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Date:   Tue, 12 Jan 2021 01:38:39 +0100
From:   Borislav Petkov <>
To:     Nick Desaulniers <>
Cc:     Andy Lutomirski <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        Fangrui Song <>,
        Arnd Bergmann <>,
        Josh Poimboeuf <>,,
        "H. Peter Anvin" <>,
        Nathan Chancellor <>,,
Subject: Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk

On Mon, Jan 11, 2021 at 12:38:06PM -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 optimization 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].
> We can omit the .L prefix on a label to 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.
> Cc: Fangrui Song <>
> Link:
> Link:
> Link:
> Link:
> Reported-by: Arnd Bergmann <>
> Suggested-by: Josh Poimboeuf <>
> Signed-off-by: Nick Desaulniers <>
> ---
> Changes v2 -> v3:
> * rework to use STB_LOCAL rather than STB_GLOBAL by dropping .L prefix,
>   as per Josh.

Ok so I read a bit around those links above...

Are you trying to tell me here that we can't use .L-prefixed local
labels anymore because, well, clang's assembler is way too overzealous
when stripping symbols to save whopping KiBs of memory?!

Btw Josh made sense to me when asking for a flag or so to keep .text.

And I see --generate-unused-section-symbols=[yes|no] for binutils.

So why isn't there a patch using that switch on clang too instead of the
kernel having to dance yet again for some tool?



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