[PATCH sched_ext/for-6.12] sched_ext: Allow p->scx.disallow only while loading

[Tejun Heo]

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.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: "Zhangqiao (2012 lab)" <zhangqiao22@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20240711110720.1285-1-zhangqiao22@xxxxxxxxxx
Fixes: 7bb6f0810ecf ("sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT")
---
 include/linux/sched/ext.h |   11 ++++++-----
 kernel/sched/ext.c        |   14 ++++++++------
 2 files changed, 14 insertions(+), 11 deletions(-)

--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -181,11 +181,12 @@ 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 */
 
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3399,18 +3399,17 @@ static int scx_ops_init_task(struct task
 
 	scx_set_task_state(p, SCX_TASK_INIT);
 
-	if (p->scx.disallow) {
+	if (!fork && p->scx.disallow) {
 		struct rq *rq;
 		struct rq_flags 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.
+		 * 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;
@@ -3418,6 +3417,9 @@ static int scx_ops_init_task(struct task
 		}
 
 		task_rq_unlock(rq, p, &rf);
+	} else if (p->scx.disallow) {
+		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;