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    Subject[79/89] tcp: fix tcp_trim_head() to adjust segment count with skb MSS
    3.2-stable review patch.  If anyone has any objections, please let me know.


    From: Neal Cardwell <>

    [ Upstream commit 5b35e1e6e9ca651e6b291c96d1106043c9af314a ]

    This commit fixes tcp_trim_head() to recalculate the number of
    segments in the skb with the skb's existing MSS, so trimming the head
    causes the skb segment count to be monotonically non-increasing - it
    should stay the same or go down, but not increase.

    Previously tcp_trim_head() used the current MSS of the connection. But
    if there was a decrease in MSS between original transmission and ACK
    (e.g. due to PMTUD), this could cause tcp_trim_head() to
    counter-intuitively increase the segment count when trimming bytes off
    the head of an skb. This violated assumptions in tcp_tso_acked() that
    tcp_trim_head() only decreases the packet count, so that packets_acked
    in tcp_tso_acked() could underflow, leading tcp_clean_rtx_queue() to
    pass u32 pkts_acked values as large as 0xffffffff to

    As an aside, if tcp_trim_head() had really wanted the skb to reflect
    the current MSS, it should have called tcp_set_skb_tso_segs()
    unconditionally, since a decrease in MSS would mean that a
    single-packet skb should now be sliced into multiple segments.

    Signed-off-by: Neal Cardwell <>
    Acked-by: Nandita Dukkipati <>
    Acked-by: Ilpo Järvinen <>
    Signed-off-by: David S. Miller <>
    Signed-off-by: Greg Kroah-Hartman <>
    net/ipv4/tcp_output.c | 6 ++----
    1 file changed, 2 insertions(+), 4 deletions(-)

    --- a/net/ipv4/tcp_output.c
    +++ b/net/ipv4/tcp_output.c
    @@ -1138,11 +1138,9 @@ int tcp_trim_head(struct sock *sk, struc
    sk_mem_uncharge(sk, len);
    sock_set_flag(sk, SOCK_QUEUE_SHRUNK);

    - /* Any change of skb->len requires recalculation of tso
    - * factor and mss.
    - */
    + /* Any change of skb->len requires recalculation of tso factor. */
    if (tcp_skb_pcount(skb) > 1)
    - tcp_set_skb_tso_segs(sk, skb, tcp_current_mss(sk));
    + tcp_set_skb_tso_segs(sk, skb, tcp_skb_mss(skb));

    return 0;

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      Last update: 2012-02-01 22:29    [W:0.024 / U:82.388 seconds]
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