[PATCH 15/23] perf pmu: Use scnprintf() in format_alias()

[Arnaldo Carvalho de Melo]

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

format_alias() accumulates strings into a caller-provided buffer using
snprintf().  On truncation, snprintf() returns the would-have-written
count, which can exceed the buffer size.  When used is larger than len,
buf + used computes a pointer past the buffer end — undefined behavior
even though sub_non_neg() clamps the subsequent write size to zero.

Switch to scnprintf() which returns the actual number of bytes written,
keeping used within bounds.

Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Fixes: aaea36174991ff39 ("perf tools: Extend format_alias() to include event parameters")
Cc: Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx>
Cc: Ian Rogers <irogers@xxxxxxxxxx>
Assisted-by: Claude Opus 4.6 <noreply@xxxxxxxxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/pmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
index 4ddccc863727d6ca..2fce38140eb8af2a 100644
--- a/tools/perf/util/pmu.c
+++ b/tools/perf/util/pmu.c
@@ -2155,11 +2155,11 @@ static char *format_alias(char *buf, int len, const struct perf_pmu *pmu,
 		snprintf(buf, len, "%.*s/%s/", (int)pmu_name_len, pmu->name, alias->name);
 		return buf;
 	}
-	used = snprintf(buf, len, "%.*s/%s", (int)pmu_name_len, pmu->name, alias->name);
+	used = scnprintf(buf, len, "%.*s/%s", (int)pmu_name_len, pmu->name, alias->name);
 
 	list_for_each_entry(term, &terms.terms, list) {
 		if (term->type_val == PARSE_EVENTS__TERM_TYPE_STR)
-			used += snprintf(buf + used, sub_non_neg(len, used),
+			used += scnprintf(buf + used, sub_non_neg(len, used),
 					",%s=%s", term->config,
 					term->val.str);
 	}
-- 
2.54.0