[PATCH net-next 2/2] udp: do not expect udp headers in recv cmsg IP_CMSG_CHECKSUM

[Willem de Bruijn]

From: Willem de Bruijn <will...@google.com>

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 <will...@google.com>
---
 net/ipv4/ip_sockglue.c | 3 ++-
 net/ipv4/udp.c         | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

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 @@ static void ip_cmsg_recv_checksum(struct msghdr *msg, 
struct sk_buff *skb,
                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);
 }
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index d2d294b..f186313 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1375,7 +1375,7 @@ try_again:
                *addr_len = sizeof(*sin);
        }
        if (inet->cmsg_flags)
-               ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr));
+               ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr) + off);
 
        err = copied;
        if (flags & MSG_TRUNC)
-- 
2.8.0.rc3.226.g39d4020