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 [off-list ref] 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. 
 >  
 >  
 >