Subject: Re: [PATCH bpf-next V8 4/8] bpf: add BPF-helper for MTU checking

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 14 Dec 2020 14:51:28 +0100

On Thu, 3 Dec 2020 00:23:14 +0100
Daniel Borkmann <[email-masked]> wrote:

> On 11/27/20 7:06 PM, Jesper Dangaard Brouer wrote:
> [...]
> > +static struct net_device *__dev_via_ifindex(struct net_device *dev_curr,
> > +					    u32 ifindex)
> > +{
> > +	struct net *netns = dev_net(dev_curr);
> > +
> > +	/* Non-redirect use-cases can use ifindex=0 and save ifindex lookup */
> > +	if (ifindex == 0)
> > +		return dev_curr;
> > +
> > +	return dev_get_by_index_rcu(netns, ifindex);
> > +}
> > +
> > +BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
> > +	   u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
> > +{
> > +	int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> > +	struct net_device *dev = skb->dev;
> > +	int len;
> > +	int mtu;
> > +
> > +	if (flags & ~(BPF_MTU_CHK_SEGS))  
> 
> nit: unlikely() (similar for XDP case)

ok

> > +		return -EINVAL;
> > +
> > +	dev = __dev_via_ifindex(dev, ifindex);
> > +	if (!dev)  
> 
> nit: unlikely() (ditto XDP)

ok

> > +		return -ENODEV;
> > +
> > +	mtu = READ_ONCE(dev->mtu);
> > +
> > +	/* TC len is L2, remove L2-header as dev MTU is L3 size */
> > +	len = skb->len - ETH_HLEN;  
> 
> s/ETH_HLEN/dev->hard_header_len/ ?

ok
 
> > +	len += len_diff; /* len_diff can be negative, minus result pass check */
> > +	if (len <= mtu) {
> > +		ret = BPF_MTU_CHK_RET_SUCCESS;  
> 
> Wouldn't it be more intuitive to do ...
> 
>     len_dev = READ_ONCE(dev->mtu) + dev->hard_header_len + VLAN_HLEN;
>     len_skb = skb->len + len_diff;
>     if (len_skb <= len_dev) {
>        ret = BPF_MTU_CHK_RET_SUCCESS;
>        got out;
>     }

Yes, that is more intuitive to read.

> > +		goto out;
> > +	}
> > +	/* At this point, skb->len exceed MTU, but as it include length of all
> > +	 * segments, it can still be below MTU.  The SKB can possibly get
> > +	 * re-segmented in transmit path (see validate_xmit_skb).  Thus, user
> > +	 * must choose if segs are to be MTU checked.  Last SKB "headlen" is
> > +	 * checked against MTU.
> > +	 */
> > +	if (skb_is_gso(skb)) {
> > +		ret = BPF_MTU_CHK_RET_SUCCESS;
> > +
> > +		if (flags & BPF_MTU_CHK_SEGS &&
> > +		    skb_gso_validate_network_len(skb, mtu)) {
> > +			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
> > +			goto out;  
> 
> Maybe my lack of coffe, but looking at ip_exceeds_mtu() for example, shouldn't
> the above test be on !skb_gso_validate_network_len() instead?

Yes, you are right!

> skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu) would indicate that
> it does /not/ exceed mtu.
> 
> > +		}
> > +
> > +		len = skb_headlen(skb) - ETH_HLEN + len_diff;  
> 
> How does this work with GRO when we invoke this helper at tc ingress, e.g. when
> there is still non-linear data in skb_shinfo(skb)->frags[]?

In case of skb_is_gso() then this code will check the linear part
skb_headlen(skb) against the MTU.  I though this was an improvement
from what we have today, where skb_is_gso() packets will skip all
checks, which have caused a lot of confusion by end-users.

I will put this under the BPF_MTU_CHK_SEGS flag (in V9) as I understand
from you comment, you don't think this is correct at tc ingress.

> > +		if (len > mtu) {
> > +			ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> > +			goto out;
> > +		}
> > +	}
> > +out:
> > +	/* BPF verifier guarantees valid pointer */
> > +	*mtu_len = mtu;
> > +
> > +	return ret;
> > +}
[...]

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