ksmbd: skip lock-range check on equal size to avoid size==0 underflow

When size equals the current i_size (including 0), the code used to call
check_lock_range(filp, i_size, size - 1, WRITE), which computes `size - 1`
and can underflow for size==0. Skip the equal case.

Cc: stable@vger.kernel.org
Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: llllIIIllll

fs/smb/server/vfs.c

@@ -324,6 +324,9 @@ static int check_lock_range(struct file *filp, loff_t start, loff_t end,
 	struct file_lock_context *ctx = locks_inode_context(file_inode(filp));
 	int error = 0;
 
+	if (start == end)
+		return 0;
+
 	if (!ctx || list_empty_careful(&ctx->flc_posix))
 		return 0;
 
@@ -828,7 +831,7 @@ int ksmbd_vfs_truncate(struct ksmbd_work *work,
 		if (size < inode->i_size) {
 			err = check_lock_range(filp, size,
 					       inode->i_size - 1, WRITE);
-		} else {
+		} else if (size > inode->i_size) {
 			err = check_lock_range(filp, inode->i_size,
 					       size - 1, WRITE);
 		}