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authorAlan Cox <alan@llwyncelyn.cymru>2017-06-02 15:49:30 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-06-03 12:43:46 +0300
commit8a8dabf2dd68caff842d38057097c23bc514ea6e (patch)
tree98e019474882e4f4ea47ac77e162c86f3540c726 /drivers/tty/tty_ldisc.c
parent47f58e32a27c647de0963386d2714d570b38e3d3 (diff)
downloadlinux-8a8dabf2dd68caff842d38057097c23bc514ea6e.tar.xz
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/tty_ldisc.c')
-rw-r--r--drivers/tty/tty_ldisc.c44
1 files changed, 30 insertions, 14 deletions
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));
}
}