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Date: Sat, 29 Jun 2002 00:33:33 +0200 (CEST)
From: Samuel Thibault <Samuel.Thibault@ens-lyon.fr>
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Reply-To: Samuel Thibault <samuel.thibault@fnac.net>
To: Alan.Cox@linux.org, <linux-net@vger.kernel.org>,
       <linux-kernel@vger.kernel.org>
Subject: recv(...,MSG_TRUNC)
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(Previous mail seemed to be garbaged by mime attachment)

Hello,

man recv says, about flags :

       MSG_TRUNC
              Return  the real length of the packet, even when it
              was longer than the passed buffer. Only  valid  for
              packet sockets.

But it is neither implemented in ipv4/udp.c, nor in ipv6/udp.c, although
it is in tcp.c, for instance !

By searching with google, I could read old manpages where it didn't exist,
but I find it very useful, especially in conjunction with MSG_PEEK, for
trying to read a packet with a little buffer, and then really get it with
an appropriate buffer.

So here's a patch which cures 2.4.18, and also works on 2.5 kernels.

Best regards,

Samuel Thibault

diff -urN linux-2.4.18/net/ipv4/udp.c linux-2.4.18-cor/net/ipv4/udp.c
--- linux-2.4.18/net/ipv4/udp.c	Mon Jun 10 23:34:59 2002
+++ linux-2.4.18-cor/net/ipv4/udp.c	Mon Jun 10 23:35:31 2002
@@ -680,7 +680,7 @@
   	}
 	if (sk->protinfo.af_inet.cmsg_flags)
 		ip_cmsg_recv(msg, skb);
-	err = copied;
+	err = (flags&MSG_TRUNC) ? skb->len - sizeof(struct udphdr) : copied;

 out_free:
   	skb_free_datagram(sk, skb);
diff -urN linux-2.4.18/net/ipv6/udp.c linux-2.4.18-cor/net/ipv6/udp.c
--- linux-2.4.18/net/ipv6/udp.c	Mon Jun 10 23:35:07 2002
+++ linux-2.4.18-cor/net/ipv6/udp.c	Mon Jun 10 23:35:36 2002
@@ -432,7 +432,7 @@
 			}
 		}
   	}
-	err = copied;
+	err = (flags&MSG_TRUNC) ? skb->len - sizeof(struct udphdr) : copied;

 out_free:
 	skb_free_datagram(sk, skb);

-
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