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Subject[PATCH 2/2] tcp: miscounts due to tcp_fragment pcount reset

It seems that trivial reset of pcount to one was not sufficient
in tcp_retransmit_skb. Multiple counters experience a positive
miscount when skb's pcount gets lowered without the necessary
adjustments (depending on skb's sacked bits which exactly), at
worst a packets_out miscount can crash at RTO if the write queue
is empty!

Triggering this requires mss change, so bidir tcp or mtu probe or
like.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Tested-by: Uwe Bugla <uwe.bugla@gmx.de>
---
Markus probably tested this as well though I didn't hear any reports back
yet.

net/ipv4/tcp_output.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f1db89b..53300fa 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1893,7 +1893,12 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
if (tcp_fragment(sk, skb, cur_mss, cur_mss))
return -ENOMEM; /* We'll try again later. */
} else {
- tcp_init_tso_segs(sk, skb, cur_mss);
+ int oldpcount = tcp_skb_pcount(skb);
+
+ if (unlikely(oldpcount > 1)) {
+ tcp_init_tso_segs(sk, skb, cur_mss);
+ tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
+ }
}

tcp_retrans_try_collapse(sk, skb, cur_mss);
--
1.5.2.2
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