Re: [PATCH v3 net-next 05/16] tcp: switch back to proper tcp_skb_cb size check in tcp_init()

From: "Neal Cardwell" <ncardwell@google.com>
To: "David Miller" <davem@davemloft.net>
Cc: <redacted>, "Eric Dumazet" <redacted>, "Soheil Hassas Yeganeh" <redacted>, "Neal
Cardwell" [off-list ref], "Yuchung Cheng" [off-list ref]
Sent: Sunday, September 18, 2016 6:03:42 PM
Subject: [PATCH v3 net-next 05/16] tcp: switch back to proper tcp_skb_cb size check in tcp_init()

From: Eric Dumazet <redacted>

Revert to the tcp_skb_cb size check that tcp_init() had before commit
b4772ef879a8 ("net: use common macro for assering skb->cb[] available
size in protocol families"). As related commit 744d5a3e9fe2 ("net:
move skb->dropcount to skb->cb[]") explains, the
sock_skb_cb_check_size() mechanism was added to ensure that there is
space for dropcount, "for protocol families using it". But TCP is not
a protocol using dropcount, so tcp_init() doesn't need to provision
space for dropcount in the skb->cb[], and thus we can revert to the
older form of the tcp_skb_cb size check. Doing so allows TCP to use 4
more bytes of the skb->cb[] space.

Fixes: b4772ef879a8 ("net: use common macro for assering skb->cb[] available
size in protocol families")
Signed-off-by: Eric Dumazet <redacted>
Signed-off-by: Soheil Hassas Yeganeh <redacted>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <redacted>
---
 net/ipv4/tcp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 5b0b49c..53798e1 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -3244,11 +3244,12 @@ static void __init tcp_init_mem(void)
 
 void __init tcp_init(void)
 {
-	unsigned long limit;
 	int max_rshare, max_wshare, cnt;
+	unsigned long limit;
+	struct sk_buff *skb;
 	unsigned int i;
 
-	sock_skb_cb_check_size(sizeof(struct tcp_skb_cb));
+	BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > sizeof(skb->cb));