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References: <20210402132602.3659282-1-eric.dumazet@gmail.com>
 <20210411134329.GA132317@roeck-us.net> <CANn89iJ+RjYPY11zUtvmMkOp1E2DKLuAk2q0LHUbcJpbcZVSjw@mail.gmail.com>
 <0f63dc52-ea72-16b6-7dcd-efb24de0c852@roeck-us.net> <CANn89iJa8KAnfWvUB8Jr8hsG5x_Amg90DbpoAHiuNZigv75MEA@mail.gmail.com>
 <c1d15bd0-8b62-f7c0-0f2e-1d527827f451@roeck-us.net>
In-Reply-To: <c1d15bd0-8b62-f7c0-0f2e-1d527827f451@roeck-us.net>
From:   Eric Dumazet <edumazet@google.com>
Date:   Sun, 11 Apr 2021 23:43:33 +0200
Message-ID: <CANn89iK-AO4MpWQzh_VkMjUgdcsP4ibaV4RhsDF9RHcuC+_=-g@mail.gmail.com>
Subject: Re: [PATCH net] virtio_net: Do not pull payload in skb->head
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org
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On Sun, Apr 11, 2021 at 11:32 PM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On 4/11/21 2:23 PM, Eric Dumazet wrote:
> > On Sun, Apr 11, 2021 at 10:37 PM Guenter Roeck <linux@roeck-us.net> wrote:
> >>
> >> On 4/11/21 8:06 AM, Eric Dumazet wrote:
> >>> On Sun, Apr 11, 2021 at 3:43 PM Guenter Roeck <linux@roeck-us.net> wrote:
> >>>
> >>>> This patch causes a virtio-net interface failure when booting sh4 images
> >>>> in qemu. The test case is nothing special: Just try to get an IP address
> >>>> using udhcpc. If it fails, udhcpc reports:
> >>>>
> >>>> udhcpc: started, v1.33.0
> >>>> udhcpc: sending discover
> >>>> FAIL
> >>>>
> >>>
> >>> Can you investigate where the incoming packet is dropped ?
> >>>
> >>
> >> Unless I am missing something, packets are not dropped. It looks more
> >> like udhcpc gets bad indigestion in the receive path and exits immediately.
> >> Plus, it doesn't happen all the time; sometimes it receives the discover
> >> response and is able to obtain an IP address.
> >>
> >> Overall this is quite puzzling since udhcpc exits immediately when the problem
> >> is seen, no matter which option I give it on the command line; it should not
> >> really do that.
> >
> >
> > Could you strace both cases and report differences you can spot ?
> >
> > strace -o STRACE -f -s 1000 udhcpc
> >
>
> I'll give it a try. It will take a while; I'll need to add strace to my root
> file systems first.
>
> As a quick hack, I added some debugging into the kernel; it looks like
> the data part of the dhcp discover response may get lost with your patch
> in place.

Data is not lost, the payload is whole contained in skb frags, which
was expected from my patch.

Maybe this sh arch does something wrong in this case.

This could be checksuming...

