Subject: Re: [PATCH 17/29] l2tp: avoid precidence issues in L2TP_SKB_CB macro

From: Tom Parkin <[email-masked]>
Date: Tue, 28 Jul 2020 20:31:32 +0100

On  Tue, Jul 28, 2020 at 11:08:45 -0700, Joe Perches wrote:
> On Tue, 2020-07-28 at 09:21 -0700, Joe Perches wrote:
> > On Tue, 2020-07-21 at 18:32 +0100, Tom Parkin wrote:
> > > checkpatch warned about the L2TP_SKB_CB macro's use of its argument: add
> > > braces to avoid the problem.
> > []
> > > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> > []
> > > @@ -93,7 +93,7 @@ struct l2tp_skb_cb {
> > >  	unsigned long		expires;
> > >  };
> > >  
> > > -#define L2TP_SKB_CB(skb)	((struct l2tp_skb_cb *)&skb->cb[sizeof(struct inet_skb_parm)])
> > > +#define L2TP_SKB_CB(skb)	((struct l2tp_skb_cb *)&(skb)->cb[sizeof(struct inet_skb_parm)])
> > 
> > Likely better to use a static inline.
> > 
> > Something like:
> > 
> > static inline struct l2tp_skb_cb *L2TP_SKB_SB(struct sk_buff *skb)
> > {
> > 	return &skb->cb[sizeof(struct inet+skb_parm)];
> > }
> 
> More precisely:
> ---
>  net/l2tp/l2tp_core.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index e723828e458b..78ad6d8405c4 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -93,7 +93,10 @@ struct l2tp_skb_cb {
>  	unsigned long		expires;
>  };
>  
> -#define L2TP_SKB_CB(skb)	((struct l2tp_skb_cb *)&(skb)->cb[sizeof(struct inet_skb_parm)])
> +static inline struct l2tp_skb_cb *L2TP_SKB_CB(struct sk_buff *skb)
> +{
> +	return (struct l2tp_skb_cb *)&skb->cb[sizeof(struct inet_skb_parm)];
> +}
>  
>  static struct workqueue_struct *l2tp_wq;
>  
> 

Thanks Joe.  I can see this is better since we get some type checking
from the compiler for the function argument.

The patchset has been applied already, but I can try to integrate this
change in the future.