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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
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