netdev - RE: [PATCH v2] net: bridge: add compile-time assert for cb struct size

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Date:	Tue, 3 Mar 2015 13:50:59 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Florian Westphal' <fw@...len.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v2] net: bridge: add compile-time assert for cb struct
 size

From: Florian Westphal
> make build fail if structure no longer fits into ->cb storage.
> 
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  Change since v1: use FIELD_SIZEOF
> 
>  checkpatch complains about >80 chars, but line break is ugly here.
> 
> diff --git a/net/bridge/br.c b/net/bridge/br.c
> index fb57ab6..02c24cf 100644
> --- a/net/bridge/br.c
> +++ b/net/bridge/br.c
> @@ -190,6 +190,8 @@ static int __init br_init(void)
>  {
>  	int err;
> 
> +	BUILD_BUG_ON(sizeof(struct br_input_skb_cb) > FIELD_SIZEOF(struct sk_buff, cb));
> +

What about using something based on:
#define GET_CB(skb, type) ((type *)((skb)->cb + (sizeof (char[(sizeof (type) <= sizeof (skb->cb)) ? 1 : -1] - 1)))
to access skb->sb?

Then all invalid uses are likely to be picked up.

	David

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