[PATCH] tools/sched_ext: scx_flatcg: Fix uninitialized stats on allocation failure

[luoliang]

From: Liang Luo <luoliang@xxxxxxxxxx>

In fcg_read_stats(), the memset() that zeroes the output @stats array
sits after the calloc() failure check. When calloc() fails, the
function returns without writing @stats.

The caller in main() declares acc_stats uninitialized, passes it as
the @stats argument, and then reads it unconditionally:

    __u64 acc_stats[FCG_NR_STATS];
    fcg_read_stats(skel, acc_stats);
    stats[i] = acc_stats[i] - last_stats[i];   // reads garbage

Because fcg_read_stats() returns void, the caller cannot detect the
failure. Reading the uninitialized array is undefined behavior, and
the garbage is further copied into last_stats via memcpy(), corrupting
the baseline used by the next interval.

This regression was introduced by commit cabd76bbc036 ("tools/sched_ext:
scx_flatcg: fix potential stack overflow from VLA in fcg_read_stats"),
which replaced the VLA with calloc() and inserted the failure check
before the existing memset().

Move the memset() above the calloc() failure check so @stats is always
zeroed regardless of allocation outcome.

Fixes: cabd76bbc036 ("tools/sched_ext: scx_flatcg: fix potential stack overflow from VLA in fcg_read_stats")
Signed-off-by: Liang Luo <luoliang@xxxxxxxxxx>
---
 tools/sched_ext/scx_flatcg.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/sched_ext/scx_flatcg.c b/tools/sched_ext/scx_flatcg.c
index de2bef86d64d..7799782b76d1 100644
--- a/tools/sched_ext/scx_flatcg.c
+++ b/tools/sched_ext/scx_flatcg.c
@@ -105,12 +105,12 @@ static void fcg_read_stats(struct scx_flatcg *skel, __u64 *stats)
 	__u64 *cnts;
 	__u32 idx;
 
+	memset(stats, 0, sizeof(stats[0]) * FCG_NR_STATS);
+
 	cnts = calloc(skel->rodata->nr_cpus, sizeof(__u64));
 	if (!cnts)
 		return;
 
-	memset(stats, 0, sizeof(stats[0]) * FCG_NR_STATS);
-
 	for (idx = 0; idx < FCG_NR_STATS; idx++) {
 		int ret, cpu;
 
-- 
2.43.0