On Sat, 23 Jan 2021 02:47:28 +0100
Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> On 1/18/21 5:54 PM, Jesper Dangaard Brouer wrote:
> > BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
> > bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
> > by adjusting fib_params 'tot_len' with the packet length plus the expected
> > encap size. (Just like the bpf_check_mtu helper supports). He discovered
> > that for SKB ctx the param->tot_len was not used, instead skb->len was used
> > (via MTU check in is_skb_forwardable() that checks against netdev MTU).
> >
> > Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
> > zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
> > check is done like XDP code-path, which checks against FIB-dst MTU.
> >
> > V10:
> > - Use same method as XDP for 'tot_len' MTU check
> >
> > Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
> > Reported-by: Carlo Carraro <colrack@xxxxxxxxx>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > ---
> > net/core/filter.c | 13 ++++++++++---
> > 1 file changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 5beadd659091..d5e6f395cf64 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -5569,6 +5569,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
> > {
> > struct net *net = dev_net(skb->dev);
> > int rc = -EAFNOSUPPORT;
> > + bool check_mtu = false;
> >
> > if (plen < sizeof(*params))
> > return -EINVAL;
> > @@ -5576,22 +5577,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
> > if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
> > return -EINVAL;
> >
> > + if (params->tot_len)
> > + check_mtu = true;
> > +
> > switch (params->family) {
> > #if IS_ENABLED(CONFIG_INET)
> > case AF_INET:
> > - rc = bpf_ipv4_fib_lookup(net, params, flags, false);
> > + rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);
> > break;
> > #endif
> > #if IS_ENABLED(CONFIG_IPV6)
> > case AF_INET6:
> > - rc = bpf_ipv6_fib_lookup(net, params, flags, false);
> > + rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);
> > break;
> > #endif
> > }
> >
> > - if (!rc) {
> > + if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
> > struct net_device *dev;
> >
> > + /* When tot_len isn't provided by user,
> > + * check skb against net_device MTU
> > + */
> > dev = dev_get_by_index_rcu(net, params->ifindex);
> > if (!is_skb_forwardable(dev, skb))
> > rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
>
> Btw, looking at some of the old feedback, looks like [0] got missed somehow. Would
> be nice if we could simplify this rather ugly bit with refetching dev for tc.
>
> [0] https://lore.kernel.org/bpf/f959017b-5d3c-5cdb-a016-c467a3c9a2fc@xxxxxxxxxxxxx/
> https://lore.kernel.org/bpf/f8ff26f0-b1b6-6dd1-738d-4c592a8efdb0@xxxxxxxxx/
I have tried to incorporate the ideas from [0]. If you notice this
code path is only called in-case params->tot_len is zero (!check_mtu).
Then if params->tot_len does contain something, then the check_mtu code
path of bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup().
I agree, that it is ugly refetching dev. I'll look at moving the
lookup dev_get_by_index_rcu() call one level-up, as both
bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup() does this call. (I
cannot move the check into the functions due skb is not avail in XDP
case)
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
