linux-kernel - RE: [PATCH net-next v02 1/2] af_packet: allow fanout

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Message-ID: <670977bac8b03_247429294f6@willemb.c.googlers.com.notmuch>
Date: Fri, 11 Oct 2024 15:08:42 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Gur Stavi <gur.stavi@...wei.com>, 
 'Willem de Bruijn' <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 netdev@...r.kernel.org, 
 pabeni@...hat.com, 
 shuah@...nel.org
Subject: RE: [PATCH net-next v02 1/2] af_packet: allow fanout_add when socket
 is not RUNNING

Gur Stavi wrote:
> > Gur Stavi wrote:
> > > >
> > > > If we don't care about opening up fanout groups to ETH_P_NONE, then
> > > > patch v2 seems sufficient. If explicitly blocking this, the ENXIO
> > > > return can be added, but ideally without touching the other lines.
> > >
> > > I don't think that allowing ETH_P_NONE is relevant.
> > > In my opinion the 2 options that should be considered to fail
> > > fanout_add are:
> > > 1. Testing proto == 0
> > > 2. Testing proto == 0 || ifindex == -1
> > >
> > > The only corner case that is caught by [2] and missed by [1] is
> > > the "unlisted" case during do_bind. It is such a rare case that
> > > probably no one will ever encounter bind "unlisted" followed by
> > > FANOUT_ADD. And this is not a dangerous corner case that leads to
> > > system crash.
> > >
> > > However, being a purist, I see the major goal of code review to promote
> > > correctness by identifying corner cases while improving style is a
> > > secondary priority. Since we did identify this corner case in our
> > > discussion I think we should still use [2].
> > > I don't consider the code complex. In fact, to me, the ifindex clause
> > > is a more understandable direct reason for failure than the proto which
> > > is indirect. Having the ifindex clause helps figuring out the proto
> > > clause.
> > 
> > It's interesting that the unlisted fix does not return ENODEV, but
> > returns success and leaves the socket in an unbound state, equivalent
> > to binding to ETH_P_NONE and ifindex 0. This seems surprising behavior
> > to the caller.
> > 
> > On rereading that, I still do not see a purpose of special ifindex -1.
> > 
> >
> 
> Can this code be relevant?
> 
> 		case NETDEV_UP:
> 			if (dev->ifindex == po->ifindex) {
> 				spin_lock(&po->bind_lock);
> 				if (po->num)
> 					register_prot_hook(sk);
> 				spin_unlock(&po->bind_lock);
> 			}
> 			break;
> 
> Perhaps, although the socket failed to (re) find the device, the device
> is still aware of the socket and we need the ifindex condition to fail.

But the behavior is the same for ifindex -1 and 0. Devices always have
an ifindex >= 1.