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Subject: Re: AW: AW: PROBLEM: Kernel Oops in UDP stack
To:     Marcel Hellwig <mhellwig@mut-group.com>,
        'Eric Dumazet' <eric.dumazet@gmail.com>,
        David Laight <David.Laight@ACULAB.COM>,
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        "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
        Matthias Wystrik <mwystrik@mut-group.com>
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From:   Eric Dumazet <eric.dumazet@gmail.com>
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Date:   Thu, 2 Aug 2018 08:07:11 -0700
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On 08/02/2018 06:57 AM, Marcel Hellwig wrote:
>>> There are actually 2 faults, difficult to quickly sort out the merged tracebacks.
>>> You are also running a rather old kernel: Linux version 3.4.113.
>>>
>>> It may well be that whichever ethernet driver generated the misaligned 
>>> frame has since been fixed.
>>
>> A misalign frame driver problem would have faulted earlier in IP stack, much before we perform the copy to user space in udp_recvmsg()
>>
> 
> JFYI: we are talking about the lpc_eth driver[0] #57c10b6 , which is not the newest, but all newer did not fix a major problem (at least the commit messages are not screaming: WARNING, UNALIGNED MEMORY!). Is there a diagram/document how a ip packet travels down the code? From the MAC/phy driver to udp_recvmsg? It's not that obvious for me, but maybe it is something I can work with.
> 
> 
> [0]: https://elixir.bootlin.com/linux/v3.4.113/source/drivers/net/ethernet/nxp/lpc_eth.c
> 
> Regards,
> Marcel
> 


Well, this driver does not use NET_IP_ALIGN reservation, meaning IP header is not 4-byte aligned.

No idea why mis-alignments are okay in IP layer, but not in UDP

You could try to patch it to use netdev_alloc_skb_ip_align() instead of dev_alloc_skb()