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Subject[PATCH 4/4] 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.


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))
-
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