct bpf_prog_stats *)0x603fe3a1f6d0,
+            .work = (struct work_struct){
+                    .data = (atomic_long_t){
+                            .counter = (long)0,
+                    },
+                    .entry = (struct list_head){
+                            .next = (struct list_head *)0x0,
+                            .prev = (struct list_head *)0x0,
+                    },
+                    .func = (work_func_t)0x0,
+            },
+            .rcu = (struct callback_head){
+                    .next = (struct callback_head *)0x0,
+                    .func = (void (*)(struct callback_head *))0x0,
+            },
+    }
+
+
+.. Links
+.. _drgn/doc: https://drgn.readthedocs.io/en/latest/
+.. _drgn/tools: https://github.com/osandov/drgn/tree/master/tools
+.. _bpf_inspect.py:
+   https://github.com/osandov/drgn/blob/master/tools/bpf_inspect.py
diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
new file mode 100644
index 000000000..4f2874b72
--- /dev/null
+++ b/Documentation/bpf/index.rst
@@ -0,0 +1,90 @@
+=================
+BPF Documentation
+=================
+
+This directory contains documentation for the BPF (Berkeley Packet
+Filter) facility, with a focus on the extended BPF version (eBPF).
+
+This kernel side documentation is still work in progress. The main
+textual documentation is (for historical reasons) described in
+:ref:`networking-filter`, which describe both classical and extended
+BPF instruction-set.
+The Cilium project also maintains a `BPF and XDP Reference Guide`_
+that goes into great technical depth about the BPF Architecture.
+
+The primary info for the bpf syscall is available in the `man-pages`_
+for `bpf(2)`_.
+
+BPF Type Format (BTF)
+=====================
+
+.. toctree::
+   :maxdepth: 1
+
+   btf
+
+
+Frequently asked questions (FAQ)
+================================
+
+Two sets of Questions and Answers (Q&A) are maintained.
+
+.. toctree::
+   :maxdepth: 1
+
+   bpf_design_QA
+   bpf_devel_QA
+
+
+Helper functions
+================
+
+* `bpf-helpers(7)`_ maintains a list of helpers available to eBPF programs.
+
+
+Program types
+=============
+
+.. toctree::
+   :maxdepth: 1
+
+   prog_cgroup_sockopt
+   prog_cgroup_sysctl
+   prog_flow_dissector
+   bpf_lsm
+   prog_sk_lookup
+
+
+Map types
+=========
+
+.. toctree::
+   :maxdepth: 1
+
+   map_cgroup_storage
+
+
+Testing and debugging BPF
+=========================
+
+.. toctree::
+   :maxdepth: 1
+
+   drgn
+   s390
+
+
+Other
+=====
+
+.. toctree::
+   :maxdepth: 1
+
+   ringbuf
+
+.. Links:
+.. _networking-filter: ../networking/filter.rst
+.. _man-pages: https://www.kernel.org/doc/man-pages/
+.. _bpf(2): https://man7.org/linux/man-pages/man2/bpf.2.html
+.. _bpf-helpers(7): https://man7.org/linux/man-pages/man7/bpf-helpers.7.html
+.. _BPF and XDP Reference Guide: https://docs.cilium.io/en/latest/bpf/
diff --git a/Documentation/bpf/map_cgroup_storage.rst b/Documentation/bpf/map_cgroup_storage.rst
new file mode 100644
index 000000000..cab954301
--- /dev/null
+++ b/Documentation/bpf/map_cgroup_storage.rst
@@ -0,0 +1,169 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2020 Google LLC.
+
+===========================
+BPF_MAP_TYPE_CGROUP_STORAGE
+===========================
+
+The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized
+storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that
+attach to cgroups; the programs are made available by the same Kconfig. The
+storage is identified by the cgroup the program is attached to.
+
+The map provide a local storage at the cgroup that the BPF program is attached
+to. It provides a faster and simpler access than the general purpose hash
+table, which performs a hash table lookups, and requires user to track live
+cgroups on their own.
+
+This document describes the usage and semantics of the
+``BPF_MAP_TYPE_CGROUP_STORAGE`` map type. Some of its behaviors was changed in
+Linux 5.9 and this document will describe the differences.
+
+Usage
+=====
+
+The map uses key of type of either ``__u64 cgroup_inode_id`` or
+``struct bpf_cgroup_storage_key``, declared in ``linux/bpf.h``::
+
+    struct bpf_cgroup_storage_key {
+            __u64 cgroup_inode_id;
+            __u32 attach_type;
+    };
+
+``cgroup_inode_id`` is the inode id of the cgroup directory.
+``attach_type`` is the the program's attach type.
+
+Linux 5.9 added support for type ``__u64 cgroup_inode_id`` as the key type.
+When this key type is used, then all attach types of the particular cgroup and
+map will share the same storage. Otherwise, if the type is
+``struct bpf_cgroup_storage_key``, then programs of different attach types
+be isolated and see different storages.
+
+To access the storage in a program, use ``bpf_get_local_storage``::
+
+    void *bpf_get_local_storage(void *map, u64 flags)
+
+``flags`` is reserved for future use and must be 0.
