linux-kernel - Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk

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Date:   Tue, 12 Jan 2021 01:38:39 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
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 <maskray@...gle.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/pipermail/binutils/2020-December/114671.html
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> 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?

:-\

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette