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

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Date:	Tue, 03 Mar 2015 06:07:44 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'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

On Tue, 2015-03-03 at 13:50 +0000, David Laight wrote:
> 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.

We want a build error, not a runtime one.


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