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Author SHA1 Message Date
Nick Desaulniers
4c973f7507 x86/entry: Emit a symbol for register restoring thunk 
commit 5e6dca82bcaa49348f9e5fcb48df4881f6d6c4ae upstream.

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]
Cc: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 2021-02-03 23:28:40 +01:00
Will Deacon
6535a39ffa Documentation: Call out example SYM_FUNC_* usage as x86-specific 
The example given in asm-annotations.rst to describe the constraints that
a function should meet in order to be annotated with a SYM_FUNC_* macro
is x86-specific, and not necessarily applicable to architectures using
branch-and-link style calling conventions such as arm64.

Tweak the example text to call out the x86-specific text.

Cc: Mark Brown <broonie@kernel.org>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200115184305.1187-1-will@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
 2020-01-16 12:53:16 -07:00
Borislav Petkov
0f42c1ad44 x86/ftrace: Get rid of function_hook 
History lesson courtesy of Steve:

"When ftrace first was introduced to the kernel, it used gcc's
mcount profiling mechanism. The mcount mechanism would add a call to
"mcount" at the start of every function but after the stack frame was
set up. Later, in gcc 4.6, gcc introduced -mfentry, that would create a
call to "__fentry__" instead of "mcount", before the stack frame was
set up. In order to handle both cases, ftrace defined a macro
"function_hook" that would be either "mcount" or "__fentry__" depending
on which one was being used.

The Linux kernel no longer supports the "mcount" method, thus there's
no reason to keep the "function_hook" define around. Simply use
"__fentry__", as there is no ambiguity to the name anymore."

Drop it everywhere.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/20191018124800.0a7006bb@gandalf.local.home
 2019-10-25 10:52:22 +02:00
Jiri Slaby
ffedeeb780 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
 2019-10-18 09:48:11 +02:00