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

On 1/14/21 3:36 PM, Jesper Dangaard Brouer wrote:
[...]

quoted

quoted

+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) {

[...]

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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.

quoted

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.