[PATCH] udp: change MSG_TRUNC return behaviour for MSG_PEEK in recvmsg

[Torin Carey <torin@tcarey.uk>]

Make UDP recvmsg only return the MSG_TRUNC flag if the read does not
copy the tail end of the datagram.  Specifically, this targets MSG_PEEK
when we're using a positive peek offset.

The current behaviour means that if we have a positive peek offset `off`
and we're reading `r` bytes from a datagram of `ulen` length, we respond
with MSG_TRUNC if and only if `r <= ulen - off`.  This is odd behaviour
as we return MSG_TRUNC if the user requests exactly `ulen - off` which
has no truncation.

The behaviour could be corrected in two ways:

This patch returns MSG_TRUNC only for tail-end truncation and not head
truncation.  This is more consistent with recv(2):
> MSG_TRUNC
>     indicates that the trailing portion of a datagram was discarded
>     because the datagram was larger than the buffer supplied.
although this isn't written with SO_PEEK_OFF in mind.

The second option is to always return MSG_TRUNC if `off > 0` like the
man-pages socket(7) page states:
> For datagram sockets, if the "peek offset" points to the middle of a
> packet, the data returned will be marked with the MSG_TRUNC flag.

Signed-off-by: Torin Carey <torin@tcarey.uk>
---
 net/ipv4/udp.c | 2 +-
 net/ipv6/udp.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 319dd7bbfe33..e57740a2c308 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1855,7 +1855,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
 	copied = len;
 	if (copied > ulen - off)
 		copied = ulen - off;
-	else if (copied < ulen)
+	else if (copied < ulen - off)
 		msg->msg_flags |= MSG_TRUNC;

 	/*
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 14a94cddcf0b..d6c0eed94564 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -348,7 +348,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	copied = len;
 	if (copied > ulen - off)
 		copied = ulen - off;
-	else if (copied < ulen)
+	else if (copied < ulen - off)
 		msg->msg_flags |= MSG_TRUNC;

 	is_udp4 = (skb->protocol == htons(ETH_P_IP));
--
2.34.1