1Assembler Annotations 2===================== 3 4Copyright (c) 2017-2019 Jiri Slaby 5 6This document describes the new macros for annotation of data and code in 7assembly. In particular, it contains information about ``SYM_FUNC_START``, 8``SYM_FUNC_END``, ``SYM_CODE_START``, and similar. 9 10Rationale 11--------- 12Some code like entries, trampolines, or boot code needs to be written in 13assembly. The same as in C, such code is grouped into functions and 14accompanied with data. Standard assemblers do not force users into precisely 15marking these pieces as code, data, or even specifying their length. 16Nevertheless, assemblers provide developers with such annotations to aid 17debuggers throughout assembly. On top of that, developers also want to mark 18some functions as *global* in order to be visible outside of their translation 19units. 20 21Over time, the Linux kernel has adopted macros from various projects (like 22``binutils``) to facilitate such annotations. So for historic reasons, 23developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other 24annotations in assembly. Due to the lack of their documentation, the macros 25are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was 26intended 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 29only ends of *standard* functions. 30 31When these macros are used correctly, they help assemblers generate a nice 32object 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 41This is not only important for debugging purposes. When there are properly 42annotated objects like this, tools can be run on them to generate more useful 43information. In particular, on properly annotated objects, ``objtool`` can be 44run to check and fix the object if needed. Currently, ``objtool`` can report 45missing frame pointer setup/destruction in functions. It can also 46automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>` 47for most code. Both of these are especially important to support reliable 48stack traces which are in turn necessary for :doc:`Kernel live patching 49<livepatch/livepatch>`. 50 51Caveat and Discussion 52--------------------- 53As one might realize, there were only three macros previously. That is indeed 54insufficient to cover all the combinations of cases: 55 56* standard/non-standard function 57* code/data 58* global/local symbol 59 60There was a discussion_ and instead of extending the current ``ENTRY/END*`` 61macros, 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 69Macros Description 70------------------ 71 72The new macros are prefixed with the ``SYM_`` prefix and can be divided into 73three main groups: 74 751. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with 76 standard C calling conventions, i.e. the stack contains a return address at 77 the predefined place and a return from the function can happen in a 78 standard way. When frame pointers are enabled, save/restore of frame 79 pointer shall happen at the start/end of a function, respectively, too. 80 81 Checking tools like ``objtool`` should ensure such marked functions conform 82 to these rules. The tools can also easily annotate these functions with 83 debugging information (like *ORC data*) automatically. 84 852. ``SYM_CODE_*`` -- special functions called with special stack. Be it 86 interrupt handlers with special stack content, trampolines, or startup 87 functions. 88 89 Checking tools mostly ignore checking of these functions. But some debug 90 information still can be generated automatically. For correct debug data, 91 this code needs hints like ``UNWIND_HINT_REGS`` provided by developers. 92 933. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to 94 ``.text``. Data do not contain instructions, so they have to be treated 95 specially by the tools: they should not treat the bytes as instructions, 96 nor assign any debug information to them. 97 98Instruction Macros 99~~~~~~~~~~~~~~~~~~ 100This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above. 101 102* ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the 103 most frequent markings**. They are used for functions with standard calling 104 conventions -- global and local. Like in C, they both align the functions to 105 architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants 106 for special cases where developers do not want this implicit alignment. 107 108 ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are 109 also offered as an assembler counterpart to the *weak* attribute known from 110 C. 111 112 All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks 113 the sequence of instructions as a function and computes its size to the 114 generated object file. Second, it also eases checking and processing such 115 object files as the tools can trivially find exact function boundaries. 116 117 So in most cases, developers should write something like in the following 118 example, having some asm instructions in between the macros, of course:: 119 120 SYM_FUNC_START(function_hook) 121 ... asm insns ... 122 SYM_FUNC_END(function_hook) 123 124 In fact, this kind of annotation corresponds to the now deprecated ``ENTRY`` 125 and ``ENDPROC`` macros. 126 127* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those 128 who decided to have two or more names for one function. The typical use is:: 129 130 SYM_FUNC_START_ALIAS(__memset) 131 SYM_FUNC_START(memset) 132 ... asm insns ... 133 SYM_FUNC_END(memset) 134 SYM_FUNC_END_ALIAS(__memset) 135 136 In this example, one can call ``__memset`` or ``memset`` with the same 137 result, except the debug information for the instructions is generated to 138 the object file only once -- for the non-``ALIAS`` case. 139 140* ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in 141 special cases -- if you know what you are doing. This is used exclusively 142 for interrupt handlers and similar where the calling convention is not the C 143 one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC`` 144 category above:: 145 146 SYM_CODE_START_LOCAL(bad_put_user) 147 ... asm insns ... 148 SYM_CODE_END(bad_put_user) 149 150 Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``. 151 152 To some extent, this category corresponds to deprecated ``ENTRY`` and 153 ``END``. Except ``END`` had several other meanings too. 154 155* ``SYM_INNER_LABEL*`` is used to denote a label inside some 156 ``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``. They are very similar 157 to C labels, except they can be made global. An example of use:: 158 159 SYM_CODE_START(ftrace_caller) 160 /* save_mcount_regs fills in first two parameters */ 161 ... 162 163 SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL) 164 /* Load the ftrace_ops into the 3rd parameter */ 165 ... 166 167 SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) 168 call ftrace_stub 169 ... 170 retq 171 SYM_CODE_END(ftrace_caller) 172 173Data Macros 174~~~~~~~~~~~ 175Similar to instructions, there is a couple of macros to describe data in the 176assembly. 177 178* ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data 179 and shall be used in conjunction with either ``SYM_DATA_END``, or 180 ``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that 181 people can use ``lstack`` and (local) ``lstack_end`` in the following 182 example:: 183 184 SYM_DATA_START_LOCAL(lstack) 185 .skip 4096 186 SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end) 187 188* ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line 189 data:: 190 191 SYM_DATA(HEAP, .long rm_heap) 192 SYM_DATA(heap_end, .long rm_stack) 193 194 In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END`` 195 internally. 196 197Support Macros 198~~~~~~~~~~~~~~ 199All the above reduce themselves to some invocation of ``SYM_START``, 200``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using 201these. 202 203Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also 204``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a 205symbol marked by them. They are used either in ``_LABEL`` variants of the 206earlier macros, or in ``SYM_START``. 207 208 209Overriding Macros 210~~~~~~~~~~~~~~~~~ 211Architecture can also override any of the macros in their own 212``asm/linkage.h``, including macros specifying the type of a symbol 213(``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``). As every macro 214described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough 215to define the macros differently in the aforementioned architecture-dependent 216header. 217