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Subject[PATCH 3.18 085/104] include/linux/swap.h: use offsetof() instead of custom __swapoffset macro
Date
[ Upstream commit a4046c06be50a4f01d435aa7fe57514818e6cc82 ]

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 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

Link: http://lkml.kernel.org/r/20190312081902.223764-1-pihsun@chromium.org
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.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 1dc0e886227d..054cddb52482 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -143,9 +143,9 @@ 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? */
--
2.19.1


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