+
+There is no implicit synchronization. Storages of ``BPF_MAP_TYPE_CGROUP_STORAGE``
+can be accessed by multiple programs across different CPUs, and user should
+take care of synchronization by themselves. The bpf infrastructure provides
+``struct bpf_spin_lock`` to synchronize the storage. See
+``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
+
+Examples
+========
+
+Usage with key type as ``struct bpf_cgroup_storage_key``::
+
+    #include <bpf/bpf.h>
+
+    struct {
+            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
+            __type(key, struct bpf_cgroup_storage_key);
+            __type(value, __u32);
+    } cgroup_storage SEC(".maps");
+
+    int program(struct __sk_buff *skb)
+    {
+            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
+            __sync_fetch_and_add(ptr, 1);
+
+            return 0;
+    }
+
+Userspace accessing map declared above::
+
+    #include <linux/bpf.h>
+    #include <linux/libbpf.h>
+
+    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
+    {
+            struct bpf_cgroup_storage_key = {
+                    .cgroup_inode_id = cgrp,
+                    .attach_type = type,
+            };
+            __u32 value;
+            bpf_map_lookup_elem(bpf_map__fd(map), &key, &value);
+            // error checking omitted
+            return value;
+    }
+
+Alternatively, using just ``__u64 cgroup_inode_id`` as key type::
+
+    #include <bpf/bpf.h>
+
+    struct {
+            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
+            __type(key, __u64);
+            __type(value, __u32);
+    } cgroup_storage SEC(".maps");
+
+    int program(struct __sk_buff *skb)
+    {
+            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
+            __sync_fetch_and_add(ptr, 1);
+
+            return 0;
+    }
+
+And userspace::
+
+    #include <linux/bpf.h>
+    #include <linux/libbpf.h>
+
+    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
+    {
+            __u32 value;
+            bpf_map_lookup_elem(bpf_map__fd(map), &cgrp, &value);
+            // error checking omitted
+            return value;
+    }
+
+Semantics
+=========
+
+``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE`` is a variant of this map type. This
+per-CPU variant will have different memory regions for each CPU for each
+storage. The non-per-CPU will have the same memory region for each storage.
+
+Prior to Linux 5.9, the lifetime of a storage is precisely per-attachment, and
+for a single ``CGROUP_STORAGE`` map, there can be at most one program loaded
+that uses the map. A program may be attached to multiple cgroups or have
+multiple attach types, and each attach creates a fresh zeroed storage. The
+storage is freed upon detach.
+
+There is a one-to-one association between the map of each type (per-CPU and
+non-per-CPU) and the BPF program during load verification time. As a result,
+each map can only be used by one BPF program and each BPF program can only use
+one storage map of each type. Because of map can only be used by one BPF
+program, sharing of this cgroup's storage with other BPF programs were
+impossible.
+
+Since Linux 5.9, storage can be shared by multiple programs. When a program is
+attached to a cgroup, the kernel would create a new storage only if the map
+does not already contain an entry for the cgroup and attach type pair, or else
+the old storage is reused for the new attachment. If the map is attach type
+shared, then attach type is simply ignored during comparison. Storage is freed
+only when either the map or the cgroup attached to is being freed. Detaching
+will not directly free the storage, but it may cause the reference to the map
+to reach zero and indirectly freeing all storage in the map.
+
+The map is not associated with any BPF program, thus making sharing possible.
+However, the BPF program can still only associate with one map of each type
+(per-CPU and non-per-CPU). A BPF program cannot use more than one
+``BPF_MAP_TYPE_CGROUP_STORAGE`` or more than one
+``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE``.
+
+In all versions, userspace may use the the attach parameters of cgroup and
+attach type pair in ``struct bpf_cgroup_storage_key`` as the key to the BPF map
+APIs to read or update the storage for a given attachment. For Linux 5.9
+attach type shared storages, only the first value in the struct, cgroup inode
+id, is used during comparison, so userspace may just specify a ``__u64``
+directly.
+
+The storage is bound at attach time. Even if the program is attached to parent
+and triggers in child, the storage still belongs to the parent.
+
+Userspace cannot create a new entry in the map or delete an existing entry.
+Program test runs always use a temporary storage.
diff --git a/Documentation/bpf/prog_cgroup_sockopt.rst b/Documentation/bpf/prog_cgroup_sockopt.rst
new file mode 100644
index 000000000..172f95720
--- /dev/null
+++ b/Documentation/bpf/prog_cgroup_sockopt.rst
@@ -0,0 +1,107 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================
+BPF_PROG_TYPE_CGROUP_SOCKOPT
+============================
+
+``BPF_PROG_TYPE_CGROUP_SOCKOPT`` program type can be attached to two
+cgroup hooks:
+
+* ``BPF_CGROUP_GETSOCKOPT`` - called every time process executes ``getsockopt``
+  system call.
+* ``BPF_CGROUP_SETSOCKOPT`` - called every time process executes ``setsockopt``
+  system call.
+
+The context (``struct bpf_sockopt``) has associated socket (``sk``) and
+all input arguments: ``level``, ``optname``, ``optval`` and ``optlen``.
+
+BPF_CGROUP_SETSOCKOPT
+=====================
+
+``BPF_CGROUP_SETSOCKOPT`` is triggered *before* the kernel handling of
+sockopt and it has writable context: it can modify the supplied arguments
+before passing them down to the kernel. This hook has access to the cgroup
+and socket local storage.
+
+If BPF program sets ``optlen`` to -1, the control will be returned
+back to the userspace after all other BPF programs in the cgroup
+chain finish (i.e. kernel ``setsockopt`` handling will *not* be executed).
+
+Note, that ``optlen`` can not be increased beyond the user-supplied
+value. It can only be decreased or set to -1. Any other value will
+trigger ``EFAULT``.
+
+Return Type
+-----------
+
+* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
+* ``1`` - success, continue with next BPF program in the cgroup chain.
+
+BPF_CGROUP_GETSOCKOPT
+=====================
+
+``BPF_CGROUP_GETSOCKOPT`` is triggered *after* the kernel handing of
+sockopt. The BPF hook can observe ``optval``, ``optlen`` and ``retval``
+if it's interested in whatever kernel has returned. BPF hook can override
+the values above, adjust ``optlen`` and reset ``retval`` to 0. If ``optlen``
+has been increased above initial ``getsockopt`` value (i.e. userspace
+buffer is too small), ``EFAULT`` is returned.
+
+This hook has access to the cgroup and socket local storage.
