Re: possible ERESTARTNOHAND leak into userspace

[Oleg Nesterov <oleg@redhat.com>]

On 12/23, Michal Hocko wrote:
>
> Hi,
> this has already been discussed few years back with reports that select
> returned with ERESTARTNOHAND in multi-threaded applications
> (http://forum.soft32.com/linux/PATCH-select-fix-sys_select-leak-ERESTARTNOHAND-userspace-ftopict338572.html)
>
> Dave has come up with a possible explanation of the race but there was
> no further follow up with a conclusion.
>
> Just for reference:
>         Thread_A				Thread_B
> 	CPU0					CPU1
> 						syscall_XYZ
> core_sys_select
>   ret = -ERESTARTNOHAND;
>   if (signal_pending(current))
>      					do_notify_resume
>      					  do_signal (clear signal pending)

"clear signal pending" can't affect Thread_A. Even if it steals
the signal sent to Thread_A.

>   return ret;
> return from syscall
> no pending signal

please see above. Only the task itself can clear its TIF_DIGPENDING.

> return ERESTARTNOHAND

do_signal() should take care and restart the syscall.

> The race window is rather small and hard to trigger but we have seen
> reports where people really saw select returning ERESTARTNOHAND (on
> 2.6.16 based kernel - x86_64).
> I am not able to reproduce that myself neither with .16 kernel nor with
> the current vanilla so I am not sure whether the problem has been fixed
> already. But I do not see what prevents the race with vanilla.

I hope the problem was already fixed, at least I do not see anything
wrong in core_sys_select().

Oleg.