linux/Documentation/asm-annotations.rst

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linkage: Introduce new macros for assembler symbols Introduce new C macros for annotations of functions and data in assembly. There is a long-standing mess in macros like ENTRY, END, ENDPROC and similar. They are used in different manners and sometimes incorrectly. So introduce macros with clear use to annotate assembly as follows: a) Support macros for the ones below SYM_T_FUNC -- type used by assembler to mark functions SYM_T_OBJECT -- type used by assembler to mark data SYM_T_NONE -- type used by assembler to mark entries of unknown type They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE respectively. According to the gas manual, this is the most portable way. I am not sure about other assemblers, so this can be switched back to %function and %object if this turns into a problem. Architectures can also override them by something like ", @function" if they need. SYM_A_ALIGN, SYM_A_NONE -- align the symbol? SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols b) Mostly internal annotations, used by the ones below SYM_ENTRY -- use only if you have to (for non-paired symbols) SYM_START -- use only if you have to (for paired symbols) SYM_END -- use only if you have to (for paired symbols) c) Annotations for code SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code SYM_INNER_LABEL -- only for labels in the middle of code SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one function SYM_FUNC_START_ALIAS -- use where there are two global names for one function SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function SYM_FUNC_START -- use for global functions SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment SYM_FUNC_START_LOCAL -- use for local functions SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment SYM_FUNC_START_WEAK -- use for weak functions SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, SYM_FUNC_START_WEAK, ... For functions with special (non-C) calling conventions: SYM_CODE_START -- use for non-C (special) functions SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment SYM_CODE_START_LOCAL -- use for local non-C (special) functions SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, w/o alignment SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START d) For data SYM_DATA_START -- global data symbol SYM_DATA_START_LOCAL -- local data symbol SYM_DATA_END -- the end of the SYM_DATA_START symbol SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol SYM_DATA -- start+end wrapper around simple global data SYM_DATA_LOCAL -- start+end wrapper around simple local data ========== The macros allow to pair starts and ends of functions and mark functions correctly in the output ELF objects. All users of the old macros in x86 are converted to use these in further patches. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Len Brown <len.brown@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-arch@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-pm@vger.kernel.org Cc: Mark Rutland <mark.rutland@arm.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will@kernel.org> Cc: x86-ml <x86@kernel.org> Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191011115108.12392-2-jslaby@suse.cz
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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. 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.
linkage: Introduce new macros for assembler symbols Introduce new C macros for annotations of functions and data in assembly. There is a long-standing mess in macros like ENTRY, END, ENDPROC and similar. They are used in different manners and sometimes incorrectly. So introduce macros with clear use to annotate assembly as follows: a) Support macros for the ones below SYM_T_FUNC -- type used by assembler to mark functions SYM_T_OBJECT -- type used by assembler to mark data SYM_T_NONE -- type used by assembler to mark entries of unknown type They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE respectively. According to the gas manual, this is the most portable way. I am not sure about other assemblers, so this can be switched back to %function and %object if this turns into a problem. Architectures can also override them by something like ", @function" if they need. SYM_A_ALIGN, SYM_A_NONE -- align the symbol? SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols b) Mostly internal annotations, used by the ones below SYM_ENTRY -- use only if you have to (for non-paired symbols) SYM_START -- use only if you have to (for paired symbols) SYM_END -- use only if you have to (for paired symbols) c) Annotations for code SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code SYM_INNER_LABEL -- only for labels in the middle of code SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one function SYM_FUNC_START_ALIAS -- use where there are two global names for one function SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function SYM_FUNC_START -- use for global functions SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment SYM_FUNC_START_LOCAL -- use for local functions SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment SYM_FUNC_START_WEAK -- use for weak functions SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, SYM_FUNC_START_WEAK, ... For functions with special (non-C) calling conventions: SYM_CODE_START -- use for non-C (special) functions SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment SYM_CODE_START_LOCAL -- use for local non-C (special) functions SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, w/o alignment SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START d) For data SYM_DATA_START -- global data symbol SYM_DATA_START_LOCAL -- local data symbol SYM_DATA_END -- the end of the SYM_DATA_START symbol SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol SYM_DATA -- start+end wrapper around simple global data SYM_DATA_LOCAL -- start+end wrapper around simple local data ========== The macros allow to pair starts and ends of functions and mark functions correctly in the output ELF objects. All users of the old macros in x86 are converted to use these in further patches. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Len Brown <len.brown@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-arch@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-pm@vger.kernel.org Cc: Mark Rutland <mark.rutland@arm.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will@kernel.org> Cc: x86-ml <x86@kernel.org> Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191011115108.12392-2-jslaby@suse.cz
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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.
