linux-kernel - Re: [PATCH] perf ftrace: Fix undefined behavior in cmp_profile

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Message-ID: <CAM9d7cgL-1rET97eVU2qpz5-V5XqeCX1N92wTwR5y2sp_4sjog@mail.gmail.com>
Date: Mon, 9 Dec 2024 09:02:24 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org, 
	mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org, 
	irogers@...gle.com, kan.liang@...ux.intel.com, adrian.hunter@...el.com, 
	jserv@...s.ncku.edu.tw, chuang@...nycu.edu.tw, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] perf ftrace: Fix undefined behavior in cmp_profile_data()

Hello,

On Mon, Dec 9, 2024 at 5:42 AM Kuan-Wei Chiu <visitorckw@...il.com> wrote:
>
> The comparison function cmp_profile_data() 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.
>
> When v1 and v2 are equal, the function incorrectly returns 1, breaking
> symmetry and transitivity. This causes undefined behavior, which can
> lead to memory corruption in certain versions of glibc [1].
>
> Fix the issue by returning 0 when v1 and v2 are equal, ensuring
> compliance with the C standard and preventing undefined behavior.
>
> Link: https://www.qualys.com/2024/01/30/qsort.txt [1]
> Fixes: 0f223813edd0 ("perf ftrace: Add 'profile' command")
> Fixes: 74ae366c37b7 ("perf ftrace profile: Add -s/--sort option")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>

Reviewed-by: Namhyung Kim <namhyung@...nel.org>

Thanks for the fix.
Namhyung

> ---
>  tools/perf/builtin-ftrace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index 272d3c70810e..a56cf8b0a7d4 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -1151,8 +1151,9 @@ static int cmp_profile_data(const void *a, const void *b)
>
>         if (v1 > v2)
>                 return -1;
> -       else
> +       if (v1 < v2)
>                 return 1;
> +       return 0;
>  }
>
>  static void print_profile_result(struct perf_ftrace *ftrace)
> --
> 2.34.1
>