[PATCH] tools/sched_ext: scx_simple/scx_cpu0: fix potential stack overflow from VLA in read_stats

[David Carlier]

read_stats() in both scx_simple and scx_cpu0 had a VLA allocating
2 * nr_cpus * 8 bytes on the stack, risking stack overflow on large
CPU counts.

Apply the same fix as commit cabd76bbc036 ("tools/sched_ext:
scx_flatcg: fix potential stack overflow from VLA in fcg_read_stats"):
use a single heap allocation, reuse it across all stat indices, and
free it at the end.

Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
---
 tools/sched_ext/scx_cpu0.c   | 12 +++++++++---
 tools/sched_ext/scx_simple.c | 12 +++++++++---
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/tools/sched_ext/scx_cpu0.c b/tools/sched_ext/scx_cpu0.c
index a6fba9978b9c..0b412d2eb3f0 100644
--- a/tools/sched_ext/scx_cpu0.c
+++ b/tools/sched_ext/scx_cpu0.c
@@ -41,21 +41,27 @@ static void read_stats(struct scx_cpu0 *skel, __u64 *stats)
 {
 	int nr_cpus = libbpf_num_possible_cpus();
 	assert(nr_cpus > 0);
-	__u64 cnts[2][nr_cpus];
+	__u64 *cnts;
 	__u32 idx;
 
+	cnts = calloc(nr_cpus, sizeof(__u64));
+	if (!cnts)
+		return;
+
 	memset(stats, 0, sizeof(stats[0]) * 2);
 
 	for (idx = 0; idx < 2; idx++) {
 		int ret, cpu;
 
 		ret = bpf_map_lookup_elem(bpf_map__fd(skel->maps.stats),
-					  &idx, cnts[idx]);
+					  &idx, cnts);
 		if (ret < 0)
 			continue;
 		for (cpu = 0; cpu < nr_cpus; cpu++)
-			stats[idx] += cnts[idx][cpu];
+			stats[idx] += cnts[cpu];
 	}
+
+	free(cnts);
 }
 
 int main(int argc, char **argv)
diff --git a/tools/sched_ext/scx_simple.c b/tools/sched_ext/scx_simple.c
index c3b48611712b..b6ef4cca425a 100644
--- a/tools/sched_ext/scx_simple.c
+++ b/tools/sched_ext/scx_simple.c
@@ -43,21 +43,27 @@ static void read_stats(struct scx_simple *skel, __u64 *stats)
 {
 	int nr_cpus = libbpf_num_possible_cpus();
 	assert(nr_cpus > 0);
-	__u64 cnts[2][nr_cpus];
+	__u64 *cnts;
 	__u32 idx;
 
+	cnts = calloc(nr_cpus, sizeof(__u64));
+	if (!cnts)
+		return;
+
 	memset(stats, 0, sizeof(stats[0]) * 2);
 
 	for (idx = 0; idx < 2; idx++) {
 		int ret, cpu;
 
 		ret = bpf_map_lookup_elem(bpf_map__fd(skel->maps.stats),
-					  &idx, cnts[idx]);
+					  &idx, cnts);
 		if (ret < 0)
 			continue;
 		for (cpu = 0; cpu < nr_cpus; cpu++)
-			stats[idx] += cnts[idx][cpu];
+			stats[idx] += cnts[cpu];
 	}
+
+	free(cnts);
 }
 
 int main(int argc, char **argv)
-- 
2.51.0