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authorJonathan Corbet <corbet@lwn.net>2022-09-27 10:05:57 -0600
committerJonathan Corbet <corbet@lwn.net>2022-09-29 12:55:06 -0600
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tree9d81c49cde1c2a98e3cde6f0f8495aed30876fc9 /Documentation/core-api
parent90c0bf89dec0c66d2e6538857060586073427059 (diff)
downloadlinux-f4bf1cd4ac9c8c4610b687e49a1ba691ab286235.tar.bz2
docs: move asm-annotations.rst into core-api
This one file should not really be in the top-level documentation directory. core-api/ may not be a perfect fit but seems to be best, so move it there. Adjust a couple of internal document references to make them location-independent, and point checkpatch.pl at the new location. Cc: Jiri Slaby <jirislaby@kernel.org> Cc: Joe Perches <joe@perches.com> Reviewed-by: David Vernet <void@manifault.com> Acked-by: Jani Nikula <jani.nikula@intel.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net> Acked-by: Randy Dunlap <rdunlap@infradead.org> Link: https://lore.kernel.org/r/20220927160559.97154-6-corbet@lwn.net Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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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 the ORC unwinder
+(Documentation/x86/orc-unwinder.rst)
+for most code. Both of these are especially important to support reliable
+stack traces which are in turn necessary for kernel live patching
+(Documentation/livepatch/livepatch.rst).
+
+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://lore.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_ALIAS``, ``SYM_FUNC_ALIAS_LOCAL``, and ``SYM_FUNC_ALIAS_WEAK`` can
+ be used to define multiple names for a function. The typical use is::
+
+ SYM_FUNC_START(__memset)
+ ... asm insns ...
+ SYN_FUNC_END(__memset)
+ SYM_FUNC_ALIAS(memset, __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.
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index dc95df462eea..f5d8e3779fe8 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -23,6 +23,7 @@ it.
printk-formats
printk-index
symbol-namespaces
+ asm-annotations
Data structures and low-level utilities
=======================================