[PATCH] 2.6.13rc3: RLIMIT_RTPRIO broken

[Andreas Steinmetz]

RLIMIT_RTPRIO is supposed to grant non privileged users the right to use
SCHED_FIFO/SCHED_RR scheduling policies with priorites bounded by the
RLIMIT_RTPRIO value via sched_setscheduler(). This is usually used by
audio users.

Unfortunately this is broken in 2.6.13rc3 as you can see in the excerpt
from sched_setscheduler below:

        /*
         * Allow unprivileged RT tasks to decrease priority:
         */
        if (!capable(CAP_SYS_NICE)) {
                /* can't change policy */
                if (policy != p->policy)
                        return -EPERM;

After the above unconditional test which causes sched_setscheduler to
fail with no regard to the RLIMIT_RTPRIO value the following check is made:

               /* can't increase priority */
                if (policy != SCHED_NORMAL &&
                    param->sched_priority > p->rt_priority &&
                    param->sched_priority >
                                p->signal->rlim[RLIMIT_RTPRIO].rlim_cur)
                        return -EPERM;

Thus I do believe that the RLIMIT_RTPRIO value must be taken into
account for the policy check, especially as the RLIMIT_RTPRIO limit is
of no use without this change.

The attached patch fixes this problem. I would appreciate it if the fix
would make it into 2.6.13.
-- 
Andreas Steinmetz                       SPAMmers use robotrap@xxxxxxxx
--- linux.orig/kernel/sched.c	2005-07-22 19:45:05.000000000 +0200
+++ linux/kernel/sched.c	2005-07-22 19:45:42.000000000 +0200
@@ -3528,7 +3528,8 @@
 	 */
 	if (!capable(CAP_SYS_NICE)) {
 		/* can't change policy */
-		if (policy != p->policy)
+		if (policy != p->policy &&
+			!p->signal->rlim[RLIMIT_RTPRIO].rlim_cur)
 			return -EPERM;
 		/* can't increase priority */
 		if (policy != SCHED_NORMAL &&