+
+Note, that the only acceptable value to set to ``retval`` is 0 and the
+original value that the kernel returned. Any other value will trigger
+``EFAULT``.
+
+Return Type
+-----------
+
+* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
+* ``1`` - success: copy ``optval`` and ``optlen`` to userspace, return
+  ``retval`` from the syscall (note that this can be overwritten by
+  the BPF program from the parent cgroup).
+
+Cgroup Inheritance
+==================
+
+Suppose, there is the following cgroup hierarchy where each cgroup
+has ``BPF_CGROUP_GETSOCKOPT`` attached at each level with
+``BPF_F_ALLOW_MULTI`` flag::
+
+  A (root, parent)
+   \
+    B (child)
+
+When the application calls ``getsockopt`` syscall from the cgroup B,
+the programs are executed from the bottom up: B, A. First program
+(B) sees the result of kernel's ``getsockopt``. It can optionally
+adjust ``optval``, ``optlen`` and reset ``retval`` to 0. After that
+control will be passed to the second (A) program which will see the
+same context as B including any potential modifications.
+
+Same for ``BPF_CGROUP_SETSOCKOPT``: if the program is attached to
+A and B, the trigger order is B, then A. If B does any changes
+to the input arguments (``level``, ``optname``, ``optval``, ``optlen``),
+then the next program in the chain (A) will see those changes,
+*not* the original input ``setsockopt`` arguments. The potentially
+modified values will be then passed down to the kernel.
+
+Large optval
+============
+When the ``optval`` is greater than the ``PAGE_SIZE``, the BPF program
+can access only the first ``PAGE_SIZE`` of that data. So it has to options:
+
+* Set ``optlen`` to zero, which indicates that the kernel should
+  use the original buffer from the userspace. Any modifications
+  done by the BPF program to the ``optval`` are ignored.
+* Set ``optlen`` to the value less than ``PAGE_SIZE``, which
+  indicates that the kernel should use BPF's trimmed ``optval``.
+
+When the BPF program returns with the ``optlen`` greater than
+``PAGE_SIZE``, the userspace will receive ``EFAULT`` errno.
+
+Example
+=======
+
+See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example
+of BPF program that handles socket options.
diff --git a/Documentation/bpf/prog_cgroup_sysctl.rst b/Documentation/bpf/prog_cgroup_sysctl.rst
new file mode 100644
index 000000000..677d6c637
--- /dev/null
+++ b/Documentation/bpf/prog_cgroup_sysctl.rst
@@ -0,0 +1,125 @@
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+===========================
+BPF_PROG_TYPE_CGROUP_SYSCTL
+===========================
+
+This document describes ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program type that
+provides cgroup-bpf hook for sysctl.
+
+The hook has to be attached to a cgroup and will be called every time a
+process inside that cgroup tries to read from or write to sysctl knob in proc.
+
+1. Attach type
+**************
+
+``BPF_CGROUP_SYSCTL`` attach type has to be used to attach
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` program to a cgroup.
+
+2. Context
+**********
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` provides access to the following context from
+BPF program::
+
+    struct bpf_sysctl {
+        __u32 write;
+        __u32 file_pos;
+    };
+
+* ``write`` indicates whether sysctl value is being read (``0``) or written
+  (``1``). This field is read-only.
+
+* ``file_pos`` indicates file position sysctl is being accessed at, read
+  or written. This field is read-write. Writing to the field sets the starting
+  position in sysctl proc file ``read(2)`` will be reading from or ``write(2)``
+  will be writing to. Writing zero to the field can be used e.g. to override
+  whole sysctl value by ``bpf_sysctl_set_new_value()`` on ``write(2)`` even
+  when it's called by user space on ``file_pos > 0``. Writing non-zero
+  value to the field can be used to access part of sysctl value starting from
+  specified ``file_pos``. Not all sysctl support access with ``file_pos !=
+  0``, e.g. writes to numeric sysctl entries must always be at file position
+  ``0``. See also ``kernel.sysctl_writes_strict`` sysctl.
+
+See `linux/bpf.h`_ for more details on how context field can be accessed.
+
+3. Return code
+**************
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` program must return one of the following
+return codes:
+
+* ``0`` means "reject access to sysctl";
+* ``1`` means "proceed with access".
+
+If program returns ``0`` user space will get ``-1`` from ``read(2)`` or
+``write(2)`` and ``errno`` will be set to ``EPERM``.
+
+4. Helpers
+**********
+
+Since sysctl knob is represented by a name and a value, sysctl specific BPF
+helpers focus on providing access to these properties:
+
+* ``bpf_sysctl_get_name()`` to get sysctl name as it is visible in
+  ``/proc/sys`` into provided by BPF program buffer;
+
+* ``bpf_sysctl_get_current_value()`` to get string value currently held by
+  sysctl into provided by BPF program buffer. This helper is available on both
+  ``read(2)`` from and ``write(2)`` to sysctl;
+
+* ``bpf_sysctl_get_new_value()`` to get new string value currently being
+  written to sysctl before actual write happens. This helper can be used only
+  on ``ctx->write == 1``;
+
+* ``bpf_sysctl_set_new_value()`` to override new string value currently being
+  written to sysctl before actual write happens. Sysctl value will be
+  overridden starting from the current ``ctx->file_pos``. If the whole value
+  has to be overridden BPF program can set ``file_pos`` to zero before calling
+  to the helper. This helper can be used only on ``ctx->write == 1``. New
+  string value set by the helper is treated and verified by kernel same way as
+  an equivalent string passed by user space.
+
+BPF program sees sysctl value same way as user space does in proc filesystem,
+i.e. as a string. Since many sysctl values represent an integer or a vector
+of integers, the following helpers can be used to get numeric value from the
+string:
+
+* ``bpf_strtol()`` to convert initial part of the string to long integer
+  similar to user space `strtol(3)`_;
+* ``bpf_strtoul()`` to convert initial part of the string to unsigned long
+  integer similar to user space `strtoul(3)`_;
+
+See `linux/bpf.h`_ for more details on helpers described here.
+
+5. Examples
+***********
+
+See `test_sysctl_prog.c`_ for an example of BPF program in C that access
+sysctl name and value, parses string value to get vector of integers and uses
+the result to make decision whether to allow or deny access to sysctl.
+
+6. Notes
+********
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` is intended to be used in **trusted** root
+environment, for example to monitor sysctl usage or catch unreasonable values
+an application, running as root in a separate cgroup, is trying to set.
+
+Since `task_dfl_cgroup(current)` is called at `sys_read` / `sys_write` time it
+may return results different from that at `sys_open` time, i.e. process that
+opened sysctl file in proc filesystem may differ from process that is trying
+to read from / write to it and two such processes may run in different
+cgroups, what means ``BPF_PROG_TYPE_CGROUP_SYSCTL`` should not be used as a
+security mechanism to limit sysctl usage.
+
+As with any cgroup-bpf program additional care should be taken if an
+application running as root in a cgroup should not be allowed to
+detach/replace BPF program attached by administrator.
+
+.. Links
+.. _linux/bpf.h: ../../include/uapi/linux/bpf.h
+.. _strtol(3): http://man7.org/linux/man-pages/man3/strtol.3p.html
+.. _strtoul(3): http://man7.org/linux/man-pages/man3/strtoul.3p.html
+.. _test_sysctl_prog.c:
+   ../../tools/testing/selftests/bpf/progs/test_sysctl_prog.c
diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
new file mode 100644
index 000000000..4d86780ab
--- /dev/null
+++ b/Documentation/bpf/prog_flow_dissector.rst
@@ -0,0 +1,147 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================
+BPF_PROG_TYPE_FLOW_DISSECTOR
+============================
+
+Overview
+========
+
+Flow dissector is a routine that parses metadata out of the packets. It's
+used in the various places in the networking subsystem (RFS, flow hash, etc).
+
+BPF flow dissector is an attempt to reimplement C-based flow dissector logic
+in BPF to gain all the benefits of BPF verifier (namely, limits on the
+number of instructions and tail calls).
+
+API
+===
+
+BPF flow dissector programs operate on an ``__sk_buff``. However, only the
+limited set of fields is allowed: ``data``, ``data_end`` and ``flow_keys``.
+``flow_keys`` is ``struct bpf_flow_keys`` and contains flow dissector input
+and output arguments.
+
+The inputs are:
+  * ``nhoff`` - initial offset of the networking header
+  * ``thoff`` - initial offset of the transport header, initialized to nhoff
+  * ``n_proto`` - L3 protocol type, parsed out of L2 header
+  * ``flags`` - optional flags
+
+Flow dissector BPF program should fill out the rest of the ``struct
+bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be
+also adjusted accordingly.
+
+The return code of the BPF program is either BPF_OK to indicate successful
+dissection, or BPF_DROP to indicate parsing error.
+
+__sk_buff->data
+===============
+
+In the VLAN-less case, this is what the initial state of the BPF flow
+dissector looks like::
+
+  +------+------+------------+-----------+
+  | DMAC | SMAC | ETHER_TYPE | L3_HEADER |
+  +------+------+------------+-----------+
+                              ^
+                              |
+                              +-- flow dissector starts here
+
+
+.. code:: c
+
+  skb->data + flow_keys->nhoff point to the first byte of L3_HEADER
+  flow_keys->thoff = nhoff
+  flow_keys->n_proto = ETHER_TYPE
+
+In case of VLAN, flow dissector can be called with the two different states.
+
+Pre-VLAN parsing::
+
+  +------+------+------+-----+-----------+-----------+
+  | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
+  +------+------+------+-----+-----------+-----------+
+                        ^
+                        |
+                        +-- flow dissector starts here
+
+.. code:: c
+
+  skb->data + flow_keys->nhoff point the to first byte of TCI
+  flow_keys->thoff = nhoff
+  flow_keys->n_proto = TPID
+
+Please note that TPID can be 802.1AD and, hence, BPF program would
+have to parse VLAN information twice for double tagged packets.
+
+
+Post-VLAN parsing::
+
+  +------+------+------+-----+-----------+-----------+
+  | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
+  +------+------+------+-----+-----------+-----------+
+                                          ^
+                                          |
+                                          +-- flow dissector starts here
+
+.. code:: c
+
+  skb->data + flow_keys->nhoff point the to first byte of L3_HEADER
+  flow_keys->thoff = nhoff
+  flow_keys->n_proto = ETHER_TYPE
+
+In this case VLAN information has been processed before the flow dissector
+and BPF flow dissector is not required to handle it.
+
+
+The takeaway here is as follows: BPF flow dissector program can be called with
+the optional VLAN header and should gracefully handle both cases: when single
+or double VLAN is present and when it is not present. The same program
+can be called for both cases and would have to be written carefully to
+handle both cases.
+
+
+Flags
+=====
+
+``flow_keys->flags`` might contain optional input flags that work as follows:
+
+* ``BPF_FLOW_DISSECTOR_F_PARSE_1ST_FRAG`` - tells BPF flow dissector to
+  continue parsing first fragment; the default expected behavior is that
+  flow dissector returns as soon as it finds out that the packet is fragmented;
+  used by ``eth_get_headlen`` to estimate length of all headers for GRO.
+* ``BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL`` - tells BPF flow dissector to
+  stop parsing as soon as it reaches IPv6 flow label; used by
+  ``___skb_get_hash`` and ``__skb_get_hash_symmetric`` to get flow hash.