x86/entry: Emit a symbol for register restoring thunk Arnd found a randconfig that produces the warning: arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at offset 0x3e when building with LLVM_IAS=1 (Clang's integrated assembler). Josh notes: With the LLVM assembler not generating section symbols, objtool has no way to reference this code when it generates ORC unwinder entries, because this code is outside of any ELF function. The limitation now being imposed by objtool is that all code must be contained in an ELF symbol. And .L symbols don't create such symbols. So basically, you can use an .L symbol *inside* a function or a code segment, you just can't use the .L symbol to contain the code using a SYM_*_START/END annotation pair. Fangrui notes that this optimization is helpful for reducing image size when compiling with -ffunction-sections and -fdata-sections. I have observed on the order of tens of thousands of symbols for the kernel images built with those flags. A patch has been authored against GNU binutils to match this behavior of not generating unused section symbols ([1]), so this will also become a problem for users of GNU binutils once they upgrade to 2.36. Omit the .L prefix on a label so that the assembler will emit an entry into the symbol table for the label, with STB_LOCAL binding. This enables objtool to generate proper unwind info here with LLVM_IAS=1 or GNU binutils 2.36+. [ bp: Massage commit message. ] Reported-by: Arnd Bergmann <arnd@arndb.de> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com> Suggested-by: Borislav Petkov <bp@alien8.de> Suggested-by: Mark Brown <broonie@kernel.org> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Link: https://lkml.kernel.org/r/20210112194625.4181814-1-ndesaulniers@google.com Link: https://github.com/ClangBuiltLinux/linux/issues/1209 Link: https://reviews.llvm.org/D93783 Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1408485ce69f844dcd7ded093a8 [1]
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``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.
linkage: Introduce new macros for assembler symbols Introduce new C macros for annotations of functions and data in assembly. There is a long-standing mess in macros like ENTRY, END, ENDPROC and similar. They are used in different manners and sometimes incorrectly. So introduce macros with clear use to annotate assembly as follows: a) Support macros for the ones below SYM_T_FUNC -- type used by assembler to mark functions SYM_T_OBJECT -- type used by assembler to mark data SYM_T_NONE -- type used by assembler to mark entries of unknown type They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE respectively. According to the gas manual, this is the most portable way. I am not sure about other assemblers, so this can be switched back to %function and %object if this turns into a problem. Architectures can also override them by something like ", @function" if they need. SYM_A_ALIGN, SYM_A_NONE -- align the symbol? SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols b) Mostly internal annotations, used by the ones below SYM_ENTRY -- use only if you have to (for non-paired symbols) SYM_START -- use only if you have to (for paired symbols) SYM_END -- use only if you have to (for paired symbols) c) Annotations for code SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code SYM_INNER_LABEL -- only for labels in the middle of code SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one function SYM_FUNC_START_ALIAS -- use where there are two global names for one function SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function SYM_FUNC_START -- use for global functions SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment SYM_FUNC_START_LOCAL -- use for local functions SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment SYM_FUNC_START_WEAK -- use for weak functions SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, SYM_FUNC_START_WEAK, ... For functions with special (non-C) calling conventions: SYM_CODE_START -- use for non-C (special) functions SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment SYM_CODE_START_LOCAL -- use for local non-C (special) functions SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, w/o alignment SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START d) For data SYM_DATA_START -- global data symbol SYM_DATA_START_LOCAL -- local data symbol SYM_DATA_END -- the end of the SYM_DATA_START symbol SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol SYM_DATA -- start+end wrapper around simple global data SYM_DATA_LOCAL -- start+end wrapper around simple local data ========== The macros allow to pair starts and ends of functions and mark functions correctly in the output ELF objects. All users of the old macros in x86 are converted to use these in further patches. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Len Brown <len.brown@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-arch@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-pm@vger.kernel.org Cc: Mark Rutland <mark.rutland@arm.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will@kernel.org> Cc: x86-ml <x86@kernel.org> Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191011115108.12392-2-jslaby@suse.cz
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* ``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)
