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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

I can take a look at what I can do, if the proposed approach is not
suitable.

Thanks, Roger.