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
"image" > when compiling with -ffunction-sections and -fdata-sections. I have > observerd on the order of tens of thousands of symbols for the kernel "observed" > 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. > * rename oneline to drop STB_GLOBAL in commit message. > * add link to GAS docs on .L prefix. > * drop Josh's ack since patch changed. Acked-by: Josh Poimboeuf <[email protected]> -- Josh
