The packet decoder in our Cilium Monitor utility never checks for errors
returned from the DecodeLayers function. In Hubble, we do have identical
code, but with the additional checks on that returned error code.
We should try to keep these tool utilities as aligned as possible to avoid
misunderstanding: in case of a decode error, Cilium Monitor would still
try to use the decoded info.

Since v1.1.18, DecodeLayers returns a non-nil error for an empty packet.
Skip decoding and truncate layers to avoid accidental re-use.
See google/gopacket#846

Signed-off-by: Simone Magnani <[email protected]>

This patch enabled the correct decoding of trace notifications when
emitted from an L3 device (ex. Wireguard). Prior to this patch, our
Monitor/Hubble logic always attempted to decode the raw bytes starting
from the Ethernet Layer. This of course doesn't hold true for L3 devices,
where we never see the L2 layer.

Since we already have a flag field in the trace notification with some
unused padding, let's borrow a bit to indicate whether the packet is being
logged from an L3 device or not. This could have been also achieved from
Monitor/Hubble directly, since the notification even has the ifindex, but
it would have required to add logic to keep ifindexes related to L3
interfaces up-to-date in Hubble/Monitor. IMHO, (1) this might not scale
well, and (2) it is much clearer to have all info forwarded from the
datapath. I'm not concerned about performance implication here since
no new data is being forwarded in the trace event.

Signed-off-by: Simone Magnani <[email protected]>

Similarly as the previous commit, this patch enabled the correct decoding
of drop notifications when emitted from an L3 device (ex. Wireguard).

Since we don't have a flag field or padding in drop notification struct,
let's increment the version and add a `__u32` field to hold flags.
For now, we use 1 bit to indicate IPv6 and 1 bit to indicate whether the
packet is being dropped from a L3 Device. The other 30 bits are kept as
padding the preserve the struct alignment.

Signed-off-by: Simone Magnani <[email protected]>