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| -rw-r--r-- | Documentation/asm-annotations.rst | 216 | ||||
| -rw-r--r-- | Documentation/index.rst | 8 | ||||
| -rw-r--r-- | arch/x86/include/asm/linkage.h | 10 | ||||
| -rw-r--r-- | include/linux/linkage.h | 245 |
4 files changed, 468 insertions, 11 deletions
diff --git a/Documentation/asm-annotations.rst b/Documentation/asm-annotations.rst new file mode 100644 index 000000000000..29ccd6e61fe5 --- /dev/null +++ b/Documentation/asm-annotations.rst @@ -0,0 +1,216 @@ +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 :doc:`ORC unwinder <x86/orc-unwinder>` +for most code. Both of these are especially important to support reliable +stack traces which are in turn necessary for :doc:`Kernel live patching +<livepatch/livepatch>`. + +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://lkml.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, i.e. 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. + +* ``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(function_hook) + ... asm insns ... + SYM_FUNC_END(function_hook) + + In fact, this kind of annotation corresponds to the now deprecated ``ENTRY`` + and ``ENDPROC`` macros. + +* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those + who decided to have two or more names for one function. The typical use is:: + + SYM_FUNC_START_ALIAS(__memset) + SYM_FUNC_START(memset) + ... asm insns ... + SYM_FUNC_END(memset) + SYM_FUNC_END_ALIAS(__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/index.rst b/Documentation/index.rst index b843e313d2f2..2ceab197246f 100644 --- a/Documentation/index.rst +++ b/Documentation/index.rst @@ -135,6 +135,14 @@ needed). mic/index scheduler/index +Architecture-agnostic documentation +----------------------------------- + +.. toctree:: + :maxdepth: 2 + + asm-annotations + Architecture-specific documentation ----------------------------------- diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h index 14caa9d9fb7f..e07188e8d763 100644 --- a/arch/x86/include/asm/linkage.h +++ b/arch/x86/include/asm/linkage.h @@ -13,9 +13,13 @@ #ifdef __ASSEMBLY__ -#define GLOBAL(name) \ - .globl name; \ - name: +/* + * GLOBAL is DEPRECATED + * + * use SYM_DATA_START, SYM_FUNC_START, SYM_INNER_LABEL, SYM_CODE_START, or + * similar + */ +#define GLOBAL(name) SYM_ENTRY(name, SYM_L_GLOBAL, SYM_A_NONE) #if defined(CONFIG_X86_64) || defined(CONFIG_X86_ALIGNMENT_16) #define __ALIGN .p2align 4, 0x90 diff --git a/include/linux/linkage.h b/include/linux/linkage.h index 7e020782ade2..f3ae8f3dea2c 100644 --- a/include/linux/linkage.h +++ b/include/linux/linkage.h @@ -75,32 +75,58 @@ #ifdef __ASSEMBLY__ +/* SYM_T_FUNC -- type used by assembler to mark functions */ +#ifndef SYM_T_FUNC +#define SYM_T_FUNC STT_FUNC +#endif + +/* SYM_T_OBJECT -- type used by assembler to mark data */ +#ifndef SYM_T_OBJECT +#define SYM_T_OBJECT STT_OBJECT +#endif + +/* SYM_T_NONE -- type used by assembler to mark entries of unknown type */ +#ifndef SYM_T_NONE +#define SYM_T_NONE STT_NOTYPE +#endif + +/* SYM_A_* -- align the symbol? */ +#define SYM_A_ALIGN ALIGN +#define SYM_A_NONE /* nothing */ + +/* SYM_L_* -- linkage of symbols */ +#define SYM_L_GLOBAL(name) .globl name +#define SYM_L_WEAK(name) .weak name +#define SYM_L_LOCAL(name) /* nothing */ + #ifndef LINKER_SCRIPT #define ALIGN __ALIGN #define ALIGN_STR __ALIGN_STR +/* === DEPRECATED annotations === */ + #ifndef GLOBAL +/* deprecated, use SYM_DATA*, SYM_ENTRY, or similar */ #define GLOBAL(name) \ .globl name ASM_NL \ name: #endif #ifndef ENTRY +/* deprecated, use SYM_FUNC_START */ #define ENTRY(name) \ - .globl name ASM_NL \ - ALIGN ASM_NL \ - name: + SYM_FUNC_START(name) #endif #endif /* LINKER_SCRIPT */ #ifndef WEAK +/* deprecated, use SYM_FUNC_START_WEAK* */ #define WEAK(name) \ - .weak name ASM_NL \ - ALIGN ASM_NL \ - name: + SYM_FUNC_START_WEAK(name) #endif #ifndef END +/* deprecated, use SYM_FUNC_END, SYM_DATA_END, or SYM_END */ #define END(name) \ .size name, .-name #endif @@ -110,11 +136,214 @@ * static analysis tools such as stack depth analyzer. */ #ifndef ENDPROC +/* deprecated, use SYM_FUNC_END */ #define ENDPROC(name) \ - .type name, @function ASM_NL \ - END(name) + SYM_FUNC_END(name) +#endif + +/* === generic annotations === */ + +/* SYM_ENTRY -- use only if you have to for non-paired symbols */ +#ifndef SYM_ENTRY +#define SYM_ENTRY(name, linkage, align...) \ + linkage(name) ASM_NL \ + align ASM_NL \ + name: +#endif + +/* SYM_START -- use only if you have to */ +#ifndef SYM_START +#define SYM_START(name, linkage, align...) \ + SYM_ENTRY(name, linkage, align) +#endif + +/* SYM_END -- use only if you have to */ +#ifndef SYM_END +#define SYM_END(name, sym_type) \ + .type name sym_type ASM_NL \ + .size name, .-name +#endif + +/* === code annotations === */ + +/* + * FUNC -- C-like functions (proper stack frame etc.) + * CODE -- non-C code (e.g. irq handlers with different, special stack etc.) + * + * Objtool validates stack for FUNC, but not for CODE. + * Objtool generates debug info for both FUNC & CODE, but needs special + * annotations for each CODE's start (to describe the actual stack frame). + * + * ALIAS -- does not generate debug info -- the aliased function will + */ + +/* SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code */ +#ifndef SYM_INNER_LABEL_ALIGN +#define SYM_INNER_LABEL_ALIGN(name, linkage) \ + .type name SYM_T_NONE ASM_NL \ + SYM_ENTRY(name, linkage, SYM_A_ALIGN) +#endif + +/* SYM_INNER_LABEL -- only for labels in the middle of code */ +#ifndef SYM_INNER_LABEL +#define SYM_INNER_LABEL(name, linkage) \ + .type name SYM_T_NONE ASM_NL \ + SYM_ENTRY(name, linkage, SYM_A_NONE) +#endif + +/* + * SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one + * function + */ +#ifndef SYM_FUNC_START_LOCAL_ALIAS +#define SYM_FUNC_START_LOCAL_ALIAS(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) +#endif + +/* + * SYM_FUNC_START_ALIAS -- use where there are two global names for one + * function + */ +#ifndef SYM_FUNC_START_ALIAS +#define SYM_FUNC_START_ALIAS(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) +#endif + +/* SYM_FUNC_START -- use for global functions */ +#ifndef SYM_FUNC_START +/* + * The same as SYM_FUNC_START_ALIAS, but we will need to distinguish these two + * later. + */ +#define SYM_FUNC_START(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) +#endif + +/* SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment */ +#ifndef SYM_FUNC_START_NOALIGN +#define SYM_FUNC_START_NOALIGN(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) +#endif + +/* SYM_FUNC_START_LOCAL -- use for local functions */ +#ifndef SYM_FUNC_START_LOCAL +/* the same as SYM_FUNC_START_LOCAL_ALIAS, see comment