netdev - Re: [PATCH net-next] net: wwan: core: Return poll error in case of port removal

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Message-ID: <CAMZdPi_GvNeY605HW6MfJrACViHTp20NyAtpm1Gbe-7F1=GzZA@mail.gmail.com>
Date:   Thu, 22 Apr 2021 15:37:10 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: wwan: core: Return poll error in case of
 port removal

On Thu, 22 Apr 2021 at 14:59, Leon Romanovsky <leon@...nel.org> wrote:
>
> On Thu, Apr 22, 2021 at 01:21:47PM +0200, Loic Poulain wrote:
> > Hi Leon,
> >
> > On Thu, 22 Apr 2021 at 12:49, Leon Romanovsky <leon@...nel.org> wrote:
> > >
> > > On Thu, Apr 22, 2021 at 11:43:34AM +0200, Loic Poulain wrote:
> > > > Ensure that the poll system call returns error flags when port is
> > > > removed, allowing user side to properly fail, without trying read
> > > > or write. Port removal leads to nullified port operations, add a
> > > > is_port_connected() helper to safely check the status.
> > > >
> > > > Fixes: 9a44c1cc6388 ("net: Add a WWAN subsystem")
> > > > Signed-off-by: Loic Poulain <loic.poulain@...aro.org>
> > > > ---
> > > >  drivers/net/wwan/wwan_core.c | 17 +++++++++++++++--
> > > >  1 file changed, 15 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
> > > > index 5be5e1e..c965b21 100644
> > > > --- a/drivers/net/wwan/wwan_core.c
> > > > +++ b/drivers/net/wwan/wwan_core.c
> > > > @@ -369,14 +369,25 @@ static int wwan_port_op_tx(struct wwan_port *port, struct sk_buff *skb)
> > > >       return ret;
> > > >  }
> > > >
> > > > +static bool is_port_connected(struct wwan_port *port)
> > > > +{
> > > > +     bool connected;
> > > > +
> > > > +     mutex_lock(&port->ops_lock);
> > > > +     connected = !!port->ops;
> > > > +     mutex_unlock(&port->ops_lock);
> > > > +
> > > > +     return connected;
> > > > +}
> > >
> > > The above can't be correct. What prevents to change the status of
> > > port->ops right before or after your mutex_lock/mutex_unlock?
> >
> > Nothing, this is just to protect access to the variable (probably
> > overkill though), which can be concurrently nullified in port removal,
> > and to check if the event (poll wake-up) has been caused by removal of
> > the port, no port operation (port->ops...) is actually called on that
> > condition. If the status is changed right after the check, then any
> > subsequent poll/read/write syscall will simply fail properly.
>
> Taking locks when it is not needed is not overkill, but bug.

Ok understood, so going to rework that patch properly.

> I wander if all these is_*_blocked() checks can be trusted if port->ops
> pointer flips.

The port->ops value can only flip from something (port connected) to
null (port disconnected), and testing port->ops in is_*_blocked()
prevents blocking on waitqueue once the port is removed (similarly to
e.g. virtio_console).

Regards,
Loic