[PATCH 01/25] perf/core: Reject non-positive AUX snapshot sizes

[Leo Yan <leo.yan@arm.com>]

The existing validation rejects negative copy errors but allows a zero
return. In that case no AUX data was copied, but it is incorrectly
passed to the alignment-padding check.

Reject zero together with negative errors before calculating the padding.
This reports the invalid snapshot result and avoids treating a missing
AUX payload as alignment padding.

Fixes: a4faf00d994c ("perf/aux: Allow using AUX data in perf samples")
Assisted-by: Codex:gpt-6
Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 kernel/events/core.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fe33fe15689d077e7f445c0b17d9c2e638c884a1..17355e4b8b7c6c106117153c9eb8509e6f9a41e3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8058,12 +8058,11 @@ static void perf_aux_sample_output(struct perf_event *event,
 	size = perf_pmu_snapshot_aux(rb, sampler, handle, data->aux_size);
 
 	/*
-	 * An error here means that perf_output_copy() failed (returned a
-	 * non-zero surplus that it didn't copy), which in its current
-	 * enlightened implementation is not possible. If that changes, we'd
-	 * like to know.
+	 * A negative return means that perf_output_copy() failed, while zero
+	 * means that no AUX data was copied despite a non-zero request. Neither
+	 * can be treated as alignment padding below.
 	 */
-	if (WARN_ON_ONCE(size < 0))
+	if (WARN_ON_ONCE(size <= 0))
 		goto out_put;
 
 	/*

-- 
2.34.1