> From: "Neal Cardwell" <ncardw...@google.com>
> To: "David Miller" <da...@davemloft.net>
> Cc: netdev@vger.kernel.org, "Eric Dumazet" <eduma...@google.com>, "Soheil
> Hassas Yeganeh" <soh...@google.com>, "Neal
> Cardwell" <ncardw...@google.com>, "Yuchung Cheng" <ych...@google.com>
> 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 <eduma...@google.com>
>
> 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 <eduma...@google.com>
> Signed-off-by: Soheil Hassas Yeganeh <soh...@google.com>
> Signed-off-by: Neal Cardwell <ncardw...@google.com>
> Signed-off-by: Yuchung Cheng <ych...@google.com>
> ---
> 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));
>
The skb local variable could be avoided via:
BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > FIELD_SIZEOF(struct sk_buff,
cb));
> percpu_counter_init(&tcp_sockets_allocated, 0, GFP_KERNEL);
> percpu_counter_init(&tcp_orphan_count, 0, GFP_KERNEL);
> --
> 2.8.0.rc3.226.g39d4020
>
>
