Re: [PATCH net 1/5] net: add eth_type_trans_careful()

Eric Dumazet wrote:

eth_type_trans() assumes ETH_HLEN bytes are present in skb->head.
NIC drivers always provide this, and they should keep using the fast
version.

Devices looping skbs from their transmit path back to their receive
path get skbs built by the stack instead. Their link-layer header
might have been popped by tc actions (act_vlan pop_eth, act_ife decode)
or OVS actions (pop_eth + pop_mpls / pop_nsh), leaving fewer than
ETH_HLEN bytes in the linear part. eth_type_trans() then hits
the BUG() in __skb_pull().

Add eth_type_trans_careful(), which first pulls the Ethernet header,
and returns a drop reason on failure, so that the few devices exposed
to this can drop such packets.

Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Reviewed-by: Willem de Bruijn <willemb@google.com>