null_blk: Suppress an UBSAN complaint triggered when setting 'memory_…

…backed'

[ Upstream commit b9853b4 ]

Although it is not clear to me why UBSAN complains when 'memory_backed'
is set, this patch suppresses the UBSAN complaint that is triggered when
setting that configfs attribute.

UBSAN: Undefined behaviour in drivers/block/null_blk_main.c:327:1
load of value 16 is not a valid value for type '_Bool'
CPU: 2 PID: 8396 Comm: check Not tainted 5.6.0-rc1-dbg+ #14
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
 dump_stack+0xa5/0xe6
 ubsan_epilogue+0x9/0x26
 __ubsan_handle_load_invalid_value+0x6d/0x76
 nullb_device_memory_backed_store.cold+0x2c/0x38 [null_blk]
 configfs_write_file+0x1c4/0x250 [configfs]
 __vfs_write+0x4c/0x90
 vfs_write+0x145/0x2c0
 ksys_write+0xd7/0x180
 __x64_sys_write+0x47/0x50
 do_syscall_64+0x6f/0x2f0
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Chaitanya Kulkarni <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

drivers/block/null_blk_main.c

@@ -276,7 +276,7 @@ nullb_device_##NAME##_store(struct config_item *item, const char *page,	\
 {									\
 	int (*apply_fn)(struct nullb_device *dev, TYPE new_value) = APPLY;\
 	struct nullb_device *dev = to_nullb_device(item);		\
-	TYPE uninitialized_var(new_value);				\
+	TYPE new_value = 0;						\
 	int ret;							\
 									\
 	ret = nullb_device_##TYPE##_attr_store(&new_value, page, count);\