linux-kernel - [33/35] ipv6/udp: Use the correct variable to determine non-blocking condition

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Date:	Fri, 08 Jul 2011 15:50:22 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Xufeng Zhang <xufeng.zhang@...driver.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [33/35] ipv6/udp: Use the correct variable to determine non-blocking condition

2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------


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

[ Upstream commit 32c90254ed4a0c698caa0794ebb4de63fcc69631 ]

udpv6_recvmsg() function is not using the correct variable to determine
whether or not the socket is in non-blocking operation, this will lead
to unexpected behavior when a UDP checksum error occurs.

Consider a non-blocking udp receive scenario: when udpv6_recvmsg() is
called by sock_common_recvmsg(), MSG_DONTWAIT bit of flags variable in
udpv6_recvmsg() is cleared by "flags & ~MSG_DONTWAIT" in this call:

    err = sk->sk_prot->recvmsg(iocb, sk, msg, size, flags & MSG_DONTWAIT,
                   flags & ~MSG_DONTWAIT, &addr_len);

i.e. with udpv6_recvmsg() getting these values:

	int noblock = flags & MSG_DONTWAIT
	int flags = flags & ~MSG_DONTWAIT

So, when udp checksum error occurs, the execution will go to
csum_copy_err, and then the problem happens:

    csum_copy_err:
            ...............
            if (flags & MSG_DONTWAIT)
                    return -EAGAIN;
            goto try_again;
            ...............

But it will always go to try_again as MSG_DONTWAIT has been cleared
from flags at call time -- only noblock contains the original value
of MSG_DONTWAIT, so the test should be:

            if (noblock)
                    return -EAGAIN;

This is also consistent with what the ipv4/udp code does.

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: Greg Kroah-Hartman <gregkh@...e.de>
---
 net/ipv6/udp.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -304,7 +304,7 @@ csum_copy_err:
 	}
 	release_sock(sk);
 
-	if (flags & MSG_DONTWAIT)
+	if (noblock)
 		return -EAGAIN;
 	goto try_again;
 }


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