Re: [PATCH v2] perf bench: Fix undefined behavior in cmpworker()

[Kuan-Wei Chiu <visitorckw@gmail.com>]

On Thu, Jan 16, 2025 at 10:40:45AM +0000, James Clark wrote:
> 
> 
> On 07/01/2025 7:39 am, Kuan-Wei Chiu wrote:
> > The comparison function cmpworker() violates the C standard's
> > requirements for qsort() comparison functions, which mandate symmetry
> > and transitivity:
> > 
> > Symmetry: If x < y, then y > x.
> > Transitivity: If x < y and y < z, then x < z.
> > 
> > In its current implementation, cmpworker() incorrectly returns 0 when
> > w1->tid < w2->tid, which breaks both symmetry and transitivity. This
> > violation causes undefined behavior, potentially leading to issues such
> > as memory corruption in glibc [1].
> > 
> > Fix the issue by returning -1 when w1->tid < w2->tid, ensuring
> > compliance with the C standard and preventing undefined behavior.
> > 
> > Link: https://www.qualys.com/2024/01/30/qsort.txt [1]
> > Fixes: 121dd9ea0116 ("perf bench: Add epoll parallel epoll_wait benchmark")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> > ---
> > Changes in v2:
> > - Rewrite commit message
> > 
> >   tools/perf/bench/epoll-wait.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/bench/epoll-wait.c b/tools/perf/bench/epoll-wait.c
> > index ef5c4257844d..4868d610e9bf 100644
> > --- a/tools/perf/bench/epoll-wait.c
> > +++ b/tools/perf/bench/epoll-wait.c
> > @@ -420,7 +420,7 @@ static int cmpworker(const void *p1, const void *p2)
> >   	struct worker *w1 = (struct worker *) p1;
> >   	struct worker *w2 = (struct worker *) p2;
> > -	return w1->tid > w2->tid;
> > +	return w1->tid > w2->tid ? 1 : -1;
> 
> I suppose you can skip the 0 for equality because you know that no two tids
> are the same?
>
Yes, exactly.

> Anyone looking at this in the future might still think it's still wrong
> unless it does the full comparison. Even if it's not technically required I
> would write it like a "normal" one now that we're here:
> 
>   if (w1->tid > w2->tid) return 1;
>   if (w1->tid < w2->tid) return -1;
>   return 0;
> 
Sure. I'll make that change in v3.

Regards,
Kuan-Wei