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diff --git a/Documentation/bpf/bpf_devel_QA.rst b/Documentation/bpf/bpf_devel_QA.rst new file mode 100644 index 000000000..5b613d2a5 --- /dev/null +++ b/Documentation/bpf/bpf_devel_QA.rst @@ -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! |