tty: handle the case where we cannot restore a line discipline
authorAlan Cox <alan@llwyncelyn.cymru>
Fri, 2 Jun 2017 12:49:30 +0000 (13:49 +0100)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Sat, 3 Jun 2017 09:43:46 +0000 (18:43 +0900)
commit8a8dabf2dd68caff842d38057097c23bc514ea6e
tree98e019474882e4f4ea47ac77e162c86f3540c726
parent47f58e32a27c647de0963386d2714d570b38e3d3
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>
drivers/tty/Makefile
drivers/tty/n_null.c [new file with mode: 0644]
drivers/tty/tty_ldisc.c
include/uapi/linux/tty.h