Documentation/asm-annotations.rst

   1 Assembler Annotations
   2 =====================
   3
   4 Copyright (c) 2017-2019 Jiri Slaby
   5
   6 This document describes the new macros for annotation of data and code in
   7 assembly. In particular, it contains information about ``SYM_FUNC_START``,
   8 ``SYM_FUNC_END``, ``SYM_CODE_START``, and similar.
   9
  10 Rationale
  11 ---------
  12 Some code like entries, trampolines, or boot code needs to be written in
  13 assembly. The same as in C, such code is grouped into functions and
  14 accompanied with data. Standard assemblers do not force users into precisely
  15 marking these pieces as code, data, or even specifying their length.
  16 Nevertheless, assemblers provide developers with such annotations to aid
  17 debuggers throughout assembly. On top of that, developers also want to mark
  18 some functions as *global* in order to be visible outside of their translation
  19 units.
  20
  21 Over time, the Linux kernel has adopted macros from various projects (like
  22 ``binutils``) to facilitate such annotations. So for historic reasons,
  23 developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other
  24 annotations in assembly.  Due to the lack of their documentation, the macros
  25 are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was
  26 intended to denote the beginning of global symbols (be it data or code).
  27 ``END`` used to mark the end of data or end of special functions with
  28 *non-standard* calling convention. In contrast, ``ENDPROC`` should annotate
  29 only ends of *standard* functions.
  30
  31 When these macros are used correctly, they help assemblers generate a nice
  32 object with both sizes and types set correctly. For example, the result of
  33 ``arch/x86/lib/putuser.S``::
  34
  35    Num:    Value          Size Type    Bind   Vis      Ndx Name
  36     25: 0000000000000000    33 FUNC    GLOBAL DEFAULT    1 __put_user_1
  37     29: 0000000000000030    37 FUNC    GLOBAL DEFAULT    1 __put_user_2
  38     32: 0000000000000060    36 FUNC    GLOBAL DEFAULT    1 __put_user_4
  39     35: 0000000000000090    37 FUNC    GLOBAL DEFAULT    1 __put_user_8
  40
  41 This is not only important for debugging purposes. When there are properly
  42 annotated objects like this, tools can be run on them to generate more useful
  43 information. In particular, on properly annotated objects, ``objtool`` can be
  44 run to check and fix the object if needed. Currently, ``objtool`` can report
  45 missing frame pointer setup/destruction in functions. It can also
  46 automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>`
  47 for most code. Both of these are especially important to support reliable
  48 stack traces which are in turn necessary for :doc:`Kernel live patching
  49 <livepatch/livepatch>`.
  50
  51 Caveat and Discussion
  52 ---------------------
  53 As one might realize, there were only three macros previously. That is indeed
  54 insufficient to cover all the combinations of cases:
  55
  56 * standard/non-standard function
  57 * code/data
  58 * global/local symbol
  59
  60 There was a discussion_ and instead of extending the current ``ENTRY/END*``
  61 macros, it was decided that brand new macros should be introduced instead::
  62
  63     So how about using macro names that actually show the purpose, instead
  64     of importing all the crappy, historic, essentially randomly chosen
  65     debug symbol macro names from the binutils and older kernels?
  66
  67 .. _discussion: https://lkml.kernel.org/r/20170217104757.28588-1-jslaby@suse.cz
  68
  69 Macros Description
  70 ------------------
  71
  72 The new macros are prefixed with the ``SYM_`` prefix and can be divided into
  73 three main groups:
  74
  75 1. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with
  76    standard C calling conventions. For example, on x86, this means that the
  77    stack contains a return address at the predefined place and a return from
  78    the function can happen in a standard way. When frame pointers are enabled,
  79    save/restore of frame pointer shall happen at the start/end of a function,
  80    respectively, too.
  81
  82    Checking tools like ``objtool`` should ensure such marked functions conform
  83    to these rules. The tools can also easily annotate these functions with
  84    debugging information (like *ORC data*) automatically.
  85
  86 2. ``SYM_CODE_*`` -- special functions called with special stack. Be it
  87    interrupt handlers with special stack content, trampolines, or startup
  88    functions.
  89
  90    Checking tools mostly ignore checking of these functions. But some debug
  91    information still can be generated automatically. For correct debug data,
  92    this code needs hints like ``UNWIND_HINT_REGS`` provided by developers.
  93
  94 3. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to
  95    ``.text``. Data do not contain instructions, so they have to be treated
  96    specially by the tools: they should not treat the bytes as instructions,
  97    nor assign any debug information to them.
  98
  99 Instruction Macros
 100 ~~~~~~~~~~~~~~~~~~
 101 This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
 102
 103 ``objtool`` requires that all code must be contained in an ELF symbol. Symbol
 104 names that have a ``.L`` prefix do not emit symbol table entries. ``.L``
 105 prefixed symbols can be used within a code region, but should be avoided for
 106 denoting a range of code via ``SYM_*_START/END`` annotations.
 107
 108 * ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the
 109   most frequent markings**. They are used for functions with standard calling
 110   conventions -- global and local. Like in C, they both align the functions to
 111   architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants
 112   for special cases where developers do not want this implicit alignment.
 113
 114   ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are
 115   also offered as an assembler counterpart to the *weak* attribute known from
 116   C.
 117
 118   All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks
 119   the sequence of instructions as a function and computes its size to the
 120   generated object file. Second, it also eases checking and processing such
 121   object files as the tools can trivially find exact function boundaries.
 122
 123   So in most cases, developers should write something like in the following
 124   example, having some asm instructions in between the macros, of course::
 125
 126     SYM_FUNC_START(memset)
 127         ... asm insns ...
 128     SYM_FUNC_END(memset)
 129
 130   In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``
 131   and ``ENDPROC`` macros.
 132
 133 * ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those
 134   who decided to have two or more names for one function. The typical use is::
 135
 136     SYM_FUNC_START_ALIAS(__memset)
 137     SYM_FUNC_START(memset)
 138         ... asm insns ...
 139     SYM_FUNC_END(memset)
 140     SYM_FUNC_END_ALIAS(__memset)
 141
 142   In this example, one can call ``__memset`` or ``memset`` with the same
 143   result, except the debug information for the instructions is generated to
 144   the object file only once -- for the non-``ALIAS`` case.
 145
 146 * ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in
 147   special cases -- if you know what you are doing. This is used exclusively
 148   for interrupt handlers and similar where the calling convention is not the C
 149   one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC``
 150   category above::
 151
 152     SYM_CODE_START_LOCAL(bad_put_user)
 153         ... asm insns ...
 154     SYM_CODE_END(bad_put_user)
 155
 156   Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``.
 157
 158   To some extent, this category corresponds to deprecated ``ENTRY`` and
 159   ``END``. Except ``END`` had several other meanings too.
 160
 161 * ``SYM_INNER_LABEL*`` is used to denote a label inside some
 162   ``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``.  They are very similar
 163   to C labels, except they can be made global. An example of use::
 164
 165     SYM_CODE_START(ftrace_caller)
 166         /* save_mcount_regs fills in first two parameters */
 167         ...
 168
 169     SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)
 170         /* Load the ftrace_ops into the 3rd parameter */
 171         ...
 172
 173     SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
 174         call ftrace_stub
 175         ...
 176         retq
 177     SYM_CODE_END(ftrace_caller)
 178
 179 Data Macros
 180 ~~~~~~~~~~~
 181 Similar to instructions, there is a couple of macros to describe data in the
 182 assembly.
 183
 184 * ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data
 185   and shall be used in conjunction with either ``SYM_DATA_END``, or
 186   ``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that
 187   people can use ``lstack`` and (local) ``lstack_end`` in the following
 188   example::
 189
 190     SYM_DATA_START_LOCAL(lstack)
 191         .skip 4096
 192     SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)
 193
 194 * ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line
 195   data::
 196
 197     SYM_DATA(HEAP,     .long rm_heap)
 198     SYM_DATA(heap_end, .long rm_stack)
 199
 200   In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END``
 201   internally.
 202
 203 Support Macros
 204 ~~~~~~~~~~~~~~
 205 All the above reduce themselves to some invocation of ``SYM_START``,
 206 ``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using
 207 these.
 208
 209 Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also
 210 ``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a
 211 symbol marked by them. They are used either in ``_LABEL`` variants of the
 212 earlier macros, or in ``SYM_START``.
 213
 214
 215 Overriding Macros
 216 ~~~~~~~~~~~~~~~~~
 217 Architecture can also override any of the macros in their own
 218 ``asm/linkage.h``, including macros specifying the type of a symbol
 219 (``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``).  As every macro
 220 described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough
 221 to define the macros differently in the aforementioned architecture-dependent
 222 header.