Re: [PATCH V4 2/2] tools/perf: Use scnprintf in buffer offset calculations

On 5 Jun 2026, at 3:32 AM, Namhyung Kim [off-list ref] wrote:

On Mon, May 04, 2026 at 09:12:05PM +0530, Athira Rajeev wrote:

quoted

Replace snprintf with scnprintf in buffer offset calculations to
ensure the 'used' count will not exceed the "len".

The current logic in perf_pmu__for_each_event uses an unconditional
+ 1 increment to buf_used to account for null terminators. This can
cause a a stack buffer overflow in the subsequent scnprintf call.
When the local stack buffer buf (1024 bytes) is full, buf_used can
reach 1025. This causes the subsequent remaining space calculation
sizeof(buf) - buf_used to underflow.

Use sub_non_neg() to see if space actually existed, and only
increment the offset if remaning space is present.

Changes includes:
- Use sub_non_neg to check if space exists
- Replacing snprintf with scnprintf to ensure the return value
reflects the actual bytes written into the buffer.
- Only increment buf_used by 1 if space exists
- If a parameterized event uses a built-in perf keyword for its
parameter name (eg, config=?), the lexer parses it as a predefined
term token, which sets term->config to NULL. Add check to use
parse_events__term_type_str() if term->config is NULL.

Signed-off-by: Athira Rajeev <redacted>
---
Changelog:
v2 -> v3:
- Split the scnprintf related changes in separate patch
- Handle the overflow issues and unconditional increment
wrapped around sub_non_neg addressing review comment from Sashiko

tools/perf/util/pmu.c | 46 ++++++++++++++++++++++++++++++++-----------
1 file changed, 35 insertions(+), 11 deletions(-)

diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
index 0b8d58543f17..4b9ade1a4cf9 100644
--- a/tools/perf/util/pmu.c
+++ b/tools/perf/util/pmu.c

@@ -2129,15 +2129,19 @@ static char *format_alias(char *buf, int len, const struct perf_pmu *pmu,

pr_err("Failure to parse '%s' terms '%s': %d\n",
alias->name, alias->terms, ret);
parse_events_terms__exit(&terms);
- snprintf(buf, len, "%.*s/%s/", (int)pmu_name_len, pmu->name, alias->name);
+ scnprintf(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) {
+ const char *name = term->config;
+
+ if (!name)
+ name = parse_events__term_type_str(term->type_term);
if (term->type_val == PARSE_EVENTS__TERM_TYPE_STR)
- used += snprintf(buf + used, sub_non_neg(len, used),
- ",%s=%s", term->config,
+ used += scnprintf(buf + used, sub_non_neg(len, used),
+ ",%s=%s", name,
term->val.str);
}
parse_events_terms__exit(&terms);

@@ -2201,6 +2205,7 @@ int perf_pmu__for_each_event(struct perf_pmu *pmu, bool skip_duplicate_pmus,

int ret = 0;
struct hashmap_entry *entry;
size_t bkt;
+ size_t size_rem, len;

if (perf_pmu__is_tracepoint(pmu))
return tp_pmu__for_each_event(pmu, state, cb);

@@ -2234,17 +2239,36 @@ int perf_pmu__for_each_event(struct perf_pmu *pmu, bool skip_duplicate_pmus,

}
buf_used = strlen(buf) + 1;
}
+
info.scale_unit = NULL;
if (strlen(event->unit) || event->scale != 1.0) {
- info.scale_unit = buf + buf_used;
- buf_used += snprintf(buf + buf_used, sizeof(buf) - buf_used,
- "%G%s", event->scale, event->unit) + 1;
+ /* Check the remaining space */
+ size_rem = sub_non_neg(sizeof(buf), buf_used);
+
+ if (size_rem > 0) {
+ info.scale_unit = buf + buf_used;
+ len = scnprintf(buf + buf_used, size_rem, "%G%s",
+ event->scale, event->unit);
+ /*
+  * Increment buf_used by 1 only if
+  * it fits remaining space
+  */
+ buf_used += min(len + 1, size_rem);

Hmm.. it seems scnprintf() cannot return a number greater than or equal
to size_rem.  Can we just do like this?

buf_used += scnprintf(...) + 1;

Thanks,
Namhyung

Sure

I will address this change in next version

Thanks
Athira

quoted

+ }
}
info.desc = event->desc;
info.long_desc = event->long_desc;
- info.encoding_desc = buf + buf_used;
- buf_used += snprintf(buf + buf_used, sizeof(buf) - buf_used,
- "%.*s/%s/", (int)pmu_name_len, info.pmu_name, event->terms) + 1;
+ info.encoding_desc = NULL;
+
+ /* Check the remaining space */
+ size_rem = sub_non_neg(sizeof(buf), buf_used);
+ if (size_rem > 0) {
+ info.encoding_desc = buf + buf_used;
+ len = scnprintf(buf + buf_used, size_rem, "%.*s/%s/",
+ (int)pmu_name_len, info.pmu_name, event->terms);
+ buf_used += min(len + 1, size_rem);
+ }
+
info.str = event->terms;
info.topic = event->topic;
info.deprecated = perf_pmu_alias__check_deprecated(pmu, event);

@@ -2254,7 +2278,7 @@ int perf_pmu__for_each_event(struct perf_pmu *pmu, bool skip_duplicate_pmus,

}
if (pmu->selectable) {
info.name = buf;
- snprintf(buf, sizeof(buf), "%s//", pmu->name);
+ scnprintf(buf, sizeof(buf), "%s//", pmu->name);
info.alias = NULL;
info.scale_unit = NULL;
info.desc = NULL;
-- 
2.47.3