Adding upstream version 5.10.209.upstream/5.10.209upstream

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

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+Assembler Annotations
+=====================
+
+Copyright (c) 2017-2019 Jiri Slaby
+
+This document describes the new macros for annotation of data and code in
+assembly. In particular, it contains information about ``SYM_FUNC_START``,
+``SYM_FUNC_END``, ``SYM_CODE_START``, and similar.
+
+Rationale
+---------
+Some code like entries, trampolines, or boot code needs to be written in
+assembly. The same as in C, such code is grouped into functions and
+accompanied with data. Standard assemblers do not force users into precisely
+marking these pieces as code, data, or even specifying their length.
+Nevertheless, assemblers provide developers with such annotations to aid
+debuggers throughout assembly. On top of that, developers also want to mark
+some functions as *global* in order to be visible outside of their translation
+units.
+
+Over time, the Linux kernel has adopted macros from various projects (like
+``binutils``) to facilitate such annotations. So for historic reasons,
+developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other
+annotations in assembly.  Due to the lack of their documentation, the macros
+are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was
+intended to denote the beginning of global symbols (be it data or code).
+``END`` used to mark the end of data or end of special functions with
+*non-standard* calling convention. In contrast, ``ENDPROC`` should annotate
+only ends of *standard* functions.
+
+When these macros are used correctly, they help assemblers generate a nice
+object with both sizes and types set correctly. For example, the result of
+``arch/x86/lib/putuser.S``::
+
+   Num:    Value          Size Type    Bind   Vis      Ndx Name
+    25: 0000000000000000    33 FUNC    GLOBAL DEFAULT    1 __put_user_1
+    29: 0000000000000030    37 FUNC    GLOBAL DEFAULT    1 __put_user_2
+    32: 0000000000000060    36 FUNC    GLOBAL DEFAULT    1 __put_user_4
+    35: 0000000000000090    37 FUNC    GLOBAL DEFAULT    1 __put_user_8
+
+This is not only important for debugging purposes. When there are properly
+annotated objects like this, tools can be run on them to generate more useful
+information. In particular, on properly annotated objects, ``objtool`` can be
+run to check and fix the object if needed. Currently, ``objtool`` can report
+missing frame pointer setup/destruction in functions. It can also
+automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>`
+for most code. Both of these are especially important to support reliable
+stack traces which are in turn necessary for :doc:`Kernel live patching
+<livepatch/livepatch>`.
+
+Caveat and Discussion
+---------------------
+As one might realize, there were only three macros previously. That is indeed
+insufficient to cover all the combinations of cases:
+
+* standard/non-standard function
+* code/data
+* global/local symbol
+
+There was a discussion_ and instead of extending the current ``ENTRY/END*``
+macros, it was decided that brand new macros should be introduced instead::
+
+    So how about using macro names that actually show the purpose, instead
+    of importing all the crappy, historic, essentially randomly chosen
+    debug symbol macro names from the binutils and older kernels?
+
+.. _discussion: https://lkml.kernel.org/r/20170217104757.28588-1-jslaby@suse.cz
+
+Macros Description
+------------------
+
+The new macros are prefixed with the ``SYM_`` prefix and can be divided into
+three main groups:
+
+1. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with
+   standard C calling conventions. For example, on x86, this means that the
+   stack contains a return address at the predefined place and a return from
+   the function can happen in a standard way. When frame pointers are enabled,
+   save/restore of frame pointer shall happen at the start/end of a function,
+   respectively, too.
+
+   Checking tools like ``objtool`` should ensure such marked functions conform
+   to these rules. The tools can also easily annotate these functions with
+   debugging information (like *ORC data*) automatically.
+
+2. ``SYM_CODE_*`` -- special functions called with special stack. Be it
+   interrupt handlers with special stack content, trampolines, or startup
+   functions.
+
+   Checking tools mostly ignore checking of these functions. But some debug
+   information still can be generated automatically. For correct debug data,
+   this code needs hints like ``UNWIND_HINT_REGS`` provided by developers.
