[PATCH 4/4] sched_setscheduler: fix? policy checks

[Oleg Nesterov <oleg@tv-sign.ru>]

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.


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:

		if (is_rt_policy(policy) && !has_rt_policy(p)
				&& !rlim_rtprio)
			return -EPERM;

but this is covered by the "/* can't increase priority */" check below,
because

	is_rt_policy(policy)	=>  ->sched_priority > 0
	!has_rt_policy(p)	=>  p->rt_priority == 0


So I think we can just remove this code.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.18-rc4/kernel/sched.c~4_policy	2006-08-19 22:03:26.000000000 +0400
+++ 2.6.18-rc4/kernel/sched.c	2006-08-19 23:20:26.000000000 +0400
@@ -4079,27 +4079,21 @@ recheck:
 	 * Allow unprivileged RT tasks to decrease priority:
 	 */
 	if (!capable(CAP_SYS_NICE)) {
-		unsigned long rlim_rtprio;
-		unsigned long flags;
+		if (is_rt_policy(policy)) {
+			unsigned long rlim_rtprio;
+			unsigned long flags;
 
-		if (!lock_task_sighand(p, &flags))
-			return -ESRCH;
-		rlim_rtprio = p->signal->rlim[RLIMIT_RTPRIO].rlim_cur;
-		unlock_task_sighand(p, &flags);
+			if (!lock_task_sighand(p, &flags))
+				return -ESRCH;
+			rlim_rtprio = p->signal->rlim[RLIMIT_RTPRIO].rlim_cur;
+			unlock_task_sighand(p, &flags);
+
+			/* can't increase priority */
+			if (param->sched_priority > p->rt_priority &&
+			    param->sched_priority > rlim_rtprio)
+				return -EPERM;
+		}
 
-		/*
-		 * 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)
-			return -EPERM;
-		/* can't increase priority */
-		if (is_rt_policy(policy) &&
-		    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))