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

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Date:	Thu, 20 Dec 2012 17:50:46 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, piergiorgio.beruto@...il.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] af_unix: MSG_TRUNC support for dgram sockets

On Wed, Feb 22, 2012 at 10:24 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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>

Patch below to man-pages applied.

Thanks for CCing me, Eric.

Cheers,

Michael

--- a/man2/recv.2
+++ b/man2/recv.2
@@ -264,7 +264,7 @@ subsequent receive call will return the same data.
 For raw
 .RB ( AF_PACKET ),
 Internet datagram (since Linux 2.4.27/2.6.8),
-and netlink (since Linux 2.6.22) sockets:
+netlink (since Linux 2.6.22) and UNIX datagram (since Linux 3.4) sockets:
 return the real length of the packet or datagram,
 even when it was longer than the passed buffer.
 Not implemented for UNIX domain
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