+
+3. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to
+   ``.text``. Data do not contain instructions, so they have to be treated
+   specially by the tools: they should not treat the bytes as instructions,
+   nor assign any debug information to them.
+
+Instruction Macros
+~~~~~~~~~~~~~~~~~~
+This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
+
+``objtool`` requires that all code must be contained in an ELF symbol. Symbol
+names that have a ``.L`` prefix do not emit symbol table entries. ``.L``
+prefixed symbols can be used within a code region, but should be avoided for
+denoting a range of code via ``SYM_*_START/END`` annotations.
+
+* ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the
+  most frequent markings**. They are used for functions with standard calling
+  conventions -- global and local. Like in C, they both align the functions to
+  architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants
+  for special cases where developers do not want this implicit alignment.
+
+  ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are
+  also offered as an assembler counterpart to the *weak* attribute known from
+  C.
+
+  All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks
+  the sequence of instructions as a function and computes its size to the
+  generated object file. Second, it also eases checking and processing such
+  object files as the tools can trivially find exact function boundaries.
+
+  So in most cases, developers should write something like in the following
+  example, having some asm instructions in between the macros, of course::
+
+    SYM_FUNC_START(memset)
+        ... asm insns ...
+    SYM_FUNC_END(memset)
+
+  In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``
+  and ``ENDPROC`` macros.
+
+* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those
+  who decided to have two or more names for one function. The typical use is::
+
+    SYM_FUNC_START_ALIAS(__memset)
+    SYM_FUNC_START(memset)
+        ... asm insns ...
+    SYM_FUNC_END(memset)
+    SYM_FUNC_END_ALIAS(__memset)
+
+  In this example, one can call ``__memset`` or ``memset`` with the same
+  result, except the debug information for the instructions is generated to
+  the object file only once -- for the non-``ALIAS`` case.
+
+* ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in
+  special cases -- if you know what you are doing. This is used exclusively
+  for interrupt handlers and similar where the calling convention is not the C
+  one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC``
+  category above::
+
+    SYM_CODE_START_LOCAL(bad_put_user)
+        ... asm insns ...
+    SYM_CODE_END(bad_put_user)
+
+  Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``.
+
+  To some extent, this category corresponds to deprecated ``ENTRY`` and
+  ``END``. Except ``END`` had several other meanings too.
+
+* ``SYM_INNER_LABEL*`` is used to denote a label inside some
+  ``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``.  They are very similar
+  to C labels, except they can be made global. An example of use::
+
+    SYM_CODE_START(ftrace_caller)
+        /* save_mcount_regs fills in first two parameters */
+        ...
+
+    SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)
+        /* Load the ftrace_ops into the 3rd parameter */
+        ...
+
+    SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
+        call ftrace_stub
+        ...
+        retq
+    SYM_CODE_END(ftrace_caller)
+
+Data Macros
+~~~~~~~~~~~
+Similar to instructions, there is a couple of macros to describe data in the
+assembly.
+
+* ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data
+  and shall be used in conjunction with either ``SYM_DATA_END``, or
+  ``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that
+  people can use ``lstack`` and (local) ``lstack_end`` in the following
+  example::
+
+    SYM_DATA_START_LOCAL(lstack)
+        .skip 4096
+    SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)
+
+* ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line
+  data::
+
+    SYM_DATA(HEAP,     .long rm_heap)
+    SYM_DATA(heap_end, .long rm_stack)
+
+  In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END``
+  internally.
+
+Support Macros
+~~~~~~~~~~~~~~
+All the above reduce themselves to some invocation of ``SYM_START``,
+``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using
+these.
+
+Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also
+``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a
+symbol marked by them. They are used either in ``_LABEL`` variants of the
+earlier macros, or in ``SYM_START``.
+
+
+Overriding Macros
+~~~~~~~~~~~~~~~~~
+Architecture can also override any of the macros in their own
+``asm/linkage.h``, including macros specifying the type of a symbol
+(``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``).  As every macro
+described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough
+to define the macros differently in the aforementioned architecture-dependent
+header.