Commit 38b04ed7 authored by Qingfang Deng's avatar Qingfang Deng Committed by Paolo Abeni
Browse files

6pack: drop redundant locking and refcounting 



The TTY layer already serializes line discipline operations with
tty->ldisc_sem, so the extra disc_data_lock and refcnt in 6pack
are unnecessary.

Removing them simplifies the code and also resolves a lockdep warning
reported by syzbot. The warning did not indicate a real deadlock, since
the write-side lock was only taken in process context with hardirqs
disabled.

Reported-by: default avatar <syzbot+5fd749c74105b0e1b302@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68c858b0.050a0220.3c6139.0d1c.GAE@google.com/


Signed-off-by: default avatarQingfang Deng <dqfext@gmail.com>
Reviewed-by: default avatarDan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/20250925051059.26876-1-dqfext@gmail.com


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent 7bd80ed8

drivers/net/hamradio/6pack.c

+5 −52
Original line number Diff line number Diff line
@@ -115,8 +115,6 @@ struct sixpack { 


	struct timer_list	tx_t;

	struct timer_list	resync_t;

	refcount_t		refcnt;

	struct completion	dead;

	spinlock_t		lock;

};
@@ -353,42 +351,13 @@ static void sp_bump(struct sixpack *sp, char cmd) 


/* ----------------------------------------------------------------------- */



/*

 * We have a potential race on dereferencing tty->disc_data, because the tty

 * layer provides no locking at all - thus one cpu could be running

 * sixpack_receive_buf while another calls sixpack_close, which zeroes

 * tty->disc_data and frees the memory that sixpack_receive_buf is using.  The

 * best way to fix this is to use a rwlock in the tty struct, but for now we

 * use a single global rwlock for all ttys in ppp line discipline.

 */

static DEFINE_RWLOCK(disc_data_lock);

                                                                                

static struct sixpack *sp_get(struct tty_struct *tty)

{

	struct sixpack *sp;



	read_lock(&disc_data_lock);

	sp = tty->disc_data;

	if (sp)

		refcount_inc(&sp->refcnt);

	read_unlock(&disc_data_lock);



	return sp;

}



static void sp_put(struct sixpack *sp)

{

	if (refcount_dec_and_test(&sp->refcnt))

		complete(&sp->dead);

}



/*

 * Called by the TTY driver when there's room for more data.  If we have

 * more packets to send, we send them here.

 */

static void sixpack_write_wakeup(struct tty_struct *tty)

{

	struct sixpack *sp = sp_get(tty);

	struct sixpack *sp = tty->disc_data;

	int actual;



	if (!sp)
@@ -400,7 +369,7 @@ static void sixpack_write_wakeup(struct tty_struct *tty) 
		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);

		sp->tx_enable = 0;

		netif_wake_queue(sp->dev);

		goto out;

		return;

	}



	if (sp->tx_enable) {
@@ -408,9 +377,6 @@ static void sixpack_write_wakeup(struct tty_struct *tty) 
		sp->xleft -= actual;

		sp->xhead += actual;

	}



out:

	sp_put(sp);

}



/* ----------------------------------------------------------------------- */
@@ -430,7 +396,7 @@ static void sixpack_receive_buf(struct tty_struct *tty, const u8 *cp, 
	if (!count)

		return;



	sp = sp_get(tty);

	sp = tty->disc_data;

	if (!sp)

		return;
@@ -446,7 +412,6 @@ static void sixpack_receive_buf(struct tty_struct *tty, const u8 *cp, 
	}

	sixpack_decode(sp, cp, count1);



	sp_put(sp);

	tty_unthrottle(tty);

}
@@ -561,8 +526,6 @@ static int sixpack_open(struct tty_struct *tty) 


	spin_lock_init(&sp->lock);

	spin_lock_init(&sp->rxlock);

	refcount_set(&sp->refcnt, 1);

	init_completion(&sp->dead);



	/* !!! length of the buffers. MTU is IP MTU, not PACLEN!  */
@@ -638,19 +601,11 @@ static void sixpack_close(struct tty_struct *tty) 
{

	struct sixpack *sp;



	write_lock_irq(&disc_data_lock);

	sp = tty->disc_data;

	tty->disc_data = NULL;

	write_unlock_irq(&disc_data_lock);

	if (!sp)

		return;



	/*

	 * We have now ensured that nobody can start using ap from now on, but

	 * we have to wait for all existing users to finish.

	 */

	if (!refcount_dec_and_test(&sp->refcnt))

		wait_for_completion(&sp->dead);

	tty->disc_data = NULL;



	/* We must stop the queue to avoid potentially scribbling

	 * on the free buffers. The sp->dead completion is not sufficient
@@ -673,7 +628,7 @@ static void sixpack_close(struct tty_struct *tty) 
static int sixpack_ioctl(struct tty_struct *tty, unsigned int cmd,

		unsigned long arg)

{

	struct sixpack *sp = sp_get(tty);

	struct sixpack *sp = tty->disc_data;

	struct net_device *dev;

	unsigned int tmp, err;
@@ -725,8 +680,6 @@ static int sixpack_ioctl(struct tty_struct *tty, unsigned int cmd, 
		err = tty_mode_ioctl(tty, cmd, arg);

	}



	sp_put(sp);



	return err;

}