[PATCH 2/2] sched: allow users with rtprio rlimit to change from SCHED_IDLE policy

[Darren Hart <dvhart@linux.intel.com>]

As it stands, users with rtprio rlimit permissions can change their policy from
SCHED_OTHER to SCHED_FIFO and back. They can change to SCHED_IDLE, but not back
to SCHED_FIFO. If they have the rtprio permission, they should be able to. Once
in SCHED_FIFO, they could go back to SCHED_OTHER. This patch allows users with
rtprio permission to change out of SCHED_IDLE.

This patch allows the following test-case to pass for users with rtprio
permissions with or without the ifdef block:

int main()
{
	int ret;
	struct sched_param sp;

	/* switch to SCHED_FIFO */
	sp.sched_priority = 1;
	ret = sched_setscheduler(0, SCHED_FIFO, &sp);
	printf("setscheduler FIFO: %d\n", ret);
	if (ret) return ret;

	/* switch to SCHED_IDLE */
	sp.sched_priority = 0;
	ret = sched_setscheduler(0, SCHED_IDLE, &sp);
	printf("setscheduler IDLE: %d\n", ret);
	if (ret) return ret;

	/* switch to SCHED_FIFO */
	sp.sched_priority = 1;
	ret = sched_setscheduler(0, SCHED_FIFO, &sp);
	printf("setscheduler FIFO: %d\n", ret);
	if (ret) return ret;
	/* switch back to SCHED_OTHER */
	sp.sched_priority = 0;
	ret = sched_setscheduler(0, SCHED_OTHER, &sp);
	printf("setscheduler OTHER: %d\n", ret);

	return ret;
}

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@elte.hu>
CC: Richard Purdie <richard.purdie@linuxfoundation.org>
---
 kernel/sched.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 18d38e4..ec6943e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4822,12 +4822,16 @@ recheck:
 			    param->sched_priority > rlim_rtprio)
 				return -EPERM;
 		}
+
 		/*
-		 * Like positive nice levels, dont allow tasks to
-		 * move out of SCHED_IDLE either:
+		 * Like positive nice levels, don't allow tasks to move out of
+		 * SCHED_IDLE either. Make an exception if the user can set rt
+		 * policy normally.
 		 */
-		if (p->policy == SCHED_IDLE && policy != SCHED_IDLE)
-			return -EPERM;
+		if (p->policy == SCHED_IDLE && policy != SCHED_IDLE) {
+			if (!task_rlimit(p, RLIMIT_RTPRIO))
+				return -EPERM;
+		}
 
 		/* can't change other user's priorities */
 		if (!check_same_owner(p))
-- 
1.7.1