Please check

nstat -n
<run udhcpc>
nstat


>
> dhcp discover response with patch in place (bad):
>
> virtio_net virtio0 eth0: __udp4_lib_rcv: data 0x8ca4cc44 head 0x8ca4cc00 tail 0x8ca4cc4c len 556 datalen 548 caller ip_protocol_deliver_rcu+0xac/0x178
> 00000000: 70 c1 a9 8c 00 00 00 00 00 00 00 00 20 ee c3 7b 34 00 e0 7b 08 00 00 00 00 00 00 00 00 00 00 00  p........... ..{4..{............
> 00000020: 60 c8 ff ff ff ff ff ff 52 55 0a 00 02 02 08 00 45 10 02 40 00 00 00 00 40 11 6c 9c 0a 00 02 02  `.......RU......E..@....@.l.....
> 00000040: ff ff ff ff 00 43 00 44 02 2c e1 21 00 00 00 00 f0 6f a4 7b 00 00 80 00 ff ff ff ff 7f 45 4c 46  .....C.D.,.!.....o.{.........ELF
>                       ^^ udp header
>                                   ^^^^^ UDP length (556)
>                                               ^^ start of UDP data (dhcp discover reply)
> 00000060: 01 01 01 00 00 00 00 00 00 00 00 00 02 00 2a 00 01 00 00 00 b0 6e 40 00 34 00 00 00 2c f6 0a 00  ..............*......n@.4...,...
> 00000080: 17 00 00 00 34 00 20 00 08 00 28 00 16 00 15 00 06 00 00 00 34 00 00 00 34 00 40 00 34 00 40 00  ....4. ...(.........4...4.@.4.@.
> 000000a0: 00 01 00 00 00 01 00 00 04 00 00 00 04 00 00 00 03 00 00 00 34 01 00 00 34 01 40 00 34 01 40 00  ....................4...4.@.4.@.
> 000000c0: 15 00 00 00 15 00 00 00 04 00 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00 00 40 00 00 00 40 00  ..........................@...@.
> 000000e0: 1c e2 0a 00 1c e2 0a 00 05 00 00 00 00 00 01 00 01 00 00 00 38 ef 0a 00 38 ef 4b 00 38 ef 4b 00  ....................8...8.K.8.K.
> 00000100: 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 00  ................................
> 00000120: b8 00 00 00 00 4c f9 8f 24 02 00 00 36 00 00 00 50 e5 74 64 88 e0 0a 00 88 e0 4a 00 88 e0 4a 00  .....L..$...6...P.td......J...J.
> 00000140: 2c 00 00 00 2c 00 00 00 04 00 00 00 04 00 00 00 51 e5 74 64 00 00 00 00 00 00 00 00 00 00 00 00  ,...,...........Q.td............
> 00000160: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 00000180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 00000200: 1c 63 5a 8c 00 3e a4 8c 88 f9 50 8c 0c ce a4 8c 0c ce a4 8c 24 81 a1 8c 00 00 00 00 1c 5c 5a 8c  .cZ..>....P.........$........\Z.
> 00000220: 04 b8 a5 8c 01 00 00 00 03 00 00 00                                                              ............
>
> dhcp discover response with patch reverted (ok):
>
> virtio_net virtio0 eth0: __udp4_lib_rcv: data 0x8ca4ca44 head 0x8ca4ca00 tail 0x8ca4cb00 len 556 datalen 368 caller ip_protocol_deliver_rcu+0xac/0x178
>                                                               ^^^^^^^^^^      ^^^^^^^^^^                 ^^^
> 00000000: 4c bd ab 8c 00 00 00 00 00 00 00 00 20 2e 85 7b 34 00 e0 7b 08 00 00 00 00 00 00 00 00 00 00 00  L........... ..{4..{............
> 00000020: 40 9a ff ff ff ff ff ff 52 55 0a 00 02 02 08 00 45 10 02 40 00 00 00 00 40 11 6c 9c 0a 00 02 02  @.......RU......E..@....@.l.....
>                                                                 ^^^^^ ip length (576)
> 00000040: ff ff ff ff 00 43 00 44 02 2c 06 18 02 01 06 00 e6 fd ce 5b 00 00 00 00 00 00 00 00 0a 00 02 0f  .....C.D.,.........[............
>                       ^^ udp header
>                                   ^^^^^ UDP length (556)
>                                               ^^ start of UDP data
> 00000060: 0a 00 02 02 00 00 00 00 52 54 00 12 34 56 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ........RT..4V..................
> 00000080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000000a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000000c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000000e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 00000100: 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00  ................................
> 00000120: b8 00 00 00 00 4d f9 8f 70 01 00 00 ea 00 00 00 50 e5 74 64 88 e0 0a 00 88 e0 4a 00 88 e0 4a 00  .....M..p.......P.td......J...J.
> 00000140: 2c 00 00 00 2c 00 00 00 04 00 00 00 04 00 00 00 51 e5 74 64 00 00 00 00 00 00 00 00 00 00 00 00  ,...,...........Q.td............
> 00000160: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 00000180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 000001e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................................
> 00000200: 00 00 00 00 00 00 00 00 06 00 01 00 54 cc a4 8c 08 00 00 00 02 00 00 00 00 00 00 00 01 00 00 00  ............T...................
> 00000220: 08 00 00 00 00 00 00 00 14 cd a4 8c                                                              ............