On 2021-01-11, 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].
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d1bcae833b32f1408485ce69f844dcd7ded093a8 has been committed. The patch should be included in binutils 2.37. The maintainers are welcome to the idea, but fixing all the arch-specific tests is tricky. H.J. fixed the x86 tests and enabled this for x86. When binutils 2.37 come out, some other architectures may follow as well.
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.
Josh, I think objtool orc generate needs to synthesize STT_SECTION symbols even if they do not exist in object files. rg 'SYM_CODE.*\.L' reveals a few other .S files which may have similar problems.
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. Changes v1 -> v2: * Pick up Josh's Ack. * Add commit message info about -ffunction-sections/-fdata-sections, and link to binutils patch. arch/x86/entry/thunk_64.S | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S index ccd32877a3c4..c9a9fbf1655f 100644 --- a/arch/x86/entry/thunk_64.S +++ b/arch/x86/entry/thunk_64.S @@ -31,7 +31,7 @@ SYM_FUNC_START_NOALIGN(\name) .endif call \func - jmp .L_restore + jmp __thunk_restore SYM_FUNC_END(\name) _ASM_NOKPROBE(\name) .endm @@ -44,7 +44,7 @@ SYM_FUNC_END(\name) #endif #ifdef CONFIG_PREEMPTION -SYM_CODE_START_LOCAL_NOALIGN(.L_restore) +SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore) popq %r11 popq %r10 popq %r9 @@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore) popq %rdi popq %rbp ret - _ASM_NOKPROBE(.L_restore) -SYM_CODE_END(.L_restore) + _ASM_NOKPROBE(__thunk_restore) +SYM_CODE_END(__thunk_restore) #endif -- 2.30.0.284.gd98b1dd5eaa7-goog
