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

[Michael Kerrisk (man-pages)]

Hi Peter,

On Fri, May 9, 2014 at 10:53 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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

(Ahh -- I missed that piece in sys_sched_setparam())

> sys_sched_setattr() -- hmm, it looks like fail

Yep, I was seeing that there was no check in sched_setatr().

As I recently said, when it comes to writing a man page, show me a new
interface, and I'll show you a bug ;-).

Thanks for the clarification.

Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

Cheers,

Michael


> ---
> Subject: sched: Disallow sched_attr::sched_policy < 0
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 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@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/n/tip-b4kbwz2qh21xlngdzje00t55@xxxxxxxxxxxxxx
> ---
>  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);



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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