On 14/03/2025 8:44 am, Jan Beulich wrote: > On 14.03.2025 09:30, Roger Pau Monné wrote: >> 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? > Hmm, yes on both points; the %= would only work for .Lskip. Have you gained > understanding why there is this duplication? The breaking out of the asm() > that you do isn't going to be reliable, as in principle the compiler is > still permitted to duplicate stuff. Afaict the only reliable way is to move > the code to a separate assembly file (with the asm() merely JMPing there, > providing a pseudo-return-address by some custom means). Or to a file-scope > asm(), as those can't be duplicated.
See the simplified example in https://github.com/llvm/llvm-project/issues/92161 When I debugged this a while back, The multiple uses of wqv->esp (one explicit after the asm, one as an asm parameter) gain pointer sanitisation, so the structure looks like: ... if ( bad pointer ) __ubsan_report(); asm volatile (...); if ( bad pointer ) __ubsan_report(); ... which then got transformed to: if ( bad pointer ) { __ubsan_report(); asm volatile (...); __ubsan_report(); } else asm volatile (...); ~Andrew
