netdev - Re: [PATCH net] virtio_net: Do not pull payload in skb->head

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Date:   Sun, 11 Apr 2021 15:07:25 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio_net: Do not pull payload in skb->head

On 4/11/21 2:43 PM, Eric Dumazet wrote:
> On Sun, Apr 11, 2021 at 11:32 PM Guenter Roeck <linux@...ck-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@...ck-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@...ck-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
> 

Another tool to install.

While that builds, output from strace:

# strace -o /tmp/STRACE  -f -s 1000 udhcpc -n -q
udhcpc: started, v1.33.0
udhcpc: sending discover
udhcpc: received SIGTERM

and:

...
136   socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP)) = 5
136   bind(5, {sa_family=AF_PACKET, sll_protocol=htons(ETH_P_IP), sll_ifindex=if_nametoindex("eth0"), sll_hatype=ARPHRD_NETROM, sll_pkttype=PACKET_HOST, sll_halen=0}, 20) = 0
136   setsockopt(5, SOL_PACKET, PACKET_AUXDATA, [1], 4) = 0
136   fcntl64(5, F_SETFD, FD_CLOEXEC)   = 0
136   socket(AF_INET, SOCK_RAW, IPPROTO_RAW) = 6
136   ioctl(6, SIOCGIFINDEX, {ifr_name="eth0", }) = 0
136   ioctl(6, SIOCGIFHWADDR, {ifr_name="eth0", ifr_hwaddr={sa_family=ARPHRD_ETHER, sa_data=52:54:00:12:34:56}}) = 0
136   close(6)                          = 0
136   clock_gettime64(CLOCK_MONOTONIC, {tv_sec=161, tv_nsec=2180242}) = 0
136   write(2, "udhcpc: sending discover\n", 25) = 25
136   socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP)) = 6
136   bind(6, {sa_family=AF_PACKET, sll_protocol=htons(ETH_P_IP), sll_ifindex=if_nametoindex("eth0"), sll_hatype=ARPHRD_NETROM, sll_pkttype=PACKET_HOST, sll_halen=6, sll_addr=[0xff, 0xff, 0xff, 0xff, 0xff, 0xff]}, 20) = 0
136   sendto(6, "E\0\1H\0\0\0\0@\21y\246\0\0\0\0\377\377\377\377\0D\0C\0014\200\256\1\1\6\0\257\321\212P\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0RT\0\0224V\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0c\202Sc5\1\1=\7\1RT\0\0224V9\2\2@7\7\1\3\6\f\17\34*<\fudhcp 1.33.0\377\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 328, 0, {sa_family=AF_PACKET, sll_protocol=htons(ETH_P_IP), sll_ifindex=if_nametoindex("eth0"), sll_hatype=ARPHRD_NETROM, sll_pkttype=PACKET_HOST, sll_halen=6, sll_addr=[0xff, 0xff, 0xff, 0xff, 0xff, 0xff]}, 20

Guenter