[PATCH 10/11] perf tools: Use scnprintf() in build_id__snprintf() and hwmon read_events()

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

build_id__snprintf() and hwmon_pmu__read_events() accumulate formatted
output via snprintf(), which returns the would-have-been-written count
on truncation.  In build_id__snprintf(), this inflates the return
value beyond the buffer size.  In hwmon_pmu__read_events(), len
overshoots out_buf_len and the next 'out_buf_len - len' underflows.

Switch both to scnprintf() which returns actual bytes written.

In build_id__snprintf(), also tighten the loop guard from
'offs < bf_size' to 'offs + 1 < bf_size': since scnprintf() returns
at most size-1, offs never reaches bf_size, and the original condition
would spin doing zero-byte writes once the buffer fills.

Fixes: fccaaf6fbbc59910 ("perf build-id: Change sprintf functions to snprintf")
Fixes: 53cc0b351ec99278 ("perf hwmon_pmu: Add a tool PMU exposing events from hwmon in sysfs")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/build-id.c  |  4 ++--
 tools/perf/util/hwmon_pmu.c | 12 ++++++------
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
index 8c0a9ae932aa5798..82af3dca7e2fbb5b 100644
--- a/tools/perf/util/build-id.c
+++ b/tools/perf/util/build-id.c
@@ -93,8 +93,8 @@ int build_id__snprintf(const struct build_id *build_id, char *bf, size_t bf_size
 		return 0;
 	}
 
-	for (size_t i = 0; i < build_id->size && offs < bf_size; ++i)
-		offs += snprintf(bf + offs, bf_size - offs, "%02x", build_id->data[i]);
+	for (size_t i = 0; i < build_id->size && offs + 1 < bf_size; ++i)
+		offs += scnprintf(bf + offs, bf_size - offs, "%02x", build_id->data[i]);
 
 	return offs;
 }
diff --git a/tools/perf/util/hwmon_pmu.c b/tools/perf/util/hwmon_pmu.c
index fb3ffa8d32ad2a93..dbf6a71af47f9a42 100644
--- a/tools/perf/util/hwmon_pmu.c
+++ b/tools/perf/util/hwmon_pmu.c
@@ -442,12 +442,12 @@ static size_t hwmon_pmu__describe_items(struct hwmon_pmu *hwm, char *out_buf, si
 
 				buf[read_len] = '\0';
 				val = strtoll(buf, /*endptr=*/NULL, 10);
-				len += snprintf(out_buf + len, out_buf_len - len, "%s%s%s=%g%s",
-						len == 0 ? " " : ", ",
-						hwmon_item_strs[bit],
-						is_alarm ? "_alarm" : "",
-						(double)val / 1000.0,
-						hwmon_units[key.type]);
+				len += scnprintf(out_buf + len, out_buf_len - len, "%s%s%s=%g%s",
+						 len == 0 ? " " : ", ",
+						 hwmon_item_strs[bit],
+						 is_alarm ? "_alarm" : "",
+						 (double)val / 1000.0,
+						 hwmon_units[key.type]);
 			}
 			close(fd);
 		}
-- 
2.54.0