From: Xie He <xie.he.0141@gmail.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Linux Kernel Network Developers <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux X25 <linux-x25@vger.kernel.org>,
Brian Norris <briannorris@chromium.org>
Subject: Re: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header_len
Date: Sat, 1 Aug 2020 05:45:46 -0700 [thread overview]
Message-ID: <CAJht_EO4b=jC8KarwZyF1M3T57MrFCDvo-+Agnm9qD4pSCmODQ@mail.gmail.com> (raw)
In-Reply-To: <CA+FuTSeusqdfkqZihFhTE9vhcL5or6DEh8UffaKM2Px82z6BZQ@mail.gmail.com>
On Fri, Jul 31, 2020 at 7:33 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.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!
next prev parent reply other threads:[~2020-08-01 12:46 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-30 7:37 [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header_len Xie He
2020-07-30 8:02 ` Xie He
2020-08-04 6:53 ` Martin Schiller
2020-08-04 7:05 ` Willem de Bruijn
2020-08-04 10:07 ` Xie He
2020-08-04 10:13 ` Xie He
2020-08-04 9:48 ` Xie He
2020-07-31 1:36 ` Xie He
2020-07-31 14:12 ` Willem de Bruijn
2020-07-31 20:40 ` Xie He
2020-08-01 2:33 ` Willem de Bruijn
2020-08-01 12:45 ` Xie He [this message]
2020-08-01 13:30 ` Willem de Bruijn
2020-08-02 0:58 ` Xie He
2020-07-31 1:57 ` Xie He
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