linux-kernel - Re: [PATCH] netlink: NL_SET_ERR_MSG

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Date:   Tue, 10 Aug 2021 13:30:58 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Joe Perches <joe@...ches.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Johannes Berg <johannes.berg@...el.com>,
        Jonathan Toppins <jtoppins@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netlink: NL_SET_ERR_MSG - remove local static array

On Mon, 09 Aug 2021 10:04:00 -0700 Joe Perches wrote:
> The want was to have some separate object section for netlink messages
> so all netlink messages could be specifically listed by some tool but
> the effect is duplicating static const char arrays in the object code.
> 
> It seems unused presently so change the macro to avoid the local static
> declarations until such time as these actually are wanted and used.
> 
> This reduces object size ~8KB in an x86-64 defconfig without modules.
> 
> $ size vmlinux.o*
>    text	   data	    bss	    dec	    hex	filename
> 20110471	3460344	 741760	24312575	172faff	vmlinux.o.new
> 20119444	3460344	 741760	24321548	1731e0c	vmlinux.o.old
> 
> Signed-off-by: Joe Perches <joe@...ches.com>

I guess we can bring it back when it's needed.

> diff --git a/include/linux/netlink.h b/include/linux/netlink.h
> index 61b1c7fcc401e..4bb32ae134aa8 100644
> --- a/include/linux/netlink.h
> +++ b/include/linux/netlink.h
> @@ -89,13 +89,12 @@ struct netlink_ext_ack {
>   * to the lack of an output buffer.)
>   */
>  #define NL_SET_ERR_MSG(extack, msg) do {		\
> -	static const char __msg[] = msg;		\
>  	struct netlink_ext_ack *__extack = (extack);	\
>  							\
> -	do_trace_netlink_extack(__msg);			\
> +	do_trace_netlink_extack(msg);			\
>  							\
>  	if (__extack)					\
> -		__extack->_msg = __msg;			\
> +		__extack->_msg = msg;			\
>  } while (0)

But you've made us evaluate msg multiple times now.
Given extack is carefully evaluated only once this stands out.

>  #define NL_SET_ERR_MSG_MOD(extack, msg)			\
> @@ -111,13 +110,12 @@ struct netlink_ext_ack {
>  #define NL_SET_BAD_ATTR(extack, attr) NL_SET_BAD_ATTR_POLICY(extack, attr, NULL)
>  
>  #define NL_SET_ERR_MSG_ATTR_POL(extack, attr, pol, msg) do {	\
> -	static const char __msg[] = msg;			\
>  	struct netlink_ext_ack *__extack = (extack);		\
>  								\
> -	do_trace_netlink_extack(__msg);				\
> +	do_trace_netlink_extack(msg);				\
>  								\
>  	if (__extack) {						\
> -		__extack->_msg = __msg;				\
> +		__extack->_msg = msg;				\
>  		__extack->bad_attr = (attr);			\
>  		__extack->policy = (pol);			\

Same here.