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Before this patch, transport offset (pkt->thoff) provides an offset
relative to the network header. This is fine for the inet families
because skb->data points to the network header in such case. However,
from netdev/egress, skb->data points to the mac header (if available),
thus, pkt->thoff is missing the mac header length.
Add skb_network_offset() to the transport offset (pkt->thoff) for
netdev, so transport header mangling works as expected. Adjust payload
fast eval function to use skb->data now that pkt->thoff provides an
absolute offset. This explains why users report that matching on
egress/netdev works but payload mangling does not.
This patch implicitly fixes payload mangling for IPv4 packets in
netdev/egress given skb_store_bits() requires an offset from skb->data
to reach the transport header.
I suspect that nft_exthdr and the trace infra were also broken from
netdev/egress because they also take skb->data as start, and pkt->thoff
was not correct.
Note that IPv6 is fine because ipv6_find_hdr() already provides a
transport offset starting from skb->data, which includes
skb_network_offset().
The bridge family also uses nft_set_pktinfo_ipv4_validate(), but there
skb_network_offset() is zero, so the update in this patch does not alter
the existing behaviour.
Fixes: 42df6e1d22 ("netfilter: Introduce egress hook")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
0ae8e4cca7