tty: Properly fix memleak of alloc_pid

Cylen Yao <cylen.yao@mediatek.com>
bug: 7845126 MT67x2

Memleak is due to unreleased pid->count, which execute in function:
get_pid()(pid->count++) and put_pid()(pid->count--).

The race condition as following:
task[dumpsys]               task[adbd]
in disassociate_ctty()      in tty_signal_session_leader()
-----------------------     -------------------------
tty = get_current_tty();
// tty is not NULL
...
spin_lock_irq(&current->sighand->siglock);
put_pid(current->signal->tty_old_pgrp);
current->signal->tty_old_pgrp = NULL;
spin_unlock_irq(&current->sighand->siglock);

                            spin_lock_irq(&p->sighand->siglock);
                            ...
                            p->signal->tty = NULL;
                            ...
                            spin_unlock_irq(&p->sighand->siglock);

tty = get_current_tty();
// tty NULL, goto else branch by accident.
if (tty) {
    ...
    put_pid(tty_session);
    put_pid(tty_pgrp);
    ...
} else {
    print msg
}

in task[dumpsys], in disassociate_ctty(), tty is set NULL by task[adbd],
tty_signal_session_leader(), then it goto else branch and lack of
put_pid(), cause memleak.

move spin_unlock(sighand->siglock) after get_current_tty() can avoid
the race and fix the memleak.

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e7f816395..183bbe642 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -874,9 +874,8 @@ void disassociate_ctty(int on_exit)
 	spin_lock_irq(&current->sighand->siglock);
 	put_pid(current->signal->tty_old_pgrp);
 	current->signal->tty_old_pgrp = NULL;
-	spin_unlock_irq(&current->sighand->siglock);
 
-	tty = get_current_tty();
+	tty = tty_kref_get(current->signal->tty);
 	if (tty) {
 		unsigned long flags;
 		spin_lock_irqsave(&tty->ctrl_lock, flags);
@@ -893,6 +892,7 @@ void disassociate_ctty(int on_exit)
 #endif
 	}
 
+        spin_unlock_irq(&current->sighand->siglock);
 	/* Now clear signal->tty under the lock */
 	read_lock(&tasklist_lock);
 	session_clear_tty(task_session(current));