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

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

The comparison function cmpworker() does not comply with the C
standard's requirements for qsort() comparison functions. Specifically,
it returns 0 when w1->tid < w2->tid, which is incorrect. According to
the standard, the function must return a negative value in such cases
to preserve proper ordering.

This violation causes undefined behavior, potentially leading to issues
such as memory corruption in certain versions of 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>
---
 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;
 }
 
 int bench_epoll_wait(int argc, const char **argv)
-- 
2.34.1