netdev - Re: [PATCH] IPv6: Set SIT tunnel hard_header

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Message-ID: <CA+FuTSeqv=SJ=5sXCrWWiA=nUnLvCgX4wjcoqZm93VSyJQO1jg@mail.gmail.com>
Date:   Wed, 4 Nov 2020 14:52:50 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Oliver Herms <oliver.peter.herms@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH] IPv6: Set SIT tunnel hard_header_len to zero

On Wed, Nov 4, 2020 at 2:30 PM Oliver Herms
<oliver.peter.herms@...il.com> wrote:
>
> On 03.11.20 19:42, Willem de Bruijn wrote:
> > Thanks. Yes, this is long overdue.
> >
> > The hard_header_len issue was also recently discussed in the context
> > of GRE in commit fdafed459998 ("ip_gre: set dev->hard_header_len and
> > dev->needed_headroom properly").
> >
> > Question is whether we should reserve room in needed_headroom instead.
> > AFAIK this existing update logic in ip6_tnl_xmit is sufficient
> >
> > "
> >         /* Calculate max headroom for all the headers and adjust
> >          * needed_headroom if necessary.
> >          */
> >         max_headroom = LL_RESERVED_SPACE(dst->dev) + sizeof(struct ipv6hdr)
> >                         + dst->header_len + t->hlen;
> >         if (max_headroom > dev->needed_headroom)
> >                 dev->needed_headroom = max_headroom;
> > "I think that's enough. At least it definitely works in my test and prod environment.
> Would be good to get another opinion on this though.

Sit should behave the same as other tunnels. At least in ip6_tunnel.c
and ip6_gre.c I do not see explicit initialization of needed_headroom.

> >> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
> >
> > How did you arrive at this SHA1?
> I think the legacy usage of hard_header_len in ipv6/sit.c was overseen in c54419321455.
> Please correct me if I'm wrong.

I don't see anything in that patch assign or modify hard_header_len.