Commit 62892e77 authored by Kuan-Wei Chiu's avatar Kuan-Wei Chiu Committed by Namhyung Kim
Browse files

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

 [1]
Fixes: 121dd9ea ("perf bench: Add epoll parallel epoll_wait benchmark")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarKuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: default avatarJames Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250116110842.4087530-1-visitorckw@gmail.com


Signed-off-by: default avatarNamhyung Kim <namhyung@kernel.org>
parent 035f0c27

tools/perf/bench/epoll-wait.c

+6 −1
Original line number Diff line number Diff line
@@ -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)