+* ``BPF_FLOW_DISSECTOR_F_STOP_AT_ENCAP`` - tells BPF flow dissector to stop
+  parsing as soon as it reaches encapsulated headers; used by routing
+  infrastructure.
+
+
+Reference Implementation
+========================
+
+See ``tools/testing/selftests/bpf/progs/bpf_flow.c`` for the reference
+implementation and ``tools/testing/selftests/bpf/flow_dissector_load.[hc]``
+for the loader. bpftool can be used to load BPF flow dissector program as well.
+
+The reference implementation is organized as follows:
+  * ``jmp_table`` map that contains sub-programs for each supported L3 protocol
+  * ``_dissect`` routine - entry point; it does input ``n_proto`` parsing and
+    does ``bpf_tail_call`` to the appropriate L3 handler
+
+Since BPF at this point doesn't support looping (or any jumping back),
+jmp_table is used instead to handle multiple levels of encapsulation (and
+IPv6 options).
+
+
+Current Limitations
+===================
+BPF flow dissector doesn't support exporting all the metadata that in-kernel
+C-based implementation can export. Notable example is single VLAN (802.1Q)
+and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
+for a set of information that's currently can be exported from the BPF context.
+
+When BPF flow dissector is attached to the root network namespace (machine-wide
+policy), users can't override it in their child network namespaces.
diff --git a/Documentation/bpf/prog_sk_lookup.rst b/Documentation/bpf/prog_sk_lookup.rst
new file mode 100644
index 000000000..85a305c19
--- /dev/null
+++ b/Documentation/bpf/prog_sk_lookup.rst
@@ -0,0 +1,98 @@
+.. SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+
+=====================
+BPF sk_lookup program
+=====================
+
+BPF sk_lookup program type (``BPF_PROG_TYPE_SK_LOOKUP``) introduces programmability
+into the socket lookup performed by the transport layer when a packet is to be
+delivered locally.
+
+When invoked BPF sk_lookup program can select a socket that will receive the
+incoming packet by calling the ``bpf_sk_assign()`` BPF helper function.
+
+Hooks for a common attach point (``BPF_SK_LOOKUP``) exist for both TCP and UDP.
+
+Motivation
+==========
+
+BPF sk_lookup program type was introduced to address setup scenarios where
+binding sockets to an address with ``bind()`` socket call is impractical, such
+as:
+
+1. receiving connections on a range of IP addresses, e.g. 192.0.2.0/24, when
+   binding to a wildcard address ``INADRR_ANY`` is not possible due to a port
+   conflict,
+2. receiving connections on all or a wide range of ports, i.e. an L7 proxy use
+   case.
+
+Such setups would require creating and ``bind()``'ing one socket to each of the
+IP address/port in the range, leading to resource consumption and potential
+latency spikes during socket lookup.
+
+Attachment
+==========
+
+BPF sk_lookup program can be attached to a network namespace with
+``bpf(BPF_LINK_CREATE, ...)`` syscall using the ``BPF_SK_LOOKUP`` attach type and a
+netns FD as attachment ``target_fd``.
+
+Multiple programs can be attached to one network namespace. Programs will be
+invoked in the same order as they were attached.
+
+Hooks
+=====
+
+The attached BPF sk_lookup programs run whenever the transport layer needs to
+find a listening (TCP) or an unconnected (UDP) socket for an incoming packet.
+
+Incoming traffic to established (TCP) and connected (UDP) sockets is delivered
+as usual without triggering the BPF sk_lookup hook.
+
+The attached BPF programs must return with either ``SK_PASS`` or ``SK_DROP``
+verdict code. As for other BPF program types that are network filters,
+``SK_PASS`` signifies that the socket lookup should continue on to regular
+hashtable-based lookup, while ``SK_DROP`` causes the transport layer to drop the
+packet.
+
+A BPF sk_lookup program can also select a socket to receive the packet by
+calling ``bpf_sk_assign()`` BPF helper. Typically, the program looks up a socket
+in a map holding sockets, such as ``SOCKMAP`` or ``SOCKHASH``, and passes a
+``struct bpf_sock *`` to ``bpf_sk_assign()`` helper to record the
+selection. Selecting a socket only takes effect if the program has terminated
+with ``SK_PASS`` code.
+
+When multiple programs are attached, the end result is determined from return
+codes of all the programs according to the following rules:
+
+1. If any program returned ``SK_PASS`` and selected a valid socket, the socket
+   is used as the result of the socket lookup.
+2. If more than one program returned ``SK_PASS`` and selected a socket, the last
+   selection takes effect.
+3. If any program returned ``SK_DROP``, and no program returned ``SK_PASS`` and
+   selected a socket, socket lookup fails.
+4. If all programs returned ``SK_PASS`` and none of them selected a socket,
+   socket lookup continues on.
+
+API
+===
+
+In its context, an instance of ``struct bpf_sk_lookup``, BPF sk_lookup program
+receives information about the packet that triggered the socket lookup. Namely:
+
+* IP version (``AF_INET`` or ``AF_INET6``),
+* L4 protocol identifier (``IPPROTO_TCP`` or ``IPPROTO_UDP``),
+* source and destination IP address,
+* source and destination L4 port,
+* the socket that has been selected with ``bpf_sk_assign()``.
+
+Refer to ``struct bpf_sk_lookup`` declaration in ``linux/bpf.h`` user API
+header, and `bpf-helpers(7)
+<https://man7.org/linux/man-pages/man7/bpf-helpers.7.html>`_ man-page section
+for ``bpf_sk_assign()`` for details.
+
+Example
+=======
+
+See ``tools/testing/selftests/bpf/prog_tests/sk_lookup.c`` for the reference
+implementation.
diff --git a/Documentation/bpf/ringbuf.rst b/Documentation/bpf/ringbuf.rst
new file mode 100644
index 000000000..6a615cd62
--- /dev/null
+++ b/Documentation/bpf/ringbuf.rst
@@ -0,0 +1,206 @@
+===============
+BPF ring buffer
+===============
+
+This document describes BPF ring buffer design, API, and implementation details.
