linux/Documentation/core-api/asm-annotations.rst

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/arch/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.