On Thu, 14 Jan 2021 23:28:57 +0100
Daniel Borkmann <[email protected]> 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
