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

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Date:	Fri, 9 May 2014 10:53:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Juri Lelli <juri.lelli@...il.com>,
	Dario Faggioli <raistlin@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, rostedt@...dmis.org,
	Oleg Nesterov <oleg@...hat.com>, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	nicola.manica@...i.unitn.it, luca.abeni@...tn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	linux-man@...r.kernel.org
Subject: Re: sched_{set,get}attr() manpage

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@...radead.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@...il.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/n/tip-b4kbwz2qh21xlngdzje00t55@git.kernel.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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