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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan