Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Wed, 18 Nov 2020 16:29:35 +0100
Jesper Dangaard Brouer <brouer@redhat.com> wrote:

> BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
> bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
> by adjusting fib_params 'tot_len' with the packet length plus the
> expected encap size. (Just like the bpf_check_mtu helper supports). He
> discovered that for SKB ctx the param->tot_len was not used, instead
> skb->len was used (via MTU check in is_skb_forwardable()).
> 
> Fix this by using fib_params 'tot_len' for MTU check.  If not provided
> (e.g. zero) then keep existing behaviour intact.

Carlo pointed out (in slack) that the logic is not correctly
implemented in this patch.

I will send a V7.


> Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
> Reported-by: Carlo Carraro <colrack@gmail.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  net/core/filter.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 1ee97fdeea64..ae1fe8e6069a 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
>  
>  	if (!rc) {
>  		struct net_device *dev;
> +		u32 mtu;
>  
>  		dev = dev_get_by_index_rcu(net, params->ifindex);
> -		if (!is_skb_forwardable(dev, skb))
> +		mtu = dev->mtu;
> +
> +		/* Using tot_len for L3 MTU check if provided by user. Notice at
> +		 * this TC cls_bpf level skb->len contains L2 size, but
> +		 * is_skb_forwardable takes that into account.
> +		 */
> +		if (params->tot_len > mtu) {
>  			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> +		} else if (!is_skb_forwardable(dev, skb)) {
> +			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> +		}
>  	}
>  
>  	return rc;

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer