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@xxxxxxxxxxxxxxx
> Signed-off-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
> ---
> Changes in v3:
> - Perform a full comparison for clarity, as suggested by James.

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