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

[Daniel Borkmann <daniel@iogearbox.net>]

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.

>> 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.