netdev - Re: Linux 5.12-rc7

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Date:   Mon, 12 Apr 2021 10:31:17 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Eric Dumazet <edumazet@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Linux 5.12-rc7

On 4/12/21 9:31 AM, Eric Dumazet wrote:
> On Mon, Apr 12, 2021 at 6:28 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> On Sun, Apr 11, 2021 at 10:14 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>> Qemu test results:
>>>         total: 460 pass: 459 fail: 1
>>> Failed tests:
>>>         sh:rts7751r2dplus_defconfig:ata:net,virtio-net:rootfs
>>>
>>> The failure bisects to commit 0f6925b3e8da ("virtio_net: Do not pull payload in
>>> skb->head"). It is a spurious problem - the test passes roughly every other
>>> time. When the failure is seen, udhcpc fails to get an IP address and aborts
>>> with SIGTERM. So far I have only seen this with the "sh" architecture.
>>
>> Hmm. Let's add in some more of the people involved in that commit, and
>> also netdev.
>>
>> Nothing in there looks like it should have any interaction with
>> architecture, so that "it happens on sh" sounds odd, but maybe it's
>> some particular interaction with the qemu environment.
> 
> Yes, maybe.
> 
> I spent few hours on this, and suspect a buggy memcpy() implementation
> on SH, but this was not conclusive.
> 

I replaced all memcpy() calls in skbuff.h with calls to

static inline void __my_memcpy(unsigned char *to, const unsigned char *from,
                               unsigned int len)
{
       while (len--)
               *to++ = *from++;
}

That made no difference, so unless you have some other memcpy() in mind that
seems to be unlikely.

> By pulling one extra byte, the problem goes away.
> 
> Strange thing is that the udhcpc process does not go past sendto().
> 

I have been trying to debug that one. Unfortunately gdb doesn't work with sh,
so I can't use it to debug the problem. I'll spend some more time on this today.

Thanks,
Guenter