Re: [PATCH -next 0/3] net: cap size to original frag size when refragmenting

[Patrick McHardy <kaber@trash.net>]

On 16.04, Herbert Xu wrote:
> David Miller <davem@davemloft.net> wrote:
> > 
> > Because then there is no ambiguity at all, you preserve on output
> > exactly what you had on input.  The same geometry, the same
> > everything.  No special checks, no max frag len, none of this crap.
> > Those are all hacks trying to work around the _fundamental_ issue
> > which is that we potentially change the thing when we refrag.
> 
> Agreed.  Doing anything other than preserving the original geometry
> is simply wrong.
> 
> However, this doesn't mean that netfilter has to process each
> fragment.  What we could do is to preserve the original fragments
> in frag_list and then process the overall skb as a unit in netfilter.
> 
> On output we simply fragment according to the original frag_list.
> 
> The only thing to watch out for is to eliminate anything in the
> middle that tries to linearise the skb.

Netfilter may change the contents of the packet, even change its size.
It is *really* hard to do this while keeping the original fragments
intact.