Re: [PATCH v2] x86/entry: use STB_GLOBAL for register restoring thunk

From: Josh Poimboeuf
Date: Tue Jan 05 2021 - 21:00:03 EST

On Tue, Jan 05, 2021 at 04:43:51PM -0800, Nick Desaulniers wrote:
> 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 (use Clang's integrated assembler). Josh
> notes:
> With the LLVM assembler stripping the .text section symbol, objtool
> has no way to reference this code when it generates ORC unwinder
> entries, because this code is outside of any ELF function.
> Fangrui notes that this is helpful for reducing images size when
> compiling with -ffunction-sections and -fdata-sections. I have observerd
> 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, with a new flag
> --generate-unused-section-symbols=[yes|no].
> Use a global symbol for the thunk that way
> objtool can generate proper unwind info here with LLVM_IAS=1.

On second thought, there's no need to make the symbol global. Just
getting rid of the '.L' local label symbol prefix should be enough to
make an ELF symbol:

diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index ccd32877a3c4..c9a9fbf1655f 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -31,7 +31,7 @@ SYM_FUNC_START_NOALIGN(\name)

call \func
- jmp .L_restore
+ jmp __thunk_restore
@@ -44,7 +44,7 @@ SYM_FUNC_END(\name)

popq %r11
popq %r10
popq %r9
@@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
popq %rdi
popq %rbp
- _ASM_NOKPROBE(.L_restore)
+ _ASM_NOKPROBE(__thunk_restore)