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SubjectRe: possible ERESTARTNOHAND leak into userspace
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.



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