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    SubjectRe: [PATCH v2] x86/entry: use STB_GLOBAL for register restoring thunk
    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)
    .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
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