diff --git a/tools/lib/api/fs/fs.c b/tools/lib/api/fs/fs.c
index d16911818d4d..cbd8eab0d1df 100644
--- a/tools/lib/api/fs/fs.c
+++ b/tools/lib/api/fs/fs.c
@@ -261,8 +261,8 @@ static const char *mount_overload(struct fs *fs)
 	/* "PERF_" + name + "_ENVIRONMENT" + '\0' */
 	char upper_name[5 + name_len + 12 + 1];
 
-	snprintf(upper_name, name_len, "PERF_%s_ENVIRONMENT", fs->name);
-	mem_toupper(upper_name, name_len);
+	snprintf(upper_name, sizeof(upper_name), "PERF_%s_ENVIRONMENT", fs->name);
+	mem_toupper(upper_name, strlen(upper_name));
 
 	return getenv(upper_name) ?: *fs->mounts;
 }