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

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Message-ID: <20180607174923.13a11d08@cakuba.netronome.com>
Date:   Thu, 7 Jun 2018 17:49:23 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>, dsahern@...il.com,
        stephen@...workplumber.org, netdev@...r.kernel.org
Subject: Re: [Bug 199643] New: UBSAN: Undefined behaviour in
 ./include/net/route.h:240:2

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?