From: Alan Cox Date: Fri, 2 Jun 2017 12:49:30 +0000 (+0100) Subject: tty: handle the case where we cannot restore a line discipline X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=8a8dabf2dd68caff842d38057097c23bc514ea6e;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git 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 Reported-by: Dmitry Vyukov Tested-by: Dmitry Vyukov Signed-off-by: Greg Kroah-Hartman --- 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 +#include +#include +#include + +/* + * 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 @@ -491,6 +491,29 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) tty_ldisc_debug(tty, "%p: closed\n", 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 @@ -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)); } } diff --git a/include/uapi/linux/tty.h b/include/uapi/linux/tty.h index e7855dffd592..cf1455396df0 100644 --- a/include/uapi/linux/tty.h +++ b/include/uapi/linux/tty.h @@ -36,5 +36,6 @@ #define N_TRACEROUTER 24 /* Trace data routing for MIPI P1149.7 */ #define N_NCI 25 /* NFC NCI UART */ #define N_SPEAKUP 26 /* Speakup communication with synths */ +#define N_NULL 27 /* Null ldisc used for error handling */ #endif /* _UAPI_LINUX_TTY_H */