Re: sched_{set,get}attr() manpage

[Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

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On Fri, May 09, 2014 at 10:23:22AM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Peter,
> 
> I'm working on this text. I see the following in kernel/sched/core.c:
> 
> [[
> static int __sched_setscheduler(struct task_struct *p,
>                                 const struct sched_attr *attr,
>                                 bool user)
> {
>         ...
> 
>         int policy = attr->sched_policy;
>         ...
>         if (policy < 0) {
>                 reset_on_fork = p->sched_reset_on_fork;
>                 policy = oldpolicy = p->policy;
> ]]
> 
> What's a negative policy about? Is this something that should 
> be documented?

That's for sched_setparam(), which internally passes policy = -1, it
wasn't meant to be user visible, lemme double check that.

sys_sched_setscheduler() -- explicit check for policy < 0
sys_sched_setparam() -- explicitly passes policy=-1, not user visible
sys_sched_setattr() -- hmm, it looks like fail


---
Subject: sched: Disallow sched_attr::sched_policy < 0
From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Date: Fri May  9 10:49:03 CEST 2014

The scheduler uses policy=-1 to preserve the current policy state to
implement sys_sched_setparam(), this got exposed to userspace by
accident through sys_sched_setattr(), cure this.

Reported-by: Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Signed-off-by: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Link: http://lkml.kernel.org/n/tip-b4kbwz2qh21xlngdzje00t55-Ckxz5ZWcFp/9qxiX1TGQuw@public.gmane.org
---
 kernel/sched/core.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3711,6 +3711,9 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pi
 	if (sched_copy_attr(uattr, &attr))
 		return -EFAULT;
 
+	if (attr.sched_policy < 0)
+		return -EINVAL;
+
 	rcu_read_lock();
 	retval = -ESRCH;
 	p = find_process_by_pid(pid);

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