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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Thu, 2 May 2019 15:56:33 +0800
Message-ID: <CANMq1KCwcVawg6L1hTKXBgBi66EKdHQrvxr_chR9Kv1ifFREnA@mail.gmail.com>
Subject: Re: [PATCH] wireless: Use offsetof instead of custom macro.
To:     Pi-Hsun Shih <pihsun@chromium.org>
Cc:     open list <linux-kernel@vger.kernel.org>
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On Thu, Apr 18, 2019 at 3:50 PM Pi-Hsun Shih <pihsun@chromium.org> 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:
>
> ==================================================================
> UBSAN: Undefined behaviour in net/wireless/wext-core.c:525:14
> member access within null pointer of type 'struct iw_point'
> CPU: 3 PID: 165 Comm: kworker/u16:3 Tainted: G S      W         4.19.23 #43
> Workqueue: cfg80211 __cfg80211_scan_done [cfg80211]
> Call trace:
>  dump_backtrace+0x0/0x194
>  show_stack+0x20/0x2c
>  __dump_stack+0x20/0x28
>  dump_stack+0x70/0x94
>  ubsan_epilogue+0x14/0x44
>  ubsan_type_mismatch_common+0xf4/0xfc
>  __ubsan_handle_type_mismatch_v1+0x34/0x54
>  wireless_send_event+0x3cc/0x470
>  ___cfg80211_scan_done+0x13c/0x220 [cfg80211]
>  __cfg80211_scan_done+0x28/0x34 [cfg80211]
>  process_one_work+0x170/0x35c
>  worker_thread+0x254/0x380
>  kthread+0x13c/0x158
>  ret_from_fork+0x10/0x18
> ===================================================================
>
> Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>

The warning from clang is spurious, but in another case, we felt that
the cleanup was worth it, nevertheless
(https://lore.kernel.org/patchwork/patch/1050040/).

Reviewed-By: Nicolas Boichat <drinkcat@chromium.org>

> ---
>  include/uapi/linux/wireless.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/wireless.h b/include/uapi/linux/wireless.h
> index 86eca3208b6b..f259cca5cc2b 100644
> --- 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)
>
> --
> 2.21.0.392.gf8f6787159e-goog
>