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 v3: - Perform a full comparison for clarity, as suggested by James. tools/perf/bench/epoll-wait.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/tools/perf/bench/epoll-wait.c b/tools/perf/bench/epoll-wait.c index ef5c4257844d..20fe4f72b4af 100644 --- a/tools/perf/bench/epoll-wait.c +++ b/tools/perf/bench/epoll-wait.c @@ -420,7 +420,12 @@ 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; + + if (w1->tid > w2->tid) + return 1; + if (w1->tid < w2->tid) + return -1; + return 0; } int bench_epoll_wait(int argc, const char **argv) -- 2.34.1
