4 Copyright (c) 2017-2019 Jiri Slaby
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.
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
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.
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``::
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
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>`.
53 As one might realize, there were only three macros previously. That is indeed
54 insufficient to cover all the combinations of cases:
56 * standard/non-standard function
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::
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?
67 .. _discussion: https://lkml.kernel.org/r/20170217104757.28588-1-jslaby@suse.cz
72 The new macros are prefixed with the ``SYM_`` prefix and can be divided into
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,
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.
86 2. ``SYM_CODE_*`` -- special functions called with special stack. Be it
87 interrupt handlers with special stack content, trampolines, or startup
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.
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.
101 This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
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.
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.
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
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.
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::
126 SYM_FUNC_START(memset)
130 In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``
131 and ``ENDPROC`` macros.
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::
136 SYM_FUNC_START_ALIAS(__memset)
137 SYM_FUNC_START(memset)
140 SYM_FUNC_END_ALIAS(__memset)
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.
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``
152 SYM_CODE_START_LOCAL(bad_put_user)
154 SYM_CODE_END(bad_put_user)
156 Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``.
158 To some extent, this category corresponds to deprecated ``ENTRY`` and
159 ``END``. Except ``END`` had several other meanings too.
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::
165 SYM_CODE_START(ftrace_caller)
166 /* save_mcount_regs fills in first two parameters */
169 SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)
170 /* Load the ftrace_ops into the 3rd parameter */
173 SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
177 SYM_CODE_END(ftrace_caller)
181 Similar to instructions, there is a couple of macros to describe data in the
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
190 SYM_DATA_START_LOCAL(lstack)
192 SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)
194 * ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line
197 SYM_DATA(HEAP, .long rm_heap)
198 SYM_DATA(heap_end, .long rm_stack)
200 In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END``
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
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``.
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