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From cd4ffa93f16efea290bb70537f98f518e1927e63 Mon Sep 17 00:00:00 2001
From: Joao Martins <>
Date: Mon, 10 Jun 2019 23:12:39 +0100
Subject: [PATCH 5/5] tcp: fix fack_count accounting on tcp_shift_skb_data()
v4.15 or since commit 737ff314563 ("tcp: use sequence distance to
detect reordering") had switched from the packet-based FACK tracking
to sequence-based.
v4.14 and older still have the old logic and hence on
tcp_skb_shift_data() needs to retain its original logic and have
@fack_count in sync. In other words, we keep the increment of pcount with
tcp_skb_pcount(skb) to later used that to update fack_count. To make it
more explicit we track the new skb that gets incremented to pcount in
@next_pcount, and we get to avoid the constant invocation of
tcp_skb_pcount(skb) all together.
Fixes: a5f1faa40101 ("tcp: limit payload size of sacked skbs")
Reported-by: Alexey Kodanev <>
Reviewed-by: Jack Vogel <>
Reviewed-by: John Haxby <>
Reviewed-by: Rao Shoaib>
Signed-off-by: Joao Martins <>
Signed-off-by: Konrad Rzeszutek Wilk <>
net/ipv4/tcp_input.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fd5c2d2a1d9c..85c09829790b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1423,6 +1423,7 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *prev;
int mss;
+ int next_pcount;
int pcount = 0;
int len;
int in_sack;
@@ -1539,9 +1540,11 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
goto out;
len = skb->len;
- pcount = tcp_skb_pcount(skb);
- if (tcp_skb_shift(prev, skb, pcount, len))
- tcp_shifted_skb(sk, skb, state, pcount, len, mss, 0);
+ next_pcount = tcp_skb_pcount(skb);
+ if (tcp_skb_shift(prev, skb, next_pcount, len)) {
+ pcount += next_pcount;
+ tcp_shifted_skb(sk, skb, state, next_pcount, len, mss, 0);
+ }
state->fack_count += pcount;
return prev;
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