[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 2/7] x86/wait: prevent duplicated assembly labels
On Fri, Mar 14, 2025 at 09:24:09AM +0100, Jan Beulich wrote: > On 13.03.2025 16:30, Roger Pau Monne wrote: > > When enabling UBSAN with clang, the following error is triggered during the > > build: > > > > common/wait.c:154:9: error: symbol '.L_wq_resume' is already defined > > 154 | "push %%rbx; push %%rbp; push %%r12;" > > | ^ > > <inline asm>:1:121: note: instantiated into assembly here > > 1 | push %rbx; push %rbp; push %r12;push %r13; push %r14; push > > %r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov %rsp,%rsi;.L_wq_resume: > > rep movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop %r14; pop %r13;pop %r12; pop > > %rbp; pop %rbx > > | > > ^ > > common/wait.c:154:9: error: symbol '.L_skip' is already defined > > 154 | "push %%rbx; push %%rbp; push %%r12;" > > | ^ > > <inline asm>:1:159: note: instantiated into assembly here > > 1 | push %rbx; push %rbp; push %r12;push %r13; push %r14; push > > %r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov %rsp,%rsi;.L_wq_resume: > > rep movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop %r14; pop %r13;pop %r12; pop > > %rbp; pop %rbx > > | > > > > ^ > > 2 errors generated. > > > > The inline assembly block in __prepare_to_wait() is duplicated, thus > > leading to multiple definitions of the otherwise unique labels inside the > > assembly block. GCC extended-asm documentation notes the possibility of > > duplicating asm blocks: > > > >> Under certain circumstances, GCC may duplicate (or remove duplicates of) > >> your assembly code when optimizing. This can lead to unexpected duplicate > >> symbol errors during compilation if your asm code defines symbols or > >> labels. Using ‘%=’ (see AssemblerTemplate) may help resolve this problem. > > > > Move the assembly blocks that deal with saving and restoring the current > > CPU context into it's own explicitly non-inline functions. This prevents > > clang from duplicating the assembly blocks. Just using noinline attribute > > seems to be enough to prevent assembly duplication, in the future noclone > > might also be required if asm block duplication issues arise again. > > Wouldn't it be a far easier / less intrusive change to simply append %= to > the label names? That won't work AFAICT, as the inline asm in check_wakeup_from_wait() won't be able to make a jump to the .L_wq_resume label defined in the __prepare_to_wait() assembly block if the label is declared as .L_wq_resume%=. Also we want to make sure there's a single .L_wq_resume seeing how check_wakeup_from_wait() uses it as the restore entry point? Thanks, Roger.
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