netdev - Re: [Bug 199643] New: UBSAN: Undefined behaviour in ./include/net/route.h:240:2

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Date:   Thu, 7 Jun 2018 17:53:35 -0700
From:   David Ahern <dsahern@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>, stephen@...workplumber.org,
        netdev@...r.kernel.org
Subject: Re: [Bug 199643] New: UBSAN: Undefined behaviour in
 ./include/net/route.h:240:2

On 6/7/18 5:49 PM, Jakub Kicinski wrote:
> On Thu, 7 Jun 2018 17:28:59 -0700, Eric Dumazet wrote:
>> On 06/07/2018 05:11 PM, David Miller wrote:
>>> From: Jakub Kicinski <jakub.kicinski@...ronome.com>
>>> Date: Thu, 7 Jun 2018 17:06:23 -0700
>>>   
>>>> [  293.213661]  ip_send_unicast_reply+0x1b67/0x1d0e  
>>>
>>> This calls ip_setup_cork() which can NULL out the 'rt' route
>>> pointer.  Hmmm... :-/
>>>   
>>
>>
>> UBSAN seems unhappy  with dst being NULL in :
>>
>> dst_release(&rt->dst);
>>
>> But the code obviously is ready for dst being NULL, it is even documented :)
> 
> Oh, so the code depends on dst being the first member?  Would it make
> sense to just cast the pointer instead?
> 

I've been going the other way with 'rt to dst' and 'dst to rt'
transformations.

Perhaps UBSAN should be updated to understand that NULL + 0 is ok.