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From:   Pi-Hsun Shih <pihsun@chromium.org>
Cc:     Pi-Hsun Shih <pihsun@chromium.org>, linux-wireless@vger.kernel.org,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-kernel@vger.kernel.org (open list),
        clang-built-linux@googlegroups.com (open list:CLANG/LLVM BUILD SUPPORT)
Subject: [PATCH v2] wireless: Use offsetof instead of custom macro.
Date:   Wed,  4 Dec 2019 16:13:07 +0800
Message-Id: <20191204081307.138765-1-pihsun@chromium.org>
MIME-Version: 1.0
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Sender: linux-wireless-owner@vger.kernel.org
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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>
---

Change since v1:
 * Add #include <stddef.h>

---
 include/uapi/linux/wireless.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/wireless.h b/include/uapi/linux/wireless.h
index 86eca3208b6b..a2c006a364e0 100644
--- a/include/uapi/linux/wireless.h
+++ b/include/uapi/linux/wireless.h
@@ -74,6 +74,8 @@
 #include <linux/socket.h>		/* for "struct sockaddr" et al	*/
 #include <linux/if.h>			/* for IFNAMSIZ and co... */
 
+#include <stddef.h>                     /* for offsetof */
+
 /***************************** VERSION *****************************/
 /*
  * This constant is used to know the availability of the wireless
@@ -1090,8 +1092,7 @@ 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)
 

base-commit: c5db92909beddadddb705b92d3388ea50b01e1a2
-- 
2.24.0.393.g34dc348eaf-goog