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

On 25/02/2020 04:02, Jason Wang wrote:

On 2020/2/25 上午6:22, Willem de Bruijn wrote:

quoted

On Mon, Feb 24, 2020 at 4:00 PM Anton Ivanov
[off-list ref] wrote:

quoted

On 24/02/2020 20:20, Willem de Bruijn wrote:

quoted

On Mon, Feb 24, 2020 at 2:55 PM Anton Ivanov
[off-list ref] wrote:

quoted

On 24/02/2020 19:27, Willem de Bruijn wrote:

quoted

On Mon, Feb 24, 2020 at 8:26 AM [off-list ref] wrote:

quoted

From: Anton Ivanov <anton.ivanov@cambridgegreys.com>

Some of the locally generated frames marked as GSO which
arrive at virtio_net_hdr_from_skb() have no GSO_TYPE, no
fragments (data_len = 0) and length significantly shorter
than the MTU (752 in my experiments).

Do we understand how these packets are generated?

No, we have not been able to trace them.

The only thing we know is that this is specific to locally generated
packets. Something arriving from the network does not show this.

quoted

Else it seems this
might be papering over a deeper problem.

The stack should not create GSO packets less than or equal to
skb_shinfo(skb)->gso_size. See for instance the check in
tcp_gso_segment after pulling the tcp header:

           mss = skb_shinfo(skb)->gso_size;
           if (unlikely(skb->len <= mss))
                   goto out;

What is the gso_type, and does it include SKB_GSO_DODGY?

0 - not set.

Thanks for the follow-up details. Is this something that you can trigger easily?

Yes, if you have a UML instance handy.

Running iperf between the host and a UML guest using raw socket
transport triggers it immediately.

This is my UML command line:

vmlinux mem=2048M umid=OPX \
      ubd0=OPX-3.0-Work.img \
vec0:transport=raw,ifname=p-veth0,depth=128,gro=1,mac=92:9b:36:5e:38:69 \
      root=/dev/ubda ro con=null con0=null,fd:2 con1=fd:0,fd:1

p-right is a part of a vEth pair:

ip link add l-veth0 type veth peer name p-veth0 && ifconfig p-veth0 up

iperf server is on host, iperf -c in the guest.

quoted

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. From the previous thread, these are assumed to be TCP
packets?

I had missed the original thread. If the packet has

     sinfo(skb)->gso_size = 752.
     skb->len = 818

then this is a GSO packet. Even though UML will correctly process it
as a normal 818 B packet if psock_rcv pretends that it is, treating it
like that is not strictly correct. A related question is how the setup
arrived at that low MTU size, assuming that is not explicitly
configured that low.

As of commit 51466a7545b7 ("tcp: fill shinfo->gso_type at last
moment") tcp unconditionally sets gso_type, even for non gso packets.
So either this is not a tcp packet or the field gets zeroed somewhere
along the way. I could not quickly find a possible path to
skb_gso_reset or a raw write.

It may be useful to insert tests for this condition (skb_is_gso(skb)
&& !skb_shinfo(skb)->gso_type) that call skb_dump at other points in
the network stack. For instance in __ip_queue_xmit and
__dev_queue_xmit.

+1

We meet some customer hit such condition as well which lead over MTU packet to be queued by TAP which crashes their buggy userspace application.

We suspect it's the issue of wrong gso_type vs gso_size.

Well, we now have a test case where all the code is available and 100% under our control :)

Brgds,

Thanks

quoted

Since skb segmentation fails in tcp_gso_segment for such packets, it
may also be informative to disable TSO on the veth device and see if
the test fails.

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Anton R. Ivanov
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