[PATCH v2] mm/swap: Use offsetof instead of custom __swapoffset macro.

[Pi-Hsun Shih]

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

Signed-off-by: Pi-Hsun Shih <pihsun@xxxxxxxxxxxx>
---
 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? */
-- 
2.21.0.360.g471c308f928-goog