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Date:	Wed, 27 Aug 2014 12:09:48 -0400
From:	Brian Rak <>
To:	Vlad Yasevich <>,
Subject: Re: skb_warn_bad_offload warnings with FreeBSD guests

On 8/25/2014 10:25 AM, Vlad Yasevich wrote:
> On 08/22/2014 12:19 PM, Brian Rak wrote:
>> We have a number of machines running qemu with bridged networking. We have noticed that
>> *sometimes* FreeBSD guests cause this warning to flood the host "WARNING: CPU: 5 PID: 3705
>> at net/core/dev.c:2238 skb_warn_bad_offload+0xc3/0xd0()".  I haven't been able to come up
>> with any sort of reproduction steps, it just seems to happen to some FreeBSD guests, but
>> not others.
>> A full stack trace looks like this:
>> ------------[ cut here ]------------
>> WARNING: CPU: 1 PID: 7147 at net/core/dev.c:2233 skb_warn_bad_offload+0xc3/0xd0()
>> igb: caps=(0x0000000190114bb3, 0x0000000000000000) len=2962 data_len=0 gso_size=1448
>> gso_type=5 ip_summed=0
>> Modules linked in: dm_snapshot dm_bufio ipmi_devintf xt_physdev ebt_arp ebt_ip ebtable_nat
>> ebtables cls_fw sch_sfq sch_htb tun kvm_intel kvm 8021q garp nfnetlink_queue nfnetlink_log
>> nfnetlink bluetooth rfkill bridge stp llc xt_CHECKSUM iptable_mangle ipt_REJECT
>> iptable_filter ip
>> _tables ip6t_REJECT ip6table_filter ip6_tables ipv6 iTCO_wdt iTCO_vendor_support ipmi_si
>> ipmi_msghandler microcode pcspkr i2c_i801 joydev sg lpc_ich shpchp igb dca ptp pps_core
>> hwmon ext4 jbd2 mbcache sd_mod crc_t10dif crct10dif_common video ahci libahci xhci_hcd ast
>> ttm drm_kms
>> _helper sysimgblt sysfillrect syscopyarea dm_mirror dm_region_hash dm_log dm_mod
>> CPU: 1 PID: 7147 Comm: qemu-kvm Tainted: G        W 3.15.5-1.el6.elrepo.x86_64 #1
>> Hardware name: Supermicro X10SLE-F/HF/X10SLE, BIOS 1.1 07/19/2013
>>   00000000000008b9 ffff88081fc435d8 ffffffff8163ba90 00000000000008b9
>>   ffff88081fc43628 ffff88081fc43618 ffffffff8106c30c ffffc90007a06e30
>>   0000000000000000 ffff8807f2b64000 ffff8807f2b64000 0000000000000000
>> Call Trace:
>>   <IRQ>  [<ffffffff8163ba90>] dump_stack+0x49/0x61
>>   [<ffffffff8106c30c>] warn_slowpath_common+0x8c/0xc0
>>   [<ffffffff8106c3f6>] warn_slowpath_fmt+0x46/0x50
>>   [<ffffffff8156ce93>] skb_warn_bad_offload+0xc3/0xd0
>>   [<ffffffff81574a29>] ? dev_hard_start_xmit+0x339/0x640
>>   [<ffffffff81574699>] __skb_gso_segment+0x89/0xe0
>>   [<ffffffff81574876>] dev_hard_start_xmit+0x186/0x640
>>   [<ffffffff81594f5a>] sch_direct_xmit+0xfa/0x1d0
>>   [<ffffffff81574f2f>] __dev_queue_xmit+0x1ff/0x4f0
>>   [<ffffffff81575240>] dev_queue_xmit+0x10/0x20
>>   [<ffffffffa02e6612>] br_dev_queue_push_xmit+0x82/0xb0 [bridge]
>>   [<ffffffffa02ee680>] br_nf_dev_queue_xmit+0x20/0x90 [bridge]
>>   [<ffffffffa02ef4b8>] br_nf_post_routing+0x2d8/0x300 [bridge]
>>   [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
>>   [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
>>   [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
>>   [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
>>   [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
>>   [<ffffffffa02ee6f0>] ? br_nf_dev_queue_xmit+0x90/0x90 [bridge]
>>   [<ffffffffa02e6b43>] br_forward_finish+0x43/0x60 [bridge]
>>   [<ffffffffa02ee8a8>] br_nf_forward_finish+0x1b8/0x1d0 [bridge]