linkage: Introduce new macros for assembler symbols Introduce new C macros for annotations of functions and data in assembly. There is a long-standing mess in macros like ENTRY, END, ENDPROC and similar. They are used in different manners and sometimes incorrectly. So introduce macros with clear use to annotate assembly as follows: a) Support macros for the ones below SYM_T_FUNC -- type used by assembler to mark functions SYM_T_OBJECT -- type used by assembler to mark data SYM_T_NONE -- type used by assembler to mark entries of unknown type They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE respectively. According to the gas manual, this is the most portable way. I am not sure about other assemblers, so this can be switched back to %function and %object if this turns into a problem. Architectures can also override them by something like ", @function" if they need. SYM_A_ALIGN, SYM_A_NONE -- align the symbol? SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols b) Mostly internal annotations, used by the ones below SYM_ENTRY -- use only if you have to (for non-paired symbols) SYM_START -- use only if you have to (for paired symbols) SYM_END -- use only if you have to (for paired symbols) c) Annotations for code SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code SYM_INNER_LABEL -- only for labels in the middle of code SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one function SYM_FUNC_START_ALIAS -- use where there are two global names for one function SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function SYM_FUNC_START -- use for global functions SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment SYM_FUNC_START_LOCAL -- use for local functions SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment SYM_FUNC_START_WEAK -- use for weak functions SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, SYM_FUNC_START_WEAK, ... For functions with special (non-C) calling conventions: SYM_CODE_START -- use for non-C (special) functions SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment SYM_CODE_START_LOCAL -- use for local non-C (special) functions SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, w/o alignment SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START d) For data SYM_DATA_START -- global data symbol SYM_DATA_START_LOCAL -- local data symbol SYM_DATA_END -- the end of the SYM_DATA_START symbol SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol SYM_DATA -- start+end wrapper around simple global data SYM_DATA_LOCAL -- start+end wrapper around simple local data ========== The macros allow to pair starts and ends of functions and mark functions correctly in the output ELF objects. All users of the old macros in x86 are converted to use these in further patches. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Len Brown <len.brown@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-arch@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-pm@vger.kernel.org Cc: Mark Rutland <mark.rutland@arm.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will@kernel.org> Cc: x86-ml <x86@kernel.org> Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191011115108.12392-2-jslaby@suse.cz
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... asm insns ...
SYM_FUNC_END(memset)
linkage: Introduce new macros for assembler symbols Introduce new C macros for annotations of functions and data in assembly. There is a long-standing mess in macros like ENTRY, END, ENDPROC and similar. They are used in different manners and sometimes incorrectly. So introduce macros with clear use to annotate assembly as follows: a) Support macros for the ones below SYM_T_FUNC -- type used by assembler to mark functions SYM_T_OBJECT -- type used by assembler to mark data SYM_T_NONE -- type used by assembler to mark entries of unknown type They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE respectively. According to the gas manual, this is the most portable way. I am not sure about other assemblers, so this can be switched back to %function and %object if this turns into a problem. Architectures can also override them by something like ", @function" if they need. SYM_A_ALIGN, SYM_A_NONE -- align the symbol? SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols b) Mostly internal annotations, used by the ones below SYM_ENTRY -- use only if you have to (for non-paired symbols) SYM_START -- use only if you have to (for paired symbols) SYM_END -- use only if you have to (for paired symbols) c) Annotations for code SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code SYM_INNER_LABEL -- only for labels in the middle of code SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for one function SYM_FUNC_START_ALIAS -- use where there are two global names for one function SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function SYM_FUNC_START -- use for global functions SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment SYM_FUNC_START_LOCAL -- use for local functions SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment SYM_FUNC_START_WEAK -- use for weak functions SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, SYM_FUNC_START_WEAK, ... For functions with special (non-C) calling conventions: SYM_CODE_START -- use for non-C (special) functions SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o alignment SYM_CODE_START_LOCAL -- use for local non-C (special) functions SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) functions, w/o alignment SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START d) For data SYM_DATA_START -- global data symbol SYM_DATA_START_LOCAL -- local data symbol SYM_DATA_END -- the end of the SYM_DATA_START symbol SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol SYM_DATA -- start+end wrapper around simple global data SYM_DATA_LOCAL -- start+end wrapper around simple local data ========== The macros allow to pair starts and ends of functions and mark functions correctly in the output ELF objects. All users of the old macros in x86 are converted to use these in further patches. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Borislav Petkov <bp@suse.de> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Len Brown <len.brown@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-arch@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-pm@vger.kernel.org Cc: Mark Rutland <mark.rutland@arm.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will@kernel.org> Cc: x86-ml <x86@kernel.org> Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191011115108.12392-2-jslaby@suse.cz
2019-10-11 11:50:41 +00:00
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.