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    Subject[34-longterm 103/179] ipv6/udp: Use the correct variable to determine non-blocking condition
    From: Xufeng Zhang <>

    This is a commit scheduled for the next v2.6.34 longterm release.
    If you see a problem with using this for longterm, please comment.

    commit 32c90254ed4a0c698caa0794ebb4de63fcc69631 upstream.

    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:

    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 <>
    Signed-off-by: Paul Gortmaker <>
    Signed-off-by: David S. Miller <>
    Signed-off-by: Paul Gortmaker <>
    net/ipv6/udp.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
    index 479f926..a0a6a08 100644
    --- a/net/ipv6/udp.c
    +++ b/net/ipv6/udp.c
    @@ -441,7 +441,7 @@ csum_copy_err:

    - if (flags & MSG_DONTWAIT)
    + if (noblock)
    return -EAGAIN;
    goto try_again;

     \ /
      Last update: 2012-05-15 07:02    [W:4.082 / U:0.032 seconds]
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