>>   [<ffffffffa02ef178>] br_nf_forward_ip+0x3a8/0x410 [bridge]
>>   [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
>>   [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
>>   [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
>>   [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
>>   [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
>>   [<ffffffffa02e66e4>] __br_forward+0xa4/0x100 [bridge]
>>   [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
>>   [<ffffffffa02e67d6>] br_forward+0x96/0xb0 [bridge]
>>   [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
>>   [<ffffffffa02e7997>] br_handle_frame_finish+0x197/0x3f0 [bridge]
>>   [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
>>   [<ffffffffa02ef790>] br_nf_pre_routing_finish+0x2b0/0x370 [bridge]
>>   [<ffffffffa02ef4e0>] ? br_nf_post_routing+0x300/0x300 [bridge]
>>   [<ffffffffa02ed986>] NF_HOOK_THRESH+0x56/0x60 [bridge]
>>   [<ffffffffa02eed2b>] br_nf_pre_routing+0x2fb/0x3a0 [bridge]
>>   [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
>>   [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
>>   [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
>>   [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
>>   [<ffffffffa02e7d8c>] br_handle_frame+0x19c/0x240 [bridge]
>>   [<ffffffffa02e7bf0>] ? br_handle_frame_finish+0x3f0/0x3f0 [bridge]
>>   [<ffffffff81572fa5>] __netif_receive_skb_core+0x1e5/0x620
>>   [<ffffffff81573407>] __netif_receive_skb+0x27/0x70
>>   [<ffffffff81573553>] process_backlog+0x103/0x200
>>   [<ffffffff81573d62>] net_rx_action+0x112/0x2a0
>>   [<ffffffff8107111c>] __do_softirq+0xfc/0x2b0
>>   [<ffffffff810713cd>] ? irq_exit+0xad/0xd0
>>   [<ffffffff8164a81c>] do_softirq_own_stack+0x1c/0x30
>>   <EOI>  [<ffffffff81070e75>] do_softirq+0x55/0x60
>>   [<ffffffff81571e19>] netif_rx_ni+0x39/0x70
>>   [<ffffffffa03e84e0>] tun_get_user+0x310/0x6c0 [tun]
>>   [<ffffffffa03e8995>] tun_chr_aio_write+0x85/0xa0 [tun]
>>   [<ffffffff811beb9d>] do_sync_readv_writev+0x4d/0x80
>>   [<ffffffff811c0128>] do_readv_writev+0xc8/0x2c0
>>   [<ffffffff811bebd0>] ? do_sync_readv_writev+0x80/0x80
>>   [<ffffffff811d2c45>] ? poll_select_set_timeout+0x95/0xb0
>>   [<ffffffff811c0357>] vfs_writev+0x37/0x50
>>   [<ffffffff811c0496>] SyS_writev+0x56/0xf0
>>   [<ffffffff81648ee9>] system_call_fastpath+0x16/0x1b
>> ---[ end trace d26e70ba037ab631 ]---
>> gso_type=5 and ip_summed=0 are always the same (though len, data_len, and gso_size vary).
>> What is causing this?
> The reason that the warning is triggered is ip_summed = 0 which means there is not
> checksum already in the packet and it needs to be calculated.  If the packet is GSO,
> then it needs to have partial checksum set (ip_summed == 3).
> You might try using systemtap or instrumenting tun and bridge to see what the
> ip_summed value is when this happens.
Who needs systemtap when you have strace ;)

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 ( 
) it seems that the tun device is expecting a virtio_net_hdr, but that 
structure is only 10 bytes long ( 
).  I'm assuming the last two bytes are padding, because then the rest 
of the structure decodes okay:

flags =  0
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.
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