linux-kernel - [ 074/167] [PATCH 14/26] tcp: fix tcp_trim

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Message-Id: <20120509055040.099618905@decadent.org.uk>
Date:	Wed, 09 May 2012 06:51:43 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Eric Dumazet <eric.dumazet@...il.com>,
	Vijay Subramanian <subramanian.vijay@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 074/167] [PATCH 14/26] tcp: fix tcp_trim_head()

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <eric.dumazet@...il.com>

[ Upstream commit 4fa48bf3c75069d636fc8830743c929a062e80dc ]

commit f07d960df3 (tcp: avoid frag allocation for small frames)
breaked assumption in tcp stack that skb is either linear (skb->data_len
== 0), or fully fragged (skb->data_len == skb->len)

tcp_trim_head() made this assumption, we must fix it.

Thanks to Vijay for providing a very detailed explanation.

Reported-by: Vijay Subramanian <subramanian.vijay@...il.com>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/ipv4/tcp_output.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 097e0c7..7413437 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1093,6 +1093,13 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
 {
 	int i, k, eat;
 
+	eat = min_t(int, len, skb_headlen(skb));
+	if (eat) {
+		__skb_pull(skb, eat);
+		len -= eat;
+		if (!len)
+			return;
+	}
 	eat = len;
 	k = 0;
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
@@ -1124,11 +1131,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 	if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
 		return -ENOMEM;
 
-	/* If len == headlen, we avoid __skb_pull to preserve alignment. */
-	if (unlikely(len < skb_headlen(skb)))
-		__skb_pull(skb, len);
-	else
-		__pskb_trim_head(skb, len - skb_headlen(skb));
+	__pskb_trim_head(skb, len);
 
 	TCP_SKB_CB(skb)->seq += len;
 	skb->ip_summed = CHECKSUM_PARTIAL;
-- 
1.7.10



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