[PATCH] sched: fix sched_setparam() policy=-1 logic

[Daniel Bristot de Oliveira <bristot@redhat.com>]

The scheduler uses policy=-1 to preserve the current policy state to
implement sched_setparam(). But, as (int) -1 is equals to 0xffffffff,
it's matching the if (policy & SCHED_RESET_ON_FORK) on
_sched_setscheduler(). This match changes the policy value to an
invalid value, breaking the sched_setparam() syscall.

This patch checks policy=-1 before check the SCHED_RESET_ON_FORK flag.

The following program shows the bug:

int main(void)
{
	struct sched_param param = {
		.sched_priority = 5,
	};

	sched_setscheduler(0, SCHED_FIFO, &param);
	param.sched_priority = 1;
	sched_setparam(0, &param);
	param.sched_priority = 0;
	sched_getparam(0, &param);
	if (param.sched_priority != 1)
		printf("failed priority setting (found %d instead of 1)\n",
			param.sched_priority);
	else
		printf("priority setting fine\n");
}

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org # 3.14+
Fixes: 7479f3c9cf67 "sched: Move SCHED_RESET_ON_FORK into attr::sched_flags"
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>

---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bc1638b..0acf96b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3558,9 +3558,10 @@ static int _sched_setscheduler(struct task_struct *p, int policy,
 	};
 
 	/*
-	 * Fixup the legacy SCHED_RESET_ON_FORK hack
+	 * Fixup the legacy SCHED_RESET_ON_FORK hack, except if
+	 * the policy=-1 was passed by sched_setparam().
 	 */
-	if (policy & SCHED_RESET_ON_FORK) {
+	if ((policy != -1) && (policy & SCHED_RESET_ON_FORK)) {
 		attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
 		policy &= ~SCHED_RESET_ON_FORK;
 		attr.sched_policy = policy;
-- 
1.9.3