linux-kernel - Re: [PATCH RESEND] wireless: Use offsetof instead of custom macro.

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Date:   Wed, 27 Nov 2019 19:53:55 -0800
From:   Joe Perches <joe@...ches.com>
To:     Pi-Hsun Shih <pihsun@...omium.org>
Cc:     linux-wireless@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:CLANG/LLVM BUILD SUPPORT" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH RESEND] wireless: Use offsetof instead of custom macro.

On Thu, 2019-11-28 at 11:39 +0800, Pi-Hsun Shih wrote:
> Use offsetof to calculate offset of a field to take advantage of
> compiler built-in version when possible, and avoid UBSAN warning when
> compiling with Clang:
[]
> diff --git a/include/uapi/linux/wireless.h b/include/uapi/linux/wireless.h
[]
> @@ -1090,8 +1090,7 @@ struct iw_event {
>  /* iw_point events are special. First, the payload (extra data) come at
>   * the end of the event, so they are bigger than IW_EV_POINT_LEN. Second,
>   * we omit the pointer, so start at an offset. */
> -#define IW_EV_POINT_OFF (((char *) &(((struct iw_point *) NULL)->length)) - \
> -			  (char *) NULL)
> +#define IW_EV_POINT_OFF offsetof(struct iw_point, length)
>  #define IW_EV_POINT_LEN	(IW_EV_LCP_LEN + sizeof(struct iw_point) - \
>  			 IW_EV_POINT_OFF)

This is uapi.  Is offsetof guaranteed to be available?

Perhaps this is better without using another macro

#define IW_EV_POINT_OFF	((size_t)&((struct iw_point *)NULL)->length)