[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>

1 files changed, 18 insertions, 20 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index c3c718aea618..155a33da7aa7 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4116,27 +4116,25 @@ recheck:
          * Allow unprivileged RT tasks to decrease priority:
          */
         if (!capable(CAP_SYS_NICE)) {
-                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 (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);
+
+                        /* can't set/change the rt policy */
+                        if (policy != p->policy && !rlim_rtprio)
+                                return -EPERM;
+
+                        /* 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))