netdev - [PATCH net-next] af_unix: MSG_TRUNC support for dgram sockets

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Date:	Wed, 22 Feb 2012 10:24:55 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	piergiorgio.beruto@...il.com, netdev@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: [PATCH net-next] af_unix: MSG_TRUNC support for dgram sockets

Piergiorgio Beruto expressed the need to fetch size of first datagram in
queue for AF_UNIX sockets and suggested a patch against SIOCINQ ioctl.

I suggested instead to implement MSG_TRUNC support as a recv() input
flag, as already done for RAW, UDP & NETLINK sockets.

len = recv(fd, &byte, 1, MSG_PEEK | MSG_TRUNC);

MSG_TRUNC asks recv() to return the real length of the packet, even when
is was longer than the passed buffer.

There is risk that a userland application used MSG_TRUNC by accident
(since it had no effect on af_unix sockets) and this might break after
this patch.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Tested-by: Piergiorgio Beruto <piergiorgio.beruto@...il.com>
CC: Michael Kerrisk <mtk.manpages@...il.com>
---
 net/unix/af_unix.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0be4d24..8ee85aa 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1845,7 +1845,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		if (UNIXCB(skb).fp)
 			siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
 	}
-	err = size;
+	err = (flags & MSG_TRUNC) ? skb->len - skip : size;

 	scm_recv(sock, msg, siocb->scm, flags);

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