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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>
81 lines
1.9 KiB
C
81 lines
1.9 KiB
C
#include <linux/types.h>
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#include <linux/errno.h>
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#include <linux/tty.h>
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#include <linux/module.h>
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/*
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* n_null.c - Null line discipline used in the failure path
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*
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* Copyright (C) Intel 2017
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*
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*/
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static int n_null_open(struct tty_struct *tty)
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{
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return 0;
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}
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static void n_null_close(struct tty_struct *tty)
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{
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}
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static ssize_t n_null_read(struct tty_struct *tty, struct file *file,
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unsigned char __user * buf, size_t nr)
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{
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return -EOPNOTSUPP;
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}
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static ssize_t n_null_write(struct tty_struct *tty, struct file *file,
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const unsigned char *buf, size_t nr)
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{
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return -EOPNOTSUPP;
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}
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static void n_null_receivebuf(struct tty_struct *tty,
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const unsigned char *cp, char *fp,
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int cnt)
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{
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}
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static struct tty_ldisc_ops null_ldisc = {
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.owner = THIS_MODULE,
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.magic = TTY_LDISC_MAGIC,
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.name = "n_null",
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.open = n_null_open,
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.close = n_null_close,
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.read = n_null_read,
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.write = n_null_write,
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.receive_buf = n_null_receivebuf
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};
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static int __init n_null_init(void)
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{
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BUG_ON(tty_register_ldisc(N_NULL, &null_ldisc));
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return 0;
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}
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static void __exit n_null_exit(void)
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{
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tty_unregister_ldisc(N_NULL);
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}
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module_init(n_null_init);
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module_exit(n_null_exit);
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Alan Cox");
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MODULE_ALIAS_LDISC(N_NULL);
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MODULE_DESCRIPTION("Null ldisc driver");
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