Re: [PATCH v3] virtio: Work around frames incorrectly marked as gso

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An skb_dump() + dump_stack() when the packet socket gets such a
packet may point us to the root cause and fix that.

We tried dump stack, it was not informative - it was just the recvmmsg
call stack coming from the UML until it hits the relevant recv bit in
af_packet - it does not tell us where the packet is coming from.

Quoting from the message earlier in the thread:

[ 2334.180854] Call Trace:
[ 2334.181947]  dump_stack+0x5c/0x80
[ 2334.183021]  packet_recvmsg.cold+0x23/0x49
[ 2334.184063]  ___sys_recvmsg+0xe1/0x1f0
[ 2334.185034]  ? packet_poll+0xca/0x130
[ 2334.186014]  ? sock_poll+0x77/0xb0
[ 2334.186977]  ? ep_item_poll.isra.0+0x3f/0xb0
[ 2334.187936]  ? ep_send_events_proc+0xf1/0x240
[ 2334.188901]  ? dequeue_signal+0xdb/0x180
[ 2334.189848]  do_recvmmsg+0xc8/0x2d0
[ 2334.190728]  ? ep_poll+0x8c/0x470
[ 2334.191581]  __sys_recvmmsg+0x108/0x150
[ 2334.192441]  __x64_sys_recvmmsg+0x25/0x30
[ 2334.193346]  do_syscall_64+0x53/0x140
[ 2334.194262]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

That makes sense. skb_dump might show more interesting details about
the packet.

I will add that and retest later today.

skb len=818 headroom=2 headlen=818 tailroom=908
mac=(2,14) net=(16,0) trans=16
shinfo(txflags=0 nr_frags=0 gso(size=752 type=0 segs=1))
csum(0x100024 ip_summed=3 complete_sw=0 valid=0 level=0)
hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=4 iif=0
sk family=17 type=3 proto=0

Deciphering the actual packet data gives a

TCP packet, ACK and PSH set.

The PSH flag looks like the only "interesting" thing about it in first read.

Thanks.

TCP always sets the PSH bit on a GSO packet as of commit commit
051ba67447de  ("tcp: force a PSH flag on TSO packets"), so that is
definitely informative.

The lower gso size might come from a path mtu probing depending on
tcp_base_mss, but that's definitely wild speculation. Increasing that
value to, say, 1024, could tell us.

In this case it may indeed not be a GSO packet. As 752 is the MSS + 28
B TCP header including timestamp + 20 B IPv4 header + 14B Eth header.
Which adds up to 814 already.

Not sure what those 2 B between skb->data and mac_header are. Was this
captured inside packet_rcv? network_header and transport_header both
at 16B offset is also sketchy, but again may be an artifact of where
exactly this is being read.

Perhaps this is a segment of a larger GSO packet that is retransmitted
in part. Like an mtu probe or loss probe. See for instance this in
tcp_send_loss_probe for  how a single MSS is extracted:

       if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
                if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                                          (pcount - 1) * mss, mss,
                                          GFP_ATOMIC)))
                        goto rearm_timer;
                skb = skb_rb_next(skb);
        }

Note that I'm not implicating this specific code. I don't see anything
wrong with it. Just an indication that a trace would be very
informative, as it could tell if any of these edge cases is being hit.