[ 125/143] tcp: fix tcp_trim_head() to adjust segment count with skb MSS

Willy Tarreau
2014-05-12T01:42:57Z

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

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From: Neal Cardwell <ncardwell@google.com>

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
ca_ops->pkts_acked().

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 <ncardwell@google.com>
Acked-by: Nandita Dukkipati <nanditad@google.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 5b35e1e6e9ca651e6b291c96d1106043c9af314a)
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
net/ipv4/tcp_output.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 49da29e..0fc0a73 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -949,11 +949,9 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
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;
}
--
1.7.12.2.21.g234cd45.dirty