netdev - Re: [PATCH net 1/2] net: avoid errors when trying to pop MLPS header on non-MPLS packets

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Date:   Fri, 11 Oct 2019 09:34:08 +0200
From:   Simon Horman <simon.horman@...ronome.com>
To:     Davide Caratti <dcaratti@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        John Hurley <john.hurley@...ronome.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net 1/2] net: avoid errors when trying to pop MLPS header
 on non-MPLS packets

On Thu, Oct 10, 2019 at 08:43:52PM +0200, Davide Caratti wrote:
> the following script:
> 
>  # tc qdisc add dev eth0 clsact
>  # tc filter add dev eth0 egress matchall action mpls pop
> 
> implicitly makes the kernel drop all packets transmitted by eth0, if they
> don't have a MPLS header. This behavior is uncommon: other encapsulations
> (like VLAN) just let the packet pass unmodified. Since the result of MPLS
> 'pop' operation would be the same regardless of the presence / absence of
> MPLS header(s) in the original packet, we can let skb_mpls_pop() return 0
> when dealing with non-MPLS packets.
> 
> Fixes: 2a2ea50870ba ("net: sched: add mpls manipulation actions to TC")
> Signed-off-by: Davide Caratti <dcaratti@...hat.com>

Hi Davide,

For the TC use-case I think this is correct for the reasons you explain
above.

For the OVS use-case I also think it is fine because
__ovs_nla_copy_actions() will ensure that MPLS POP only occurs
for packets with an MPLS Ethernet protocol. That is, this condition
should never occur in that use-case.

And it appears that there are no other users of this function.

I think it might be worth adding something about use-cases other than TC
to the changelog, but that aside:

Reviewed-by: Simon Horman <simon.horman@...ronome.com>

> ---
>  net/core/skbuff.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 529133611ea2..cd59ccd6da57 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5536,7 +5536,7 @@ int skb_mpls_pop(struct sk_buff *skb, __be16 next_proto)
>  	int err;
>  
>  	if (unlikely(!eth_p_mpls(skb->protocol)))
> -		return -EINVAL;
> +		return 0;
>  
>  	err = skb_ensure_writable(skb, skb->mac_len + MPLS_HLEN);
>  	if (unlikely(err))
> -- 
> 2.21.0
>