summaryrefslogtreecommitdiffstats
path: root/drivers/tty/n_null.c
diff options
context:
space:
mode:
authorAlan Cox <alan@llwyncelyn.cymru>2017-06-02 13:49:30 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-06-03 18:43:46 +0900
commit8a8dabf2dd68caff842d38057097c23bc514ea6e (patch)
tree98e019474882e4f4ea47ac77e162c86f3540c726 /drivers/tty/n_null.c
parent47f58e32a27c647de0963386d2714d570b38e3d3 (diff)
downloadlinux-8a8dabf2dd68caff842d38057097c23bc514ea6e.tar.bz2
tty: handle the case where we cannot restore a line discipline
Historically the N_TTY driver could never fail but this has become broken over time. Rather than trying to rewrite half the ldisc layer to fix the breakage introduce a second level of fallback with an N_NULL ldisc which cannot fail, and thus restore the guarantees required by the ldisc layer. We still try and fail to N_TTY first. It's much more useful to find yourself back in your old ldisc (first attempt) or in N_TTY (second attempt), and while I'm not aware of any code out there that makes those assumptions it's good to drive(r) defensively. Signed-off-by: Alan Cox <alan@linux.intel.com> Reported-by: Dmitry Vyukov <dvyukov@google.com> Tested-by: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/tty/n_null.c')
-rw-r--r--drivers/tty/n_null.c80
1 files changed, 80 insertions, 0 deletions
diff --git a/drivers/tty/n_null.c b/drivers/tty/n_null.c
new file mode 100644
index 000000000000..d63261c36e42
--- /dev/null
+++ b/drivers/tty/n_null.c
@@ -0,0 +1,80 @@
+#include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/tty.h>
+#include <linux/module.h>
+
+/*
+ * n_null.c - Null line discipline used in the failure path
+ *
+ * Copyright (C) Intel 2017
+ *
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ */
+
+static int n_null_open(struct tty_struct *tty)
+{
+ return 0;
+}
+
+static void n_null_close(struct tty_struct *tty)
+{
+}
+
+static ssize_t n_null_read(struct tty_struct *tty, struct file *file,
+ unsigned char __user * buf, size_t nr)
+{
+ return -EOPNOTSUPP;
+}
+
+static ssize_t n_null_write(struct tty_struct *tty, struct file *file,
+ const unsigned char *buf, size_t nr)
+{
+ return -EOPNOTSUPP;
+}
+
+static void n_null_receivebuf(struct tty_struct *tty,
+ const unsigned char *cp, char *fp,
+ int cnt)
+{
+}
+
+static struct tty_ldisc_ops null_ldisc = {
+ .owner = THIS_MODULE,
+ .magic = TTY_LDISC_MAGIC,
+ .name = "n_null",
+ .open = n_null_open,
+ .close = n_null_close,
+ .read = n_null_read,
+ .write = n_null_write,
+ .receive_buf = n_null_receivebuf
+};
+
+static int __init n_null_init(void)
+{
+ BUG_ON(tty_register_ldisc(N_NULL, &null_ldisc));
+ return 0;
+}
+
+static void __exit n_null_exit(void)
+{
+ tty_unregister_ldisc(N_NULL);
+}
+
+module_init(n_null_init);
+module_exit(n_null_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Alan Cox");
+MODULE_ALIAS_LDISC(N_NULL);
+MODULE_DESCRIPTION("Null ldisc driver");