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tty: handle the case where we cannot restore a line discipline

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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>
This commit is contained in:
Alan Cox
2017-06-02 13:49:30 +01:00
committed by Greg Kroah-Hartman
parent 47f58e32a2
commit 8a8dabf2dd
4 changed files with 113 additions and 15 deletions
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44
drivers/tty/tty_ldisc.c
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@@ -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));
}
}