near SYM_FUNC_START */ +#define SYM_FUNC_START_LOCAL(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) #endif +/* SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment */ +#ifndef SYM_FUNC_START_LOCAL_NOALIGN +#define SYM_FUNC_START_LOCAL_NOALIGN(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) #endif +/* SYM_FUNC_START_WEAK -- use for weak functions */ +#ifndef SYM_FUNC_START_WEAK +#define SYM_FUNC_START_WEAK(name) \ + SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN) #endif + +/* SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment */ +#ifndef SYM_FUNC_START_WEAK_NOALIGN +#define SYM_FUNC_START_WEAK_NOALIGN(name) \ + SYM_START(name, SYM_L_WEAK, SYM_A_NONE) +#endif + +/* SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function */ +#ifndef SYM_FUNC_END_ALIAS +#define SYM_FUNC_END_ALIAS(name) \ + SYM_END(name, SYM_T_FUNC) +#endif + +/* + * SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, + * SYM_FUNC_START_WEAK, ... + */ +#ifndef SYM_FUNC_END +/* the same as SYM_FUNC_END_ALIAS, see comment near SYM_FUNC_START */ +#define SYM_FUNC_END(name) \ + SYM_END(name, SYM_T_FUNC) +#endif + +/* SYM_CODE_START -- use for non-C (special) functions */ +#ifndef SYM_CODE_START +#define SYM_CODE_START(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) +#endif + +/* SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment */ +#ifndef SYM_CODE_START_NOALIGN +#define SYM_CODE_START_NOALIGN(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) +#endif + +/* SYM_CODE_START_LOCAL -- use for local non-C (special) functions */ +#ifndef SYM_CODE_START_LOCAL +#define SYM_CODE_START_LOCAL(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) +#endif + +/* + * SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, + * w/o alignment + */ +#ifndef SYM_CODE_START_LOCAL_NOALIGN +#define SYM_CODE_START_LOCAL_NOALIGN(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) +#endif + +/* SYM_CODE_END -- the end of SYM_CODE_START_LOCAL, SYM_CODE_START, ... */ +#ifndef SYM_CODE_END +#define SYM_CODE_END(name) \ + SYM_END(name, SYM_T_NONE) +#endif + +/* === data annotations === */ + +/* SYM_DATA_START -- global data symbol */ +#ifndef SYM_DATA_START +#define SYM_DATA_START(name) \ + SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) +#endif + +/* SYM_DATA_START -- local data symbol */ +#ifndef SYM_DATA_START_LOCAL +#define SYM_DATA_START_LOCAL(name) \ + SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) +#endif + +/* SYM_DATA_END -- the end of SYM_DATA_START symbol */ +#ifndef SYM_DATA_END +#define SYM_DATA_END(name) \ + SYM_END(name, SYM_T_OBJECT) +#endif + +/* SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol */ +#ifndef SYM_DATA_END_LABEL +#define SYM_DATA_END_LABEL(name, linkage, label) \ + linkage(label) ASM_NL \ + .type label SYM_T_OBJECT ASM_NL \ + label: \ + SYM_END(name, SYM_T_OBJECT) +#endif + +/* SYM_DATA -- start+end wrapper around simple global data */ +#ifndef SYM_DATA +#define SYM_DATA(name, data...) \ + SYM_DATA_START(name) ASM_NL \ + data ASM_NL \ + SYM_DATA_END(name) +#endif + +/* SYM_DATA_LOCAL -- start+end wrapper around simple local data */ +#ifndef SYM_DATA_LOCAL +#define SYM_DATA_LOCAL(name, data...) \ + SYM_DATA_START_LOCAL(name) ASM_NL \ + data ASM_NL \ + SYM_DATA_END(name) +#endif + +#endif /* __ASSEMBLY__ */ + +#endif /* _LINUX_LINKAGE_H */ |