+
+.. contents::
+    :local:
+    :depth: 2
+
+Motivation
+----------
+
+There are two distinctive motivators for this work, which are not satisfied by
+existing perf buffer, which prompted creation of a new ring buffer
+implementation.
+
+- more efficient memory utilization by sharing ring buffer across CPUs;
+- preserving ordering of events that happen sequentially in time, even across
+  multiple CPUs (e.g., fork/exec/exit events for a task).
+
+These two problems are independent, but perf buffer fails to satisfy both.
+Both are a result of a choice to have per-CPU perf ring buffer.  Both can be
+also solved by having an MPSC implementation of ring buffer. The ordering
+problem could technically be solved for perf buffer with some in-kernel
+counting, but given the first one requires an MPSC buffer, the same solution
+would solve the second problem automatically.
+
+Semantics and APIs
+------------------
+
+Single ring buffer is presented to BPF programs as an instance of BPF map of
+type ``BPF_MAP_TYPE_RINGBUF``. Two other alternatives considered, but
+ultimately rejected.
+
+One way would be to, similar to ``BPF_MAP_TYPE_PERF_EVENT_ARRAY``, make
+``BPF_MAP_TYPE_RINGBUF`` could represent an array of ring buffers, but not
+enforce "same CPU only" rule. This would be more familiar interface compatible
+with existing perf buffer use in BPF, but would fail if application needed more
+advanced logic to lookup ring buffer by arbitrary key.
+``BPF_MAP_TYPE_HASH_OF_MAPS`` addresses this with current approach.
+Additionally, given the performance of BPF ringbuf, many use cases would just
+opt into a simple single ring buffer shared among all CPUs, for which current
+approach would be an overkill.
+
+Another approach could introduce a new concept, alongside BPF map, to represent
+generic "container" object, which doesn't necessarily have key/value interface
+with lookup/update/delete operations. This approach would add a lot of extra
+infrastructure that has to be built for observability and verifier support. It
+would also add another concept that BPF developers would have to familiarize
+themselves with, new syntax in libbpf, etc. But then would really provide no
+additional benefits over the approach of using a map.  ``BPF_MAP_TYPE_RINGBUF``
+doesn't support lookup/update/delete operations, but so doesn't few other map
+types (e.g., queue and stack; array doesn't support delete, etc).
+
+The approach chosen has an advantage of re-using existing BPF map
+infrastructure (introspection APIs in kernel, libbpf support, etc), being
+familiar concept (no need to teach users a new type of object in BPF program),
+and utilizing existing tooling (bpftool). For common scenario of using a single
+ring buffer for all CPUs, it's as simple and straightforward, as would be with
+a dedicated "container" object. On the other hand, by being a map, it can be
+combined with ``ARRAY_OF_MAPS`` and ``HASH_OF_MAPS`` map-in-maps to implement
+a wide variety of topologies, from one ring buffer for each CPU (e.g., as
+a replacement for perf buffer use cases), to a complicated application
+hashing/sharding of ring buffers (e.g., having a small pool of ring buffers
+with hashed task's tgid being a look up key to preserve order, but reduce
+contention).
+
+Key and value sizes are enforced to be zero. ``max_entries`` is used to specify
+the size of ring buffer and has to be a power of 2 value.
+
+There are a bunch of similarities between perf buffer
+(``BPF_MAP_TYPE_PERF_EVENT_ARRAY``) and new BPF ring buffer semantics:
+
+- variable-length records;
+- if there is no more space left in ring buffer, reservation fails, no
+  blocking;
+- memory-mappable data area for user-space applications for ease of
+  consumption and high performance;
+- epoll notifications for new incoming data;
+- but still the ability to do busy polling for new data to achieve the
+  lowest latency, if necessary.
+
+BPF ringbuf provides two sets of APIs to BPF programs:
+
+- ``bpf_ringbuf_output()`` allows to *copy* data from one place to a ring
+  buffer, similarly to ``bpf_perf_event_output()``;
+- ``bpf_ringbuf_reserve()``/``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()``
+  APIs split the whole process into two steps. First, a fixed amount of space
+  is reserved. If successful, a pointer to a data inside ring buffer data
+  area is returned, which BPF programs can use similarly to a data inside
+  array/hash maps. Once ready, this piece of memory is either committed or
+  discarded. Discard is similar to commit, but makes consumer ignore the
+  record.
+
+``bpf_ringbuf_output()`` has disadvantage of incurring extra memory copy,
+because record has to be prepared in some other place first. But it allows to
+submit records of the length that's not known to verifier beforehand. It also
+closely matches ``bpf_perf_event_output()``, so will simplify migration
+significantly.
+
+``bpf_ringbuf_reserve()`` avoids the extra copy of memory by providing a memory
+pointer directly to ring buffer memory. In a lot of cases records are larger
+than BPF stack space allows, so many programs have use extra per-CPU array as
+a temporary heap for preparing sample. bpf_ringbuf_reserve() avoid this needs
+completely. But in exchange, it only allows a known constant size of memory to
+be reserved, such that verifier can verify that BPF program can't access memory
+outside its reserved record space. bpf_ringbuf_output(), while slightly slower
+due to extra memory copy, covers some use cases that are not suitable for
+``bpf_ringbuf_reserve()``.
+
+The difference between commit and discard is very small. Discard just marks
+a record as discarded, and such records are supposed to be ignored by consumer
+code. Discard is useful for some advanced use-cases, such as ensuring
+all-or-nothing multi-record submission, or emulating temporary
+``malloc()``/``free()`` within single BPF program invocation.
