sched_ext: Allow p->scx.disallow only while loading

From 1232da7eced620537a78f19c8cf3d4a3508e2419 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 31 Jul 2024 09:14:52 -1000

p->scx.disallow provides a way for the BPF scheduler to reject certain tasks
from attaching. It's currently allowed for both the load and fork paths;
however, the latter doesn't actually work as p->sched_class is already set
by the time scx_ops_init_task() is called during fork.

This is a convenience feature which is mostly useful from the load path
anyway. Allow it only from the load path.

v2: Trigger scx_ops_error() iff @p->policy == SCHED_EXT to make it a bit
    easier for the BPF scheduler (David).
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Zhangqiao (2012 lab)" <zhangqiao22@huawei.com>
Link: http://lkml.kernel.org/r/20240711110720.1285-1-zhangqiao22@huawei.com
Fixes: 7bb6f081 ("sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT")
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/sched/ext.h

@@ -179,11 +179,12 @@ struct sched_ext_entity {
 	 * If set, reject future sched_setscheduler(2) calls updating the policy
 	 * to %SCHED_EXT with -%EACCES.
 	 *
-	 * If set from ops.init_task() and the task's policy is already
-	 * %SCHED_EXT, which can happen while the BPF scheduler is being loaded
-	 * or by inhering the parent's policy during fork, 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.
+	 * 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.
 	 */
 	bool			disallow;	/* reject switching into SCX */
 

kernel/sched/ext.c

@@ -3396,24 +3396,29 @@ static int scx_ops_init_task(struct task_struct *p, struct task_group *tg, bool
 	scx_set_task_state(p, SCX_TASK_INIT);
 
 	if (p->scx.disallow) {
-		struct rq *rq;
-		struct rq_flags rf;
+		if (!fork) {
+			struct rq *rq;
+			struct rq_flags rf;
 
-		rq = task_rq_lock(p, &rf);
+			rq = task_rq_lock(p, &rf);
 
-		/*
-		 * We're either in fork or load path and @p->policy will be
-		 * applied right after. Reverting @p->policy here and rejecting
-		 * %SCHED_EXT transitions from scx_check_setscheduler()
-		 * guarantees that if ops.init_task() sets @p->disallow, @p can
-		 * never be in SCX.
-		 */
-		if (p->policy == SCHED_EXT) {
-			p->policy = SCHED_NORMAL;
-			atomic_long_inc(&scx_nr_rejected);
-		}
+			/*
+			 * We're in the load path and @p->policy will be applied
+			 * right after. Reverting @p->policy here and rejecting
+			 * %SCHED_EXT transitions from scx_check_setscheduler()
+			 * guarantees that if ops.init_task() sets @p->disallow,
+			 * @p can never be in SCX.
+			 */
+			if (p->policy == SCHED_EXT) {
+				p->policy = SCHED_NORMAL;
+				atomic_long_inc(&scx_nr_rejected);
+			}
 
-		task_rq_unlock(rq, p, &rf);
+			task_rq_unlock(rq, p, &rf);
+		} else if (p->policy == SCHED_EXT) {
+			scx_ops_error("ops.init_task() set task->scx.disallow for %s[%d] during fork",
+				      p->comm, p->pid);
+		}
 	}
 
 	p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;