Re: [PATCH v2 1/2] tty: add missing rcu_read_lock for task_pgrp

[Peter Hurley <peter@hurleysoftware.com>]

On 06/27/2015 08:51 PM, Patrick Donnelly wrote:
> task_pgrp requires an rcu or tasklist lock to be obtained if the returned pid
> is to be dereferenced, which kill_pgrp does. Obtain an RCU lock for the
> duration of use.

kill_pgrp() obtains tasklist_lock, so I don't see an unsafe deref.

> Signed-off-by: Patrick Donnelly <batrick@batbytes.com>
> ---
>  drivers/tty/tty_io.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 57fc6ee..fbb55db 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -388,33 +388,39 @@ EXPORT_SYMBOL_GPL(tty_find_polling_driver);
>  int tty_check_change(struct tty_struct *tty)
>  {
>  	unsigned long flags;
> +	struct pid *pgrp;
>  	int ret = 0;
>  
>  	if (current->signal->tty != tty)
>  		return 0;
>  
> -	spin_lock_irqsave(&tty->ctrl_lock, flags);
> +	rcu_read_lock();
> +	pgrp = task_pgrp(current);
>  
> +	spin_lock_irqsave(&tty->ctrl_lock, flags);
>  	if (!tty->pgrp) {
>  		printk(KERN_WARNING "tty_check_change: tty->pgrp == NULL!\n");
> -		goto out_unlock;
> +		goto out_irqunlock;
>  	}
> -	if (task_pgrp(current) == tty->pgrp)
> -		goto out_unlock;
> +	if (pgrp == tty->pgrp)
> +		goto out_irqunlock;
>  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
> +
>  	if (is_ignored(SIGTTOU))
> -		goto out;
> +		goto out_rcuunlock;
>  	if (is_current_pgrp_orphaned()) {
>  		ret = -EIO;
> -		goto out;
> +		goto out_rcuunlock;
>  	}
> -	kill_pgrp(task_pgrp(current), SIGTTOU, 1);
> +	kill_pgrp(pgrp, SIGTTOU, 1);
> +	rcu_read_unlock();
>  	set_thread_flag(TIF_SIGPENDING);
>  	ret = -ERESTARTSYS;
> -out:
>  	return ret;
> -out_unlock:
> +out_irqunlock:
>  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
> +out_rcuunlock:
> +	rcu_read_unlock();
>  	return ret;
>  }
>  
>