From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: AW: AW: PROBLEM: Kernel Oops in UDP stack
Date: Wed, 1 Aug 2018 03:49:48 -0700
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Cc: "'netdev@vger.kernel.org'" <netdev@vger.kernel.org>,
        "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
        Matthias Wystrik <mwystrik@mut-group.com>
To: Paolo Abeni <pabeni@redhat.com>,
        Marcel Hellwig <mhellwig@mut-group.com>,
        'Eric Dumazet' <eric.dumazet@gmail.com>,
        "'davem@davemloft.net'" <davem@davemloft.net>,
        "'kuznet@ms2.inr.ac.ru'" <kuznet@ms2.inr.ac.ru>,
        "'yoshfuji@linux-ipv6.org'" <yoshfuji@linux-ipv6.org>,
        "'andrew@lunn.ch'" <andrew@lunn.ch>
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On 08/01/2018 03:44 AM, Paolo Abeni wrote:
> On Wed, 2018-08-01 at 10:35 +0000, Marcel Hellwig wrote:
>>>> [<c0228adc>] (udp_recvmsg+0x284/0x33c) from [<c02306e0>] (inet_recvmsg+0x38/0x4c): net/ipv4/udp.c:1234
>>>
>>>              sin->sin_addr.s_addr = ip_hdr(skb)->saddr;
>>>
>>> Unaligned access trap (virtual address c14fe63a), so either sin or ip_hdr(skb) are not on a 32bit alignment
>>>
>>> Can you produce the disassembly of the trapping instruction ?
>>
>> https://gist.github.com/hellow554/6b11c6c0827d5db80a7e66f71f5636ff#file-net_uipv4_udp-lst-L1892-L1895
>>
>> 		sin->sin_addr.s_addr = ip_hdr(skb)->saddr;
>> c0228ad8:	e5943080 	ldr	r3, [r4, #128]	; 0x80
>> c0228adc:	e593300c 	ldr	r3, [r3, #12]
>> c0228ae0: 	e5823004	str	r3, [r2, #4]
> 
> I *think* pskb_trim_rcsum() in __udp4_lib_rcv() can copy the ipv4
> header to an unaligned address, for cloned skbs. If I understood
> correctly the relevant socket is a mcast one, so cloned skbs can land
> there.
> 

kmalloc() should return aligned pointer.

pskb_expand_head() should allocate aligned skb->head

So pskb_expand_head() should keep whatever offset was provided in the source skb 

( Driver called skb_reserve() or similar function)