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

[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@vger.kernel.org
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
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

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

[James Clark]

On 16/01/2025 11:08 am, Kuan-Wei Chiu wrote:
> 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@vger.kernel.org
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
> Changes in v3:
> - Perform a full comparison for clarity, as suggested by James.
> 

Reviewed-by: James Clark <james.clark@linaro.org>

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

[Namhyung Kim]

On Thu, 16 Jan 2025 19:08:42 +0800, Kuan-Wei Chiu wrote:

> 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.
> 
> [...]

Applied to perf-tools-next, thanks!

Best regards,
Namhyung