Re: [PATCH 2/7] x86/wait: prevent duplicated assembly labels

["Roger Pau Monné" <roger.pau@citrix.com>]

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.