netdev - [PATCH] tcp: fix retransmit of partially acked frames

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Date:	Wed, 18 Apr 2012 22:14:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Beregalov <a.beregalov@...il.com>,
	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Marc MERLIN <marc@...lins.org>
Subject: [PATCH] tcp: fix retransmit of partially acked frames

From: Eric Dumazet <edumazet@...gle.com>

Alexander Beregalov reported skb_over_panic errors and provided stack
trace.

I occurs commit a21d45726aca (tcp: avoid order-1 allocations on wifi and
tx path) added a regression, when a retransmit is done after a partial
ACK.

tcp_retransmit_skb() tries to aggregate several frames if the first one
has enough available room to hold the following ones payload. This is
controlled by /proc/sys/net/ipv4/tcp_retrans_collapse tunable (default :
enabled)

Problem is we must make sure _pskb_trim_head() doesnt fool
skb_availroom() when pulling some bytes from skb (this pull is done when
receiver ACK part of the frame).

Reported-by: Alexander Beregalov <a.beregalov@...il.com>
Cc: Marc MERLIN <marc@...lins.org>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_output.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 376b2cf..7ac6423 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1096,6 +1096,7 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
 	eat = min_t(int, len, skb_headlen(skb));
 	if (eat) {
 		__skb_pull(skb, eat);
+		skb->avail_size -= eat;
 		len -= eat;
 		if (!len)
 			return;

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