linux-kernel - Re: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header

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Date:   Sat, 1 Aug 2020 05:45:46 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead
 of hard_header_len

On Fri, Jul 31, 2020 at 7:33 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> I quickly scanned the main x.25 datapath code. Specifically
> x25_establish_link, x25_terminate_link and x25_send_frame. These all
> write this 1 byte header. It appears to be an in-band communication
> means between the network and data link layer, never actually ending
> up on the wire?

Yes, this 1-byte header is just a "fake" header that is only for
communication between the network layer and the link layer. It never
ends up on wire.

I think we can think of it as the Ethernet header for Wifi drivers.
Although Wifi doesn't actually use the Ethernet header, Wifi drivers
use a "fake" Ethernet header to communicate with code outside of the
driver. From outside, it appears that Wifi drivers use the Ethernet
header.

> > The best solution might be to implement header_ops for X.25 drivers
> > and let dev_hard_header create this 1-byte header, so that
> > hard_header_len can equal to the header length created by
> > dev_hard_header. This might be the best way to fit the logic of
> > af_packet.c. But this requires changing the interface of X.25 drivers
> > so it might be a big change.
>
> Agreed.

Actually I tried this solution today. It was easier to implement than
I originally thought. I implemented header_ops to make dev_hard_header
generate the 1-byte header. And when receiving, (according to the
requirement of af_packet.c) I pulled this 1-byte header before
submitting the packet to upper layers. Everything worked fine, except
one issue:

When receiving, af_packet.c doesn't handle 0-sized packets well. It
will drop them. This causes an AF_PACKET/DGRAM socket to receive no
indication when it is connected or disconnected. Do you think this is
a problem? Actually I'm also afraid that future changes in af_packet.c
will make 0-sized packets not able to pass when sending as well.

> Either lapbeth_xmit has to have a guard against 0 byte packets before
> reading skb->data[0], or packet sockets should not be able to generate
> those (is this actually possible today through PF_PACKET? not sure)
>
> If SOCK_DGRAM has to always select one of the three values (0x00:
> data, 0x01: establish, 0x02: terminate) the first seems most sensible.
> Though if there is no way to establish a connection with
> PF_PACKET/SOCK_DGRAM, that whole interface may still be academic.
> Maybe eventually either 0x00 or 0x01 could be selected based on
> lapb->state.. That however is out of scope of this fix.

Yes, I think the first solution may be better, because we need to have
a way to drop 0-sized DGRAM packets (as long as we need to include the
1-byte header when sending DGRAM packets) and I'm not aware
af_packet.c can do this.

Yes, I think maybe the best way is to get rid of the 1-byte header
completely and use other ways to ask the driver to connect or
disconnect, or let it connect and disconnect automatically.

> Normally a fix should aim to have a Fixes: tag, but all this code
> precedes git history, so that is not feasible here.

Thanks for pointing this out!