staging/lustre/llite: fix ll_getname user buffer copy

strncpy_from_user could return negative values on error,
so need to take those into account.
Since ll_getname is used to get a single component name from userspace
to transfer to server as-is, there's no need to allocate 4k buffer
as done by __getname. Allocate NAME_MAX+1 buffer instead to ensure
we have enough for a null terminated max valid length buffer.

This was discovered by Al Viro in https://lkml.org/lkml/2015/4/11/243Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/dir.c

@@ -1213,29 +1213,34 @@ static int quotactl_ioctl(struct ll_sb_info *sbi, struct if_quotactl *qctl)
 	return rc;
 }
 
-static char *
-ll_getname(const char __user *filename)
+/* This function tries to get a single name component,
+ * to send to the server. No actual path traversal involved,
+ * so we limit to NAME_MAX */
+static char *ll_getname(const char __user *filename)
 {
 	int ret = 0, len;
-	char *tmp = __getname();
+	char *tmp;
 
+	tmp = kzalloc(NAME_MAX + 1, GFP_KERNEL);
 	if (!tmp)
 		return ERR_PTR(-ENOMEM);
 
-	len = strncpy_from_user(tmp, filename, PATH_MAX);
-	if (len == 0)
+	len = strncpy_from_user(tmp, filename, NAME_MAX + 1);
+	if (len < 0)
+		ret = len;
+	else if (len == 0)
 		ret = -ENOENT;
-	else if (len > PATH_MAX)
+	else if (len > NAME_MAX && tmp[NAME_MAX] != 0)
 		ret = -ENAMETOOLONG;
 
 	if (ret) {
-		__putname(tmp);
+		kfree(tmp);
 		tmp =  ERR_PTR(ret);
 	}
 	return tmp;
 }
 
-#define ll_putname(filename) __putname(filename)
+#define ll_putname(filename) kfree(filename)
 
 static long ll_dir_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {