Re: [PATCH bpf-next V11 4/7] bpf: add BPF-helper for MTU checking

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

On Thu, 14 Jan 2021 23:28:57 +0100
Daniel Borkmann <daniel@iogearbox.net> wrote:

> On 1/14/21 3:36 PM, Jesper Dangaard Brouer wrote:
> [...]
> >>> +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 skb_len, dev_len;
> >>> +	int mtu;
> >>> +
> >>> +	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
> >>> +		return -EINVAL;
> >>> +
> >>> +	dev = __dev_via_ifindex(dev, ifindex);
> >>> +	if (unlikely(!dev))
> >>> +		return -ENODEV;
> >>> +
> >>> +	mtu = READ_ONCE(dev->mtu);
> >>> +
> >>> +	dev_len = mtu + dev->hard_header_len;
> >>> +	skb_len = skb->len + len_diff; /* minus result pass check */
> >>> +	if (skb_len <= dev_len) {
> >>> +		ret = BPF_MTU_CHK_RET_SUCCESS;
> >>> +		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))
> >>> +			goto out;
> >>> +
> >>> +		if (!skb_gso_validate_network_len(skb, mtu)) {
> >>> +			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
> >>> +			goto out;
> >>> +		}
> >>> +
> >>> +		skb_len = skb_headlen(skb) + len_diff;
> >>> +		if (skb_len > dev_len) {  
> [...]
> >> Do you have a particular use case for the BPF_MTU_CHK_SEGS?  
> > 
> > The complaint from Maze (and others) were that when skb_is_gso then all
> > the MTU checks are bypassed.  This flag enables checking the GSO part
> > via skb_gso_validate_network_len().  We cannot enable it per default,
> > as you say, it is universally correct in all cases.  
> 
> If there is a desire to have access to the skb_gso_validate_network_len(), I'd
> keep that behind the flag then, but would drop the skb_headlen(skb) + len_diff
> case given the mentioned case on rx where it would yield misleading results to
> users that might be unintuitive & hard to debug.

Okay, I will update the patch, and drop those lines.

> >> I also don't see the flag being used anywhere in your selftests, so I presume
> >> not as otherwise you would have added an example there?  
> > 
> > I'm using the flag in the bpf-examples code[1], this is how I've tested
> > the code path.
> > 
> > I've not found a way to generate GSO packet via the selftests
> > infrastructure via bpf_prog_test_run_xattr().  I'm
> > 
> > [1] https://github.com/xdp-project/bpf-examples/blob/master/MTU-tests/tc_mtu_enforce.c  
> 
> Haven't checked but likely something as prog_tests/skb_ctx.c might not be sufficient
> to pass it into the helper. For real case you might need a netns + veth setup like
> some of the other tests are doing and then generating TCP stream from one end to the
> other.

I have looked at prog_tests/skb_ctx.c and (as you say yourself) this is
not sufficient.  I can look into creating a netns+veth setup, but I
will appreciate if we can merge this patchset to make forward progress,
as I'm sure the netns+veth setup will require its own round of nitpicking.

I have created netns+veth test scripts before (see test_xdp_vlan.sh),
but my experience is that people/maintainers forget/don't to run these
separate shell scripts.  Thus, if I create a netns+veth test, then I
will prefer if I can integrate this into the "test_progs", as I know
that will be run by people/maintainers.

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