Re: [PATCH] select: fix sys_select to not leak ERESTARTNOHAND to userspace

[Neil Horman <nhorman@tuxdriver.com>]

On Tue, Jan 23, 2007 at 09:59:26PM -0800, David Miller wrote:
> From: Neil Horman <nhorman@tuxdriver.com>
> Date: Tue, 16 Jan 2007 15:13:32 -0500
> 
> > As it is currently written, sys_select checks its return code to convert
> > ERESTARTNOHAND to EINTR.  However, the check is within an if (tvp) clause, and
> > so if select is called from userspace with a NULL timeval, then it is possible
> > for the ERESTARTNOHAND errno to leak into userspace, which is incorrect.  This
> > patch moves that check outside of the conditional, and prevents the errno leak.
> > 
> > Signed-Off-By: Neil Horman <nhorman@tuxdriver.com>
> 
> As has been probably mentioned, this change is bogus.
> 
> core_sys_select() returns -ERESTARTNOHAND in exactly the
> case where that return value is legal, when a signal is
> pending, so that the signal dispatch code (on return from
> the system call) will "fixup" the -ERESTARTNOHAND return
> value so that userspace does not see it.
> 
> What could be happening is that, somehow, we see the signal
> pending in core_sys_select(), but for some reason by the time
> the signal dispatch code would run, the signal is cleared.
> 
> I always found this scheme a little fishy, relying on signal
> pending state to guarentee the fixup of the syscall restart
> error return values.
> 
> For example, I see nothing that prevents another thread sharing
> this signal state from clearing the signal between the syscall
> checking in the other thread and the signal dispatch check running
> in that other thread.
> 
> 	cpu 1			cpu 2
> 	check sigpending
> 				clear signal
> 	syscall return
> 	no signal panding
> 	get -ERESTARTNOHAND
> 
> Perhaps something makes this no happen, but I don't see it :)
This is exactly the theory that I've been tossing around with the person that
reported it to me.  We're still witing on a reproduer, but I'm currently working
on a multithreaded test case to try and trigger user space leaks of
ERESTARTNOHAND to user space.

Neil