From: Brian Rak <brak@gameservers•com>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: Vlad Yasevich <vyasevich@gmail•com>, netdev@vger•kernel.org
Subject: Re: skb_warn_bad_offload warnings with FreeBSD guests
Date: Wed, 27 Aug 2014 13:11:55 -0400 [thread overview]
Message-ID: <53FE115B.7000806@gameservers.com> (raw)
In-Reply-To: <1409157890.3173.71.camel@edumazet-glaptop2.roam.corp.google.com>
On 8/27/2014 12:44 PM, Eric Dumazet wrote:
> On Wed, 2014-08-27 at 12:09 -0400, Brian Rak wrote:
>
>> I managed to intercept the raw packet + headers being delivered to the
>> tun device, though I'm having some trouble making sense of it. I've got
>> this call:
>>
>> writev(33, [{"\x00\x01\x42\x00\xa0\x05\x00\x00\x00\x00\x00\x00", 12},
>> .... ], 4) = 4258
>>
>> If I ignore the first 12 bytes that were written, I end up with a 4246
>> byte packet, which matches the warning message:
>>
>> kernel: igb: caps=(0x0000000390114bb3, 0x0000000000000000) len=4246
>> data_len=4180 gso_size=1440 gso_type=5 ip_summed=0
>>
>> Looking at the code (
>> https://github.com/torvalds/linux/blob/68e370289c29e3beac99d59c6d840d470af9dfcf/drivers/net/tun.c#L1037
>> ) it seems that the tun device is expecting a virtio_net_hdr, but that
>> structure is only 10 bytes long (
>> http://lxr.free-electrons.com/source/include/uapi/linux/virtio_net.h#L73
>> ). I'm assuming the last two bytes are padding, because then the rest
>> of the structure decodes okay:
>>
>> flags = 0
>> gso_type = VIRTIO_NET_HDR_GSO_TCPV4
>> hdr_len = 66
>> gso_size = 1440
>> csum_start = 0
>> csum_offset = 0
>>
>> This matches what the warning message says, so I'm fairly confident in
>> it. If I decode the remainder of the write call (ignoring the 2 bytes
>> after the header), I'm left with a perfectly normal looking TCP packet
>> (with a 4180 byte payload).
>>
>> Looking at the packet itself, I see a valid IP checksum, and a valid TCP
>> checksum. So, it seems like FreeBSD is calculating the packet checksums
>> correctly, but I'm unsure of why Linux isn't noticing that. I thought
>> it might be related to VIRTIO_NET_HDR_F_DATA_VALID, but I can't seem to
>> find any uses of this that seem relevant (not that FreeBSD sets it anyway).
>>
>> Shouldn't the tun code be setting ip_summed after receiving a packet
>> with a valid checksum? It's not clear to me where ip_summed should be
>> getting set.
> You need VIRTIO_NET_HDR_F_NEEDS_CSUM, and to provide proper csum_start &
> csum_offset
>
Why? The packet doesn't need a checksum (it already has a valid one).
Does 'VIRTIO_NET_HDR_F_NEEDS_CSUM' not mean 'this packet needs a
checksum calculated'?
next prev parent reply other threads:[~2014-08-27 17:12 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-22 16:19 skb_warn_bad_offload warnings with FreeBSD guests Brian Rak
2014-08-25 14:25 ` Vlad Yasevich
2014-08-27 16:09 ` Brian Rak
2014-08-27 16:44 ` Eric Dumazet
2014-08-27 17:11 ` Brian Rak [this message]
2014-08-27 17:24 ` Vlad Yasevich
2014-08-27 18:25 ` Brian Rak
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=53FE115B.7000806@gameservers.com \
--to=brak@gameservers$(echo .)com \
--cc=eric.dumazet@gmail$(echo .)com \
--cc=netdev@vger$(echo .)kernel.org \
--cc=vyasevich@gmail$(echo .)com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox