Re: [PATCH] sit: reload iphdr in ipip6_rcv

[Eric Dumazet]

On Sun, 2017-06-04 at 14:43 +0800, Haishuang Yan wrote:
> Since iptunnel_pull_header() can call pskb_may_pull(),
> we must reload any pointer that was related to skb->head.
> 
> Fixes: a09a4c8dd1ec ("tunnels: Remove encapsulation offloads on decap")
> Signed-off-by: Haishuang Yan <yanhaishuang@xxxxxxxxxxxxxxxxxxxx>
> ---
>  net/ipv6/sit.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> index 61e5902..af832e7 100644
> --- a/net/ipv6/sit.c
> +++ b/net/ipv6/sit.c
> @@ -657,6 +657,7 @@ static int ipip6_rcv(struct sk_buff *skb)
>  		if (iptunnel_pull_header(skb, 0, htons(ETH_P_IPV6),
>  		    !net_eq(tunnel->net, dev_net(tunnel->dev))))
>  			goto out;
> +		iph = ip_hdr(skb);
>  
>  		err = IP_ECN_decapsulate(iph, skb);
>  		if (unlikely(err)) {

This seems unnecessary.

By the time ipip6_rcv() is called, we already have the guarantee the
IPv4 header is in skb linear part.

Otherwise we could not use iph->saddr and iph->daddr in the call to
ipip6_tunnel_lookup()

Therefore, the pskb_may_pull() is a nop in this particular case.