linux-kernel - Re: [PATCH] net: Reset skb to network header in neigh_hh

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Date:   Tue, 25 Oct 2016 17:12:22 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Abdelrhman Ahmed <ab@...hmed.com>
Cc:     davem <davem@...emloft.net>, netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: Reset skb to network header in neigh_hh_output

On Wed, 2016-10-26 at 01:57 +0200, Abdelrhman Ahmed wrote:
>  > What is the issue you want to fix exactly ? 
>  > Please describe the use case. 
> 
> When netfilter hook uses skb_push to add a specific header between network
> header and hardware header.
> For the first time(s) before caching hardware header, this header will be
> removed / overwritten by hardware header due to resetting to network header.
> After using the cached hardware header, this header will be kept as we do not
> reset. I think this behavior is inconsistent, so we need to reset in both cases.
> 
>  > Otherwise, your fix is in fact adding a critical bug. 
> 
> Could you explain more as it's not clear to me?
> 

Maybe my wording was not good here.

What I intended to say is that the 
__skb_pull(skb, skb_network_offset(skb)) might not be at the right
place.

Look at commit e1f165032c8bade3a6bdf546f8faf61fda4dd01c to find the
reason.


> 
> 
>  ---- On Fri, 07 Oct 2016 23:10:56 +0200 Eric Dumazet <eric.dumazet@...il.com> wrote ---- 
>  > On Fri, 2016-10-07 at 16:14 +0200, Abdelrhman Ahmed wrote: 
>  > > When hardware header is added without using cached one, neigh_resolve_output 
>  > > and neigh_connected_output reset skb to network header before adding it. 
>  > > When cached one is used, neigh_hh_output does not reset the skb to network 
>  > > header. 
>  > >  
>  > > The fix is to reset skb to network header before adding cached hardware header 
>  > > to keep the behavior consistent in all cases. 
>  >  
>  > What is the issue you want to fix exactly ? 
>  >  
>  > Please describe the use case. 
>  >  
>  > I highly suggest you take a look at commit 
>  >  
>  > e1f165032c8bade3a6bdf546f8faf61fda4dd01c 
>  > ("net: Fix skb_under_panic oops in neigh_resolve_output") 
>  >  
>  > Otherwise, your fix is in fact adding a critical bug. 
>  >  
>  >  
>  > 
>