On 14.03.2025 10:06, Roger Pau Monné wrote: > On Fri, Mar 14, 2025 at 09:44:10AM +0100, 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? > > Not anything else than what Andrew found in: > > https://github.com/llvm/llvm-project/issues/92161 > >> 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. > > I know. That's why I mention in the commit message that "... asm > block duplication issues arise again." > >> 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. > > Moving to a separate file was my first thought, but it seemed more > intrusive that strictly needed to workaround the issue at hand.
Maybe the file-scope asm() approach would be less intrusive overall, compared to the separate-.S-file one. Plus it may allow keeping labels non-global. > I can take a look at what I can do, if the proposed approach is not > suitable. I've made yet another suggestion in reply to Andrew's response. Jan
