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]
This commit is contained in:
Nick Desaulniers 2021-01-12 11:46:24 -08:00 коммит произвёл Borislav Petkov
Родитель 7c53f6b671
Коммит 5e6dca82bc
3 изменённых файлов: 14 добавлений и 4 удалений

Просмотреть файл

@ -100,6 +100,11 @@ Instruction Macros
~~~~~~~~~~~~~~~~~~
This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
``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.
* ``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

Просмотреть файл

@ -31,7 +31,7 @@ SYM_FUNC_START_NOALIGN(\name)
.endif
call \func
jmp .L_restore
jmp __thunk_restore
SYM_FUNC_END(\name)
_ASM_NOKPROBE(\name)
.endm
@ -44,7 +44,7 @@ SYM_FUNC_END(\name)
#endif
#ifdef CONFIG_PREEMPTION
SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
popq %r11
popq %r10
popq %r9
@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
popq %rdi
popq %rbp
ret
_ASM_NOKPROBE(.L_restore)
SYM_CODE_END(.L_restore)
_ASM_NOKPROBE(__thunk_restore)
SYM_CODE_END(__thunk_restore)
#endif

Просмотреть файл

@ -178,6 +178,11 @@
* Objtool generates debug info for both FUNC & CODE, but needs special
* annotations for each CODE's start (to describe the actual stack frame).
*
* 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.
*
* ALIAS -- does not generate debug info -- the aliased function will
*/