[PATCH v3 08/17] exit: hang up the tty before closing the files

[Christian Brauner <brauner@kernel.org>]

do_exit() closes the task's files in exit_files() and hangs up the
controlling tty of a session leader in disassociate_ctty(1) after
that. Since commit d99d38540bf0 ("fs: make close_files() synchronous")
the final __fput() of every file runs inside exit_files(). So when the
session leader holds the last open of its tty the tty is released
before disassociate_ctty() runs. tty_release() clears signal->tty for
the whole session in session_clear_tty() once the count drops to zero
and sends no signal doing so. disassociate_ctty(1) then finds neither a
tty nor a tty_old_pgrp and does nothing. The foreground process group
loses its SIGHUP:

    do_exit()
      exit_files()
        close_files()
          tty_release()             tty->count == 0
            session_clear_tty()     signal->tty = NULL, no signal
      disassociate_ctty(1)
        get_current_tty()           NULL
        signal->tty_old_pgrp        NULL, nothing sent

That only affects real ttys. For a pty the master's open keeps the
slave's count above zero. And it only affects a foreground job that
holds no descriptor to the tty anymore while its session leader exits.
Everything else is unchanged. The DTR drop on the last close happens in
tty_port_shutdown() regardless, stopped jobs get their SIGHUP from
kill_orphaned_pgrp() and signal->tty is cleared either way.

Before that commit the final __fput() ran from exit_task_work() which
comes after disassociate_ctty(). That order isn't old. Until v3.14
exit_task_work() came right after exit_files() and before v3.6 fput()
was synchronous, so the tty was always released first. Commit
c39df5fa37b0 ("exit: call disassociate_ctty() before
exit_task_namespaces()") moved disassociate_ctty() up to fix a pppd
crash and in front of exit_task_work() as a side effect. The hangup in
this case has worked since then and that's eleven years of userspace
being able to rely on it.

Hang the tty up before closing the files. This is the ordinary hangup
with the file still open: __tty_hangup() swaps in hung_up_tty_fops and
tty_release() runs from the close afterwards as it does when a modem
drops the line. disassociate_ctty() stays in front of
exit_task_namespaces() which the pppd fix needs.

Reported-by: Chris Mason <mason@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 kernel/exit.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 55dbea3b242e..9ed5eb03d0e1 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1003,10 +1003,11 @@ void __noreturn do_exit(long code)
 
 	exit_sem(tsk);
 	exit_shm(tsk);
-	exit_files(tsk);
-	exit_fs(tsk);
+	/* Hang the tty up before the last close of it can clear the session. */
 	if (group_dead)
 		disassociate_ctty(1);
+	exit_files(tsk);
+	exit_fs(tsk);
 	exit_nsproxy_namespaces(tsk);
 	exit_task_work(tsk);
 	exit_thread(tsk);

-- 
2.53.0