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@xxxxxxxxxxxxxxx > > Signed-off-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx> > > --- > > 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
