udp: do not expect udp headers in recv cmsg IP_CMSG_CHECKSUM
On udp sockets, recv cmsg IP_CMSG_CHECKSUM returns a checksum over
the packet payload. Since commit e6afc8ace6dd pulled the headers,
taking skb->data as the start of transport header is incorrect. Use
the transport header pointer.
Also, when peeking at an offset from the start of the packet, only
return a checksum from the start of the peeked data. Note that the
cmsg does not subtract a tail checkum when reading truncated data.
Fixes: e6afc8ace6dd ("udp: remove headers from UDP packets before queueing")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 89b5f3b..279471c 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -106,7 +106,8 @@
return;
if (offset != 0)
- csum = csum_sub(csum, csum_partial(skb->data, offset, 0));
+ csum = csum_sub(csum, csum_partial(skb_transport_header(skb),
+ offset, 0));
put_cmsg(msg, SOL_IP, IP_CHECKSUM, sizeof(__wsum), &csum);
}