diff options
author | Alan Cox <alan@llwyncelyn.cymru> | 2017-06-02 13:49:30 +0100 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2017-06-03 18:43:46 +0900 |
commit | 8a8dabf2dd68caff842d38057097c23bc514ea6e (patch) | |
tree | 98e019474882e4f4ea47ac77e162c86f3540c726 /drivers | |
parent | 47f58e32a27c647de0963386d2714d570b38e3d3 (diff) | |
download | linux-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')
-rw-r--r-- | drivers/tty/Makefile | 3 | ||||
-rw-r--r-- | drivers/tty/n_null.c | 80 | ||||
-rw-r--r-- | drivers/tty/tty_ldisc.c | 44 |
3 files changed, 112 insertions, 15 deletions
diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile index f02becdb3e33..8689279afdf1 100644 --- a/drivers/tty/Makefile +++ b/drivers/tty/Makefile @@ -1,6 +1,7 @@ obj-$(CONFIG_TTY) += tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \ tty_buffer.o tty_port.o tty_mutex.o \ - tty_ldsem.o tty_baudrate.o tty_jobctrl.o + tty_ldsem.o tty_baudrate.o tty_jobctrl.o \ + n_null.o obj-$(CONFIG_LEGACY_PTYS) += pty.o obj-$(CONFIG_UNIX98_PTYS) += pty.o obj-$(CONFIG_AUDIT) += tty_audit.o 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"); diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index e4603b09863a..4a04567d9aef 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -492,6 +492,29 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) } /** + * tty_ldisc_failto - helper for ldisc failback + * @tty: tty to open the ldisc on + * @ld: ldisc we are trying to fail back to + * + * Helper to try and recover a tty when switching back to the old + * ldisc fails and we need something attached. + */ + +static int tty_ldisc_failto(struct tty_struct *tty, int ld) +{ + struct tty_ldisc *disc = tty_ldisc_get(tty, ld); + int r; + + if (IS_ERR(disc)) + return PTR_ERR(disc); + tty->ldisc = disc; + tty_set_termios_ldisc(tty, ld); + if ((r = tty_ldisc_open(tty, disc)) < 0) + tty_ldisc_put(disc); + return r; +} + +/** * tty_ldisc_restore - helper for tty ldisc change * @tty: tty to recover * @old: previous ldisc @@ -502,9 +525,6 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) { - struct tty_ldisc *new_ldisc; - int r; - /* There is an outstanding reference here so this is safe */ old = tty_ldisc_get(tty, old->ops->num); WARN_ON(IS_ERR(old)); @@ -512,17 +532,13 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) tty_set_termios_ldisc(tty, old->ops->num); if (tty_ldisc_open(tty, old) < 0) { tty_ldisc_put(old); - /* This driver is always present */ - new_ldisc = tty_ldisc_get(tty, N_TTY); - if (IS_ERR(new_ldisc)) - panic("n_tty: get"); - tty->ldisc = new_ldisc; - tty_set_termios_ldisc(tty, N_TTY); - r = tty_ldisc_open(tty, new_ldisc); - if (r < 0) - panic("Couldn't open N_TTY ldisc for " - "%s --- error %d.", - tty_name(tty), r); + /* The traditional behaviour is to fall back to N_TTY, we + want to avoid falling back to N_NULL unless we have no + choice to avoid the risk of breaking anything */ + if (tty_ldisc_failto(tty, N_TTY) < 0 && + tty_ldisc_failto(tty, N_NULL) < 0) + panic("Couldn't open N_NULL ldisc for %s.", + tty_name(tty)); } } |