On Mon, 25 Jan 2021 23:27:22 +0100
Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >>> + /* 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.
> >>> + */
> >>> + 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;
> >>
> >> I think that looks okay overall now. One thing that will easily slip through
> >> is that in the helper description you mentioned 'Check cannot use len_diff.'
> >> for BPF_MTU_CHK_SEGS flag. So right now for non-zero len_diff the user
> >> will still get BPF_MTU_CHK_RET_SUCCESS if the current length check via
> >> skb_gso_validate_network_len(skb, mtu) passes. If it cannot be checked,
> >> maybe enforce len_diff == 0 for gso skbs on BPF_MTU_CHK_SEGS?
> >
> > Ok. Do you want/think this can be enforced by the verifier or are you
> > simply requesting that the helper will return -EINVAL (or another errno)?
>
> Simple -EINVAL should be fine in this case. Generally, we can detect this from
> verifier side but I don't think the extra complexity is worth it especially given
> this is dependent on BPF_MTU_CHK_SEGS and otherwise can be non-zero.
Luckily this was also my choice in V13 that I've already send out.
https://lore.kernel.org/netdev/161159457239.321749.9067604476261493815.stgit@firesoul/
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
