linux-kernel - [34-longterm 104/179] udp/recvmsg: Clear MSG_TRUNC flag when starting over for a new packet

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Date:	Mon, 14 May 2012 22:13:20 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	<stable@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: [34-longterm 104/179] udp/recvmsg: Clear MSG_TRUNC flag when starting over for a new packet

From: Xufeng Zhang <xufeng.zhang@...driver.com>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit 9cfaa8def1c795a512bc04f2aec333b03724ca2e upstream.

Consider this scenario: When the size of the first received udp packet
is bigger than the receive buffer, MSG_TRUNC bit is set in msg->msg_flags.
However, if checksum error happens and this is a blocking socket, it will
goto try_again loop to receive the next packet.  But if the size of the
next udp packet is smaller than receive buffer, MSG_TRUNC flag should not
be set, but because MSG_TRUNC bit is not cleared in msg->msg_flags before
receive the next packet, MSG_TRUNC is still set, which is wrong.

Fix this problem by clearing MSG_TRUNC flag when starting over for a
new packet.

Signed-off-by: Xufeng Zhang <xufeng.zhang@...driver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 net/ipv4/udp.c |    3 +++
 net/ipv6/udp.c |    3 +++
 2 files changed, 6 insertions(+)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index ff6a18e..7932dc6 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1203,6 +1203,9 @@ csum_copy_err:
 
 	if (noblock)
 		return -EAGAIN;
+
+	/* starting over for a new packet */
+	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index a0a6a08..a1d3d32 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -443,6 +443,9 @@ csum_copy_err:
 
 	if (noblock)
 		return -EAGAIN;
+
+	/* starting over for a new packet */
+	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }
 
-- 
1.7.9.6

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