From 12b5cd99a05f7cbc2ceb88b3b9601d404ef2236a Mon Sep 17 00:00:00 2001 From: John Stultz Date: Sat, 6 Dec 2025 02:22:03 +0000 Subject: [PATCH] sched/ext: Avoid null ptr traversal when ->put_prev_task() is called with NULL next Early when trying to get sched_ext and proxy-exe working together, I kept tripping over NULL ptr in put_prev_task_scx() on the line: if (sched_class_above(&ext_sched_class, next->sched_class)) { Which was due to put_prev_task() passes a NULL next, calling: prev->sched_class->put_prev_task(rq, prev, NULL); put_prev_task_scx() already guards for a NULL next in the switch_class case, but doesn't seem to have a guard for sched_class_above() check. I can't say I understand why this doesn't trip usually without proxy-exec. And in newer kernels there are way fewer put_prev_task(), and I can't easily reproduce the issue now even with proxy-exec. But we still have one put_prev_task() call left in core.c that seems like it could trip this, so I wanted to send this out for consideration. tj: put_prev_task() can be called with NULL @next; however, when @p is queued, that doesn't happen, so this condition shouldn't currently be triggerable. The connection isn't straightforward or necessarily reliable, so add the NULL check even if it can't currently be triggered. Link: http://lkml.kernel.org/r/20251206022218.1541878-1-jstultz@google.com Signed-off-by: John Stultz Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 073b669869cb..bd74b371f52d 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -2402,7 +2402,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, * ops.enqueue() that @p is the only one available for this cpu, * which should trigger an explicit follow-up scheduling event. */ - if (sched_class_above(&ext_sched_class, next->sched_class)) { + if (next && sched_class_above(&ext_sched_class, next->sched_class)) { WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_LAST)); do_enqueue_task(rq, p, SCX_ENQ_LAST, -1); } else { -- 2.47.3