[PATCH] sched_setscheduler: fix? policy checks

I am not sure this patch is correct: I can't understand what the current
code does, and I don't know what it was supposed to do.

The comment says:

		 * can't change policy, except between SCHED_NORMAL
		 * and SCHED_BATCH:

The code:

		if (((policy != SCHED_NORMAL && p->policy != SCHED_BATCH) &&
			(policy != SCHED_BATCH && p->policy != SCHED_NORMAL)) &&

But this is equivalent to:

		if ( (is_rt_policy(policy) && has_rt_policy(p)) &&

which means something different.  We can't _decrease_ the current
->rt_priority with such a check (if rlim[RLIMIT_RTPRIO] == 0).

Probably, it was supposed to be:

		if (	!(policy == SCHED_NORMAL && p->policy == SCHED_BATCH)  &&
			!(policy == SCHED_BATCH  && p->policy == SCHED_NORMAL)

this matches the comment, but strange: it doesn't allow to _drop_ the
realtime priority when rlim[RLIMIT_RTPRIO] == 0.

I think the right check would be:

		/* can't set/change rt policy */
		if (is_rt_policy(policy) &&
				policy != p->policy &&
				!rlim_rtprio)
			return -EPERM;
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4116,6 +4116,7 @@ recheck:
 	 * Allow unprivileged RT tasks to decrease priority:
 	 */
 	if (!capable(CAP_SYS_NICE)) {
+		if (is_rt_policy(policy)) {
 			unsigned long rlim_rtprio;
 			unsigned long flags;
 
@@ -4124,19 +4125,16 @@ recheck:
 			rlim_rtprio = p->signal->rlim[RLIMIT_RTPRIO].rlim_cur;
 			unlock_task_sighand(p, &flags);
 
-		/*
-		 * can't change policy, except between SCHED_NORMAL
-		 * and SCHED_BATCH:
-		 */
-		if (((policy != SCHED_NORMAL && p->policy != SCHED_BATCH) &&
-			(policy != SCHED_BATCH && p->policy != SCHED_NORMAL)) &&
-				!rlim_rtprio)
+			/* can't set/change the rt policy */
+			if (policy != p->policy && !rlim_rtprio)
 				return -EPERM;
+
 			/* can't increase priority */
-		if (is_rt_policy(policy) &&
-		    param->sched_priority > p->rt_priority &&
+			if (param->sched_priority > p->rt_priority &&
 			    param->sched_priority > rlim_rtprio)
 				return -EPERM;
+		}
+
 		/* can't change other user's priorities */
 		if ((current->euid != p->euid) &&
 		    (current->euid != p->uid))