On 2/2/21 5:26 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.
V13:
- Only do ifindex lookup one time, calling dev_get_by_index_rcu().
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>
Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
[...]
I was about to apply the series just now, but on a last double check there is
a subtle logic bug in here that still needs fixing unfortunately. :/ See below:
@@ -5568,7 +5565,9 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
struct bpf_fib_lookup *, params, int, plen, u32, flags)
{
struct net *net = dev_net(skb->dev);
+ struct net_device *dev;
int rc = -EAFNOSUPPORT;
+ bool check_mtu = false;
if (plen < sizeof(*params))
return -EINVAL;
@@ -5576,23 +5575,30 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
return -EINVAL;
+ dev = dev_get_by_index_rcu(net, params->ifindex);
+ if (unlikely(!dev))
+ return -ENODEV;
Based on your earlier idea, you try to avoid refetching the dev this way, so
here it's being looked up via params->ifindex provided from the BPF prog ...
+ 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, dev, params, flags, check_mtu);
... however, bpf_ipv{4,6}_fib_lookup() might change params->ifindex here to
indicate nexthop output dev:
[...]
dev = nhc->nhc_dev;
params->rt_metric = res.fi->fib_priority;
params->ifindex = dev->ifindex;
[...]
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, dev, params, flags, check_mtu);
break;
#endif
}
- if (!rc) {
- struct net_device *dev;
-
- dev = dev_get_by_index_rcu(net, params->ifindex);
+ if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
+ /* When tot_len isn't provided by user,
+ * check skb against net_device MTU
+ */
if (!is_skb_forwardable(dev, skb))
rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
... so using old cached dev from above will result in wrong MTU check &
subsequent passing of wrong params->mtu_result = dev->mtu this way. So one
way to fix is that we would need to pass &dev to bpf_ipv{4,6}_fib_lookup().
}
Thanks,
Daniel
