netdev - Re: [PATCH net-next] mpls: drop skb's dst in mpls

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Message-ID: <20201103125637.37838cf7@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 3 Nov 2020 12:56:37 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Martin Varghese <martin.varghese@...ia.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        David Ahern <dsa@...ulusnetworks.com>
Subject: Re: [PATCH net-next] mpls: drop skb's dst in mpls_forward()

On Sat, 31 Oct 2020 01:07:25 +0100 Guillaume Nault wrote:
> Commit 394de110a733 ("net: Added pointer check for
> dst->ops->neigh_lookup in dst_neigh_lookup_skb") added a test in
> dst_neigh_lookup_skb() to avoid a NULL pointer dereference. The root
> cause was the MPLS forwarding code, which doesn't call skb_dst_drop()
> on incoming packets. That is, if the packet is received from a
> collect_md device, it has a metadata_dst attached to it that doesn't
> implement any dst_ops function.
> 
> To align the MPLS behaviour with IPv4 and IPv6, let's drop the dst in
> mpls_forward(). This way, dst_neigh_lookup_skb() doesn't need to test
> ->neigh_lookup any more. Let's keep a WARN condition though, to  
> document the precondition and to ease detection of such problems in the
> future.
> 
> Signed-off-by: Guillaume Nault <gnault@...hat.com>

Applied, thanks!