[PATCH] sched/topology: fail overlapping group build when no sched_group is created

[Joseph Salisbury]

build_overlap_sched_groups() can complete without creating any sched_group
and still return success.

This can happen on overlapping NUMA topologies with unequal domain depth.
In that case build_sched_domains() may stop building some sibling domain
levels early once their span already matches the target cpu_map. Later,
when build_overlap_sched_groups() walks the span, every candidate sibling
can fail the:

	cpumask_test_cpu(i, sched_domain_span(sibling))

check, so no group is created and first remains NULL.

That leaves sd->groups unset (NULL). Later topology setup still assumes
sd->groups is valid and can dereference it from 
init_sched_groups_capacity(), sd_degenerate(), and cpu_attach_domain().

Catch the empty-group case in build_overlap_sched_groups() itself and
fail the sched-domain build through the existing error path instead of
constructing an invalid sched_domain.

Fixes: f2cb13609d53 ("sched/topology: Split out scheduler topology code from core.c into topology.c")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Codex:GPT-5
Signed-off-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxx>
---
 kernel/sched/topology.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1108,6 +1108,10 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
 		last = sg;
 		last->next = first;
 	}
+
+	if (WARN_ON_ONCE(!first))
+		return -EINVAL;
+
 	sd->groups = first;
 
 	return 0;
-- 
2.47.3