linux-stable.git/drivers/tty/n_null.c, branch v3.16.78 Linux kernel stable tree tty: handle the case where we cannot restore a line discipline 2018-10-21T07:45:38+00:00 Alan Cox alan@llwyncelyn.cymru 2017-06-02T12:49:30+00:00 a6a35a2a03c2fecc1d6dbf552a262fa7321b7adf commit 8a8dabf2dd68caff842d38057097c23bc514ea6e upstream. 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> [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings <ben@decadent.org.uk> 
commit 8a8dabf2dd68caff842d38057097c23bc514ea6e upstream.

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>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>