[PATCH] 2.6.13rc3: RLIMIT_RTPRIO broken

Andreas Steinmetz
2005-07-23T11:42:07Z

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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@domdv.de

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--- 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 &&