[PATCH v3] perf bench: Fix undefined behavior in cmpworker()

[Kuan-Wei Chiu]

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