[PATCH] perf ftrace: Fix undefined behavior in cmp_profile_data()

[Kuan-Wei Chiu]

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@xxxxxxxxxxxxxxx
Signed-off-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
---
 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