perf hwmon: Fix off-by-one null termination on sysfs reads

Three functions read sysfs files into fixed-size stack buffers using
the full buffer size, then null-terminate at buf[read_len].  If the
read fills the buffer exactly, read_len equals sizeof(buf) and the
null byte writes one past the array, corrupting an adjacent stack
variable.

Fix all three by reading sizeof(buf) - 1 bytes, reserving space for
the null terminator:

  - hwmon_pmu__read_events(): buf[128]
  - hwmon_pmu__describe_items(): buf[64]
  - evsel__hwmon_pmu_read(): buf[32]

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:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/tools/perf/util/hwmon_pmu.c b/tools/perf/util/hwmon_pmu.c
index dbf6a71af47f..d895cd74f2a0 100644
--- a/tools/perf/util/hwmon_pmu.c
+++ b/tools/perf/util/hwmon_pmu.c
@@ -289,7 +289,7 @@ static int hwmon_pmu__read_events(struct hwmon_pmu *pmu)
 			if (fd < 0)
 				continue;
 
-			read_len = read(fd, buf, sizeof(buf));
+			read_len = read(fd, buf, sizeof(buf) - 1);
 
 			while (read_len > 0 && buf[read_len - 1] == '\n')
 				read_len--;
@@ -432,7 +432,7 @@ static size_t hwmon_pmu__describe_items(struct hwmon_pmu *hwm, char *out_buf, si
 			is_alarm ? "_alarm" : "");
 		fd = openat(dir, buf, O_RDONLY);
 		if (fd > 0) {
-			ssize_t read_len = read(fd, buf, sizeof(buf));
+			ssize_t read_len = read(fd, buf, sizeof(buf) - 1);
 
 			while (read_len > 0 && buf[read_len - 1] == '\n')
 				read_len--;
@@ -816,7 +816,7 @@ int evsel__hwmon_pmu_read(struct evsel *evsel, int cpu_map_idx, int thread)
 
 	count = perf_counts(evsel->counts, cpu_map_idx, thread);
 	fd = FD(evsel, cpu_map_idx, thread);
-	len = pread(fd, buf, sizeof(buf), 0);
+	len = pread(fd, buf, sizeof(buf) - 1, 0);
 	if (len <= 0) {
 		count->lost++;
 		return -EINVAL;