netdev - Re: [PATCH net v3] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may

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Message-ID: <aW7FvS8wE2zNDDZ2@strlen.de>
Date: Tue, 20 Jan 2026 01:01:01 +0100
From: Florian Westphal <fw@...len.de>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	pabeni@...hat.com,
	syzbot+6023ea32e206eef7920a@...kaller.appspotmail.com,
	davem@...emloft.net, Mazin Al Haddad <mazin@...state.dev>
Subject: Re: [PATCH net v3] ip6_gre: use skb_vlan_inet_prepare() instead of
 pskb_inet_may_pull()

Eric Dumazet <edumazet@...gle.com> wrote:
> On Mon, Jan 19, 2026 at 6:06 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Mon, 19 Jan 2026 12:24:57 +0100 Florian Westphal wrote:
> > > From: Eric Dumazet <edumazet@...gle.com>
> > >
> > > I added skb_vlan_inet_prepare() helper in the cited commit, hinting
> > > that we would need to use it more broadly.
> >
> > I _think_ this makes GRE forwarding tests a bit unhappy:
> >
> > https://netdev.bots.linux.dev/contest.html?branch=net-next-2026-01-19--12-00&executor=vmksft-forwarding&pw-n=0&pass=0
> > --
> 
> I was unsure about ip6erspan_tunnel_xmit() change, I think I started
> full tests days ago but probably was distracted.
> 
> I had :
> 
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index d19d86ed43766bbc8ec052113be02ab231a5272c..9e214c355e6ce15fa828866ae20fa8fe321b4bf7
> 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -881,7 +881,7 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
>         __be16 payload_protocol;
>         int ret;
> 
> -       if (!pskb_inet_may_pull(skb))
> +       if (skb_vlan_inet_prepare(skb, true))
>                 goto tx_err;

It has to be either true or false depending on test case 8-/

gre_gso.sh needs this to be set to true, skbs don't have a mac
header: with "false": skb nhoff gets munged from 0 to 14.

But in mirror_gre.sh test case, skbs do have a mac header:
"true" munges nh offset from 14 to 0 and test fails.