Re: pppx(4): prevent concurency with pppx_if_destroy()

[Vitaliy Makkoveev]

> On 7 Apr 2020, at 17:43, Martin Pieuchot <m...@openbsd.org> wrote:
> 
> On 07/04/20(Tue) 17:14, Vitaliy Makkoveev wrote:
>> As Claudio Jeker noticed, NET_LOCK() can release KERNEL_LOCK(). pppx(4)
>> code has some NET_LOCK() dances which make it unsafe. [...]
> 
> The easiest way to fix that is to move if_detach() out of pppx_if_destroy().

pppxioctl() is under KERNEL_LOCK(). pppxioctl() holds NET_LOCK() it’s whole
runtime. So your suggestion is (pseudo code):

KERNEL_LOCK();

pppx_ioctl()
{
        NET_LOCK();

        switch() {
        case PIPEXDSESSION:
                NET_UNLOCK();
                /*
                 * KERNEL_LOCK() can be released here and concurrent
                 * thread with PIPEXDSESSION case will enter here
                 */
                if_detach();
                NET_LOCK();

                pppx_if_destroy();
                break;
        }

        NET_UNLOCK();
}

KERNEL_UNLOCK();

I understood well?
> 
> It generally makes sense to call if_detach() first then free/close the
> descriptor of a driver.  However some drivers have callbacks and in that
> case you might want to teardown those first then call if_detach().
> 
> if_detach() will require the NET_LOCK() for some time.  However
> pseudo-driver should start protecting their own data structure with
> different locks.
>