[PATCH 1/4] sched_ext: Reject setting disallow from init_task outside the enable path

The p->scx.disallow revert assumes the root enable path, where the switching
loop reads the reverted policy right afterwards and leaves the task off SCX.
The sub-scheduler disable path also reaches it when re-initializing the
returned tasks on a root parent. Nothing reads the policy there: the task is
enabled on root anyway and keeps running on the ext class with a silently
rewritten policy.

Kill the sched instead, matching the fork and non-root branches, and update
the disallow documentation, which equated !fork with the load path and
pointed at a stale debugfs path for nr_rejected.

Fixes: 337ec00b1d9c ("sched_ext: Implement cgroup sub-sched enabling and disabling")
Signed-off-by: Tejun Heo <[email protected]>
---
 include/linux/sched/ext.h | 10 +++++-----
 kernel/sched/ext/ext.c    |  3 +++
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 20b2343aa344..87e353f7e011 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -244,11 +244,11 @@ struct sched_ext_entity {
 	 * to %SCHED_EXT with -%EACCES.
 	 *
 	 * Can be set from ops.init_task() while the BPF scheduler is being
-	 * loaded (!scx_init_task_args->fork). If set and the task's policy is
-	 * already %SCHED_EXT, the task's policy is rejected and forcefully
-	 * reverted to %SCHED_NORMAL. The number of such events are reported
-	 * through /sys/kernel/debug/sched_ext::nr_rejected. Setting this flag
-	 * during fork is not allowed.
+	 * loaded. If set and the task's policy is already %SCHED_EXT, the
+	 * task's policy is rejected and forcefully reverted to %SCHED_NORMAL.
+	 * The number of such events are reported through
+	 * /sys/kernel/sched_ext/nr_rejected. Setting this flag from any other
+	 * ops.init_task() invocation, such as during fork, fails the scheduler.
 	 */
 	bool			disallow;	/* reject switching into SCX */
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index e3fa7b2fac9d..46f135bddd46 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -3606,6 +3606,9 @@ static int __scx_init_task(struct scx_sched *sch, struct task_struct *p, bool fo
 		} else if (unlikely(fork)) {
 			scx_error(sch, "ops.init_task() set task->scx.disallow for %s[%d] during fork",
 				  p->comm, p->pid);
+		} else if (unlikely(scx_enable_state() != SCX_ENABLING)) {
+			scx_error(sch, "ops.init_task() set task->scx.disallow for %s[%d] outside the enable path",
+				  p->comm, p->pid);
 		} else {
 			struct rq *rq;
 			struct rq_flags rf;
-- 
2.55.0