linux-kernel - Re: [PATCH v3] tty: add missing rcu_read_lock for task

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Date:	Sat, 11 Jul 2015 22:05:45 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Patrick Donnelly <batrick@...bytes.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Jiri Slaby <jslaby@...e.cz>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] tty: add missing rcu_read_lock for task_pgrp

On 06/29/2015 07:59 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.
> 
> Signed-off-by: Patrick Donnelly <batrick@...bytes.com>
> ---
>  drivers/tty/n_tty.c  | 12 ++++++++++--
>  drivers/tty/tty_io.c | 17 ++++++++++++-----
>  2 files changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index c9c27f6..0d631f8 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -2137,6 +2137,8 @@ extern ssize_t redirected_tty_write(struct file *, const char __user *,
>  
>  static int job_control(struct tty_struct *tty, struct file *file)
>  {
> +	struct pid *pgrp;
> +
>  	/* Job control check -- must be done at start and after
>  	   every sleep (POSIX.1 7.1.1.4). */
>  	/* NOTE: not yet done after every sleep pending a thorough
> @@ -2146,18 +2148,24 @@ static int job_control(struct tty_struct *tty, struct file *file)
>  	    current->signal->tty != tty)
>  		return 0;
>  
> +	rcu_read_lock();
> +	pgrp = task_pgrp(current);
> +
>  	spin_lock_irq(&tty->ctrl_lock);
> +
>  	if (!tty->pgrp)
>  		printk(KERN_ERR "n_tty_read: no tty->pgrp!\n");
> -	else if (task_pgrp(current) != tty->pgrp) {
> +	else if (pgrp != tty->pgrp) {
>  		spin_unlock_irq(&tty->ctrl_lock);
>  		if (is_ignored(SIGTTIN) || is_current_pgrp_orphaned())
>  			return -EIO;

I just realized there's a missing rcu_read_unlock() from this early return.

Regards,
Peter Hurley


> -		kill_pgrp(task_pgrp(current), SIGTTIN, 1);
> +		kill_pgrp(pgrp, SIGTTIN