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

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);

+			}
 		}
 		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