Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

Hi

I report here the issue with the previous patch.
The code is now checking against params->tot_len but then it is still
using is_skb_forwardable.
Consider this case where I shrink the packet:
skb->len == 1520
dev->mtu == 1500
params->tot_len == 1480
So the incoming pkt has len 1520, and the out interface has mtu 1500.
In this case fragmentation is not needed because params->tot_len < dev->mtu.
However the code calls is_skb_forwardable and may return false because
skb->len > dev->mtu, resulting in BPF_FIB_LKUP_RET_FRAG_NEEDED.
What I propose is using params->tot_len only if provided, without
falling back to use is_skb_forwardable when provided.
Something like this:

if (params->tot_len > 0) {
  if (params->tot_len > mtu)
    rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
} else if (!is_skb_forwardable(dev, skb)) {
  rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
}

However, doing so we are skipping more relaxed MTU checks inside
is_skb_forwardable, so I'm not sure about this.
Please comment

Il giorno ven 20 nov 2020 alle ore 09:26 Jesper Dangaard Brouer
[off-list ref] ha scritto:

On Wed, 18 Nov 2020 16:29:35 +0100
Jesper Dangaard Brouer [off-list ref] wrote:

quoted

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()).

Fix this by using fib_params 'tot_len' for MTU check.  If not provided
(e.g. zero) then keep existing behaviour intact.

Carlo pointed out (in slack) that the logic is not correctly
implemented in this patch.

I will send a V7.

quoted

Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
Reported-by: Carlo Carraro <redacted>
Signed-off-by: Jesper Dangaard Brouer <redacted>
---
 net/core/filter.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 1ee97fdeea64..ae1fe8e6069a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

      if (!rc) {
              struct net_device *dev;
+             u32 mtu;

              dev = dev_get_by_index_rcu(net, params->ifindex);
-             if (!is_skb_forwardable(dev, skb))
+             mtu = dev->mtu;
+
+             /* Using tot_len for L3 MTU check if provided by user. Notice at
+              * this TC cls_bpf level skb->len contains L2 size, but
+              * is_skb_forwardable takes that into account.
+              */
+             if (params->tot_len > mtu) {
                      rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+             } else if (!is_skb_forwardable(dev, skb)) {
+                     rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+             }
      }

      return rc;

--
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer