[PATCH AUTOSEL 3.18 10/10] include/linux/swap.h: use offsetof() instead of custom __swapoffset macro

[Sasha Levin]

From: Pi-Hsun Shih <pihsun@xxxxxxxxxxxx>

[ 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@xxxxxxxxxxxx
Signed-off-by: Pi-Hsun Shih <pihsun@xxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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