perf bench: Fix undefined behavior in cmpworker()

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
Fixes: 121dd9ea0116 ("perf bench: Add epoll parallel epoll_wait benchmark")
Cc: stable@vger.kernel.org
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250116110842.4087530-1-visitorckw@gmail.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

diff --git a/tools/perf/bench/epoll-wait.c b/tools/perf/bench/epoll-wait.c
index ef5c4257844d13c79d2ddce2473326b2c81b74e2..20fe4f72b4afccee8e445c5433d52773c20f23b1 100644 (file)
--- 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)