On Mon, Jan 11, 2021 at 4:38 PM Borislav Petkov <[email protected]> wrote: > > 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 <[email protected]> > > 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 <[email protected]> > > Suggested-by: Josh Poimboeuf <[email protected]> > > Signed-off-by: Nick Desaulniers <[email protected]> > > --- > > 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?!
To be fair: we cannot use .L-prefixed local because of the objtool limitation. The LLVM integrated assembler behavior is a good one and binutils global maintainers have agreed so H.J. went ahead and implemented it for GNU as x86. > 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? --generate-unused-section-symbols=[yes|no] as an assembler option has been rejected. > :-\ > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
