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

[Eric Dumazet]

On Mon, Sep 19, 2016 at 7:37 AM, Lance Richardson  wrote:

> The skb local variable could be avoided via:
>
> BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > FIELD_SIZEOF(struct sk_buff, 
> cb));
>

Right you are, thanks !

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

[Lance Richardson]

> 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(_sockets_allocated, 0, GFP_KERNEL);
>   percpu_counter_init(_orphan_count, 0, GFP_KERNEL);
> --
> 2.8.0.rc3.226.g39d4020
> 
> 

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

[Neal Cardwell]

From: Eric Dumazet 

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 
Signed-off-by: Soheil Hassas Yeganeh 
Signed-off-by: Neal Cardwell 
Signed-off-by: Yuchung Cheng 
---
 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));
 
percpu_counter_init(_sockets_allocated, 0, GFP_KERNEL);
percpu_counter_init(_orphan_count, 0, GFP_KERNEL);
-- 
2.8.0.rc3.226.g39d4020