+
+Each reserved record is tracked by verifier through existing
+reference-tracking logic, similar to socket ref-tracking. It is thus
+impossible to reserve a record, but forget to submit (or discard) it.
+
+``bpf_ringbuf_query()`` helper allows to query various properties of ring
+buffer.  Currently 4 are supported:
+
+- ``BPF_RB_AVAIL_DATA`` returns amount of unconsumed data in ring buffer;
+- ``BPF_RB_RING_SIZE`` returns the size of ring buffer;
+- ``BPF_RB_CONS_POS``/``BPF_RB_PROD_POS`` returns current logical possition
+  of consumer/producer, respectively.
+
+Returned values are momentarily snapshots of ring buffer state and could be
+off by the time helper returns, so this should be used only for
+debugging/reporting reasons or for implementing various heuristics, that take
+into account highly-changeable nature of some of those characteristics.
+
+One such heuristic might involve more fine-grained control over poll/epoll
+notifications about new data availability in ring buffer. Together with
+``BPF_RB_NO_WAKEUP``/``BPF_RB_FORCE_WAKEUP`` flags for output/commit/discard
+helpers, it allows BPF program a high degree of control and, e.g., more
+efficient batched notifications. Default self-balancing strategy, though,
+should be adequate for most applications and will work reliable and efficiently
+already.
+
+Design and Implementation
+-------------------------
+
+This reserve/commit schema allows a natural way for multiple producers, either
+on different CPUs or even on the same CPU/in the same BPF program, to reserve
+independent records and work with them without blocking other producers. This
+means that if BPF program was interruped by another BPF program sharing the
+same ring buffer, they will both get a record reserved (provided there is
+enough space left) and can work with it and submit it independently. This
+applies to NMI context as well, except that due to using a spinlock during
+reservation, in NMI context, ``bpf_ringbuf_reserve()`` might fail to get
+a lock, in which case reservation will fail even if ring buffer is not full.
+
+The ring buffer itself internally is implemented as a power-of-2 sized
+circular buffer, with two logical and ever-increasing counters (which might
+wrap around on 32-bit architectures, that's not a problem):
+
+- consumer counter shows up to which logical position consumer consumed the
+  data;
+- producer counter denotes amount of data reserved by all producers.
+
+Each time a record is reserved, producer that "owns" the record will
+successfully advance producer counter. At that point, data is still not yet
+ready to be consumed, though. Each record has 8 byte header, which contains the
+length of reserved record, as well as two extra bits: busy bit to denote that
+record is still being worked on, and discard bit, which might be set at commit
+time if record is discarded. In the latter case, consumer is supposed to skip
+the record and move on to the next one. Record header also encodes record's
+relative offset from the beginning of ring buffer data area (in pages). This
+allows ``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()`` to accept only the
+pointer to the record itself, without requiring also the pointer to ring buffer
+itself. Ring buffer memory location will be restored from record metadata
+header. This significantly simplifies verifier, as well as improving API
+usability.
+
+Producer counter increments are serialized under spinlock, so there is
+a strict ordering between reservations. Commits, on the other hand, are
+completely lockless and independent. All records become available to consumer
+in the order of reservations, but only after all previous records where
+already committed. It is thus possible for slow producers to temporarily hold
+off submitted records, that were reserved later.
+
+One interesting implementation bit, that significantly simplifies (and thus
+speeds up as well) implementation of both producers and consumers is how data
+area is mapped twice contiguously back-to-back in the virtual memory. This
+allows to not take any special measures for samples that have to wrap around
+at the end of the circular buffer data area, because the next page after the
+last data page would be first data page again, and thus the sample will still
+appear completely contiguous in virtual memory. See comment and a simple ASCII
+diagram showing this visually in ``bpf_ringbuf_area_alloc()``.
+
+Another feature that distinguishes BPF ringbuf from perf ring buffer is
+a self-pacing notifications of new data being availability.
+``bpf_ringbuf_commit()`` implementation will send a notification of new record
+being available after commit only if consumer has already caught up right up to
+the record being committed. If not, consumer still has to catch up and thus
+will see new data anyways without needing an extra poll notification.
+Benchmarks (see tools/testing/selftests/bpf/benchs/bench_ringbufs.c) show that
+this allows to achieve a very high throughput without having to resort to
+tricks like "notify only every Nth sample", which are necessary with perf
+buffer. For extreme cases, when BPF program wants more manual control of
+notifications, commit/discard/output helpers accept ``BPF_RB_NO_WAKEUP`` and
+``BPF_RB_FORCE_WAKEUP`` flags, which give full control over notifications of
+data availability, but require extra caution and diligence in using this API.
diff --git a/Documentation/bpf/s390.rst b/Documentation/bpf/s390.rst
new file mode 100644
index 000000000..21ecb309d
--- /dev/null
+++ b/Documentation/bpf/s390.rst
@@ -0,0 +1,205 @@
+===================
+Testing BPF on s390
+===================
+
+1. Introduction
+***************
+
+IBM Z are mainframe computers, which are descendants of IBM System/360 from
+year 1964. They are supported by the Linux kernel under the name "s390". This
+document describes how to test BPF in an s390 QEMU guest.
+
+2. One-time setup
+*****************
+
+The following is required to build and run the test suite:
+
+  * s390 GCC
+  * s390 development headers and libraries
+  * Clang with BPF support
+  * QEMU with s390 support
+  * Disk image with s390 rootfs
+
+Debian supports installing compiler and libraries for s390 out of the box.
+Users of other distros may use debootstrap in order to set up a Debian chroot::
+
+  sudo debootstrap \
+    --variant=minbase \
+    --include=sudo \
+    testing \
+    ./s390-toolchain
+  sudo mount --rbind /dev ./s390-toolchain/dev
+  sudo mount --rbind /proc ./s390-toolchain/proc
+  sudo mount --rbind /sys ./s390-toolchain/sys
+  sudo chroot ./s390-toolchain
+
+Once on Debian, the build prerequisites can be installed as follows::
+
+  sudo dpkg --add-architecture s390x
+  sudo apt-get update
+  sudo apt-get install \
+    bc \
+    bison \
+    cmake \
+    debootstrap \
+    dwarves \
+    flex \
+    g++ \
+    gcc \
+    g++-s390x-linux-gnu \
+    gcc-s390x-linux-gnu \
+    gdb-multiarch \
+    git \
+    make \
+    python3 \
+    qemu-system-misc \
+    qemu-utils \
+    rsync \
+    libcap-dev:s390x \
+    libelf-dev:s390x \
+    libncurses-dev
+
+Latest Clang targeting BPF can be installed as follows::
+
+  git clone https://github.com/llvm/llvm-project.git
+  ln -s ../../clang llvm-project/llvm/tools/
+  mkdir llvm-project-build
+  cd llvm-project-build
+  cmake \
+    -DLLVM_TARGETS_TO_BUILD=BPF \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_INSTALL_PREFIX=/opt/clang-bpf \
+    ../llvm-project/llvm
+  make
+  sudo make install
+  export PATH=/opt/clang-bpf/bin:$PATH
+
+The disk image can be prepared using a loopback mount and debootstrap::
+
+  qemu-img create -f raw ./s390.img 1G
+  sudo losetup -f ./s390.img
+  sudo mkfs.ext4 /dev/loopX
+  mkdir ./s390.rootfs
+  sudo mount /dev/loopX ./s390.rootfs
+  sudo debootstrap \
+    --foreign \
+    --arch=s390x \
+    --variant=minbase \
+    --include=" \
+      iproute2, \
+      iputils-ping, \
+      isc-dhcp-client, \
+      kmod, \
+      libcap2, \
+      libelf1, \
+      netcat, \
+      procps" \
+    testing \
+    ./s390.rootfs
+  sudo umount ./s390.rootfs
+  sudo losetup -d /dev/loopX
+
+3. Compilation
+**************
+
+In addition to the usual Kconfig options required to run the BPF test suite, it
+is also helpful to select::
+
+  CONFIG_NET_9P=y
+  CONFIG_9P_FS=y
+  CONFIG_NET_9P_VIRTIO=y
+  CONFIG_VIRTIO_PCI=y
+
+as that would enable a very easy way to share files with the s390 virtual
+machine.
+
+Compiling kernel, modules and testsuite, as well as preparing gdb scripts to
+simplify debugging, can be done using the following commands::
+
+  make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- menuconfig
+  make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- bzImage modules scripts_gdb
+  make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- \
+    -C tools/testing/selftests \
+    TARGETS=bpf \
+    INSTALL_PATH=$PWD/tools/testing/selftests/kselftest_install \
+    install
+
+4. Running the test suite
+*************************
+
+The virtual machine can be started as follows::
+
+  qemu-system-s390x \
+    -cpu max,zpci=on \
+    -smp 2 \
+    -m 4G \
+    -kernel linux/arch/s390/boot/compressed/vmlinux \
+    -drive file=./s390.img,if=virtio,format=raw \
+    -nographic \
+    -append 'root=/dev/vda rw console=ttyS1' \
+    -virtfs local,path=./linux,security_model=none,mount_tag=linux \
+    -object rng-random,filename=/dev/urandom,id=rng0 \
+    -device virtio-rng-ccw,rng=rng0 \
+    -netdev user,id=net0 \
+    -device virtio-net-ccw,netdev=net0
+
+When using this on a real IBM Z, ``-enable-kvm`` may be added for better
+performance. When starting the virtual machine for the first time, disk image
+setup must be finalized using the following command::
+
+  /debootstrap/debootstrap --second-stage
+
+Directory with the code built on the host as well as ``/proc`` and ``/sys``
+need to be mounted as follows::
+
+  mkdir -p /linux
+  mount -t 9p linux /linux
+  mount -t proc proc /proc
+  mount -t sysfs sys /sys
+
+After that, the test suite can be run using the following commands::
+
+  cd /linux/tools/testing/selftests/kselftest_install
+  ./run_kselftest.sh
+
+As usual, tests can be also run individually::
+
+  cd /linux/tools/testing/selftests/bpf
+  ./test_verifier
+
+5. Debugging
+************
+
+It is possible to debug the s390 kernel using QEMU GDB stub, which is activated
+by passing ``-s`` to QEMU.
+
+It is preferable to turn KASLR off, so that gdb would know where to find the
+kernel image in memory, by building the kernel with::
+
+  RANDOMIZE_BASE=n
+
+GDB can then be attached using the following command::
+
+  gdb-multiarch -ex 'target remote localhost:1234' ./vmlinux
+
+6. Network
+**********
+
+In case one needs to use the network in the virtual machine in order to e.g.
+install additional packages, it can be configured using::
+
+  dhclient eth0
+
+7. Links
+********
+
+This document is a compilation of techniques, whose more comprehensive
+descriptions can be found by following these links:
+
+- `Debootstrap <https://wiki.debian.org/EmDebian/CrossDebootstrap>`_
+- `Multiarch <https://wiki.debian.org/Multiarch/HOWTO>`_
+- `Building LLVM <https://llvm.org/docs/CMake.html>`_
+- `Cross-compiling the kernel <https://wiki.gentoo.org/wiki/Embedded_Handbook/General/Cross-compiling_the_kernel>`_
+- `QEMU s390x Guest Support <https://wiki.qemu.org/Documentation/Platforms/S390X>`_
+- `Plan 9 folder sharing over Virtio <https://wiki.qemu.org/Documentation/9psetup>`_
+- `Using GDB with QEMU <https://wiki.osdev.org/Kernel_Debugging#Use_GDB_with_QEMU>`_