[PATCH] fs: fix undefined behavior in bit shift for SB_NOUSER

[Gaosheng Cui]

Shifting signed 32-bit value by 31 bits is undefined, so changing most
significant bit to unsigned. The UBSAN warning calltrace like below:

UBSAN: shift-out-of-bounds in fs/namespace.c:2330:33
left shift of 1 by 31 places cannot be represented in type 'int'
Call Trace:
 <TASK>
 dump_stack_lvl+0x7d/0xa5
 dump_stack+0x15/0x1b
 ubsan_epilogue+0xe/0x4e
 __ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
 graft_tree+0x36/0xf0
 do_add_mount+0x98/0x100
 path_mount+0xbd6/0xd50
 init_mount+0x6a/0xa3
 devtmpfs_setup+0x47/0x7e
 devtmpfsd+0x1a/0x50
 kthread+0x126/0x160
 ret_from_fork+0x1f/0x30
 </TASK>

Fixes: e462ec50cb5f ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
Signed-off-by: Gaosheng Cui <cuigaosheng1@xxxxxxxxxx>
---
 include/linux/fs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 85015e21b755..a68d5310be7b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1396,7 +1396,7 @@ extern int send_sigurg(struct fown_struct *fown);
 #define SB_NOSEC	(1<<28)
 #define SB_BORN		(1<<29)
 #define SB_ACTIVE	(1<<30)
-#define SB_NOUSER	(1<<31)
+#define SB_NOUSER	(1U<<31)
 
 /* These flags relate to encoding and casefolding */
 #define SB_ENC_STRICT_MODE_FL	(1 << 0)
-- 
2.25.1