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From:   Pi-Hsun Shih <pihsun@chromium.org>
Cc:     Pi-Hsun Shih <pihsun@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Minchan Kim <minchan@kernel.org>,
        Omar Sandoval <osandov@fb.com>,
        Huang Ying <ying.huang@intel.com>,
        Aaron Lu <aaron.lu@intel.com>, Tejun Heo <tj@kernel.org>,
        Wei Yang <richard.weiyang@gmail.com>,
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset
Date:   Thu,  7 Mar 2019 17:46:50 +0800
Message-Id: <20190307094653.119425-1-pihsun@chromium.org>
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Use offsetof to calculate offset of a field to avoid UBSAN warning like:

===================================================================
UBSAN: Undefined behaviour in mm/swapfile.c:3010:38
member access within null pointer of type 'union swap_header'
CPU: 6 PID: 1833 Comm: swapon Tainted: G S                4.19.23 #43
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
 __se_sys_swapon+0x654/0x1084
 __arm64_sys_swapon+0x1c/0x24
 el0_svc_common+0xa8/0x150
 el0_svc_compat_handler+0x2c/0x38
 el0_svc_compat+0x8/0x18
==================================================================

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
---
 include/linux/swap.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index fc50e21b3b88..4bfb5c4ac108 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -157,9 +157,9 @@ struct swap_extent {
 /*
  * Max bad pages in the new format..
  */
-#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
 #define MAX_SWAP_BADPAGES \
-	((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
+	((offsetof(union swap_header, magic.magic) - \
+	  offsetof(union swap_header, info.badpages)) / sizeof(int))
 
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
-- 
2.21.0.352.gf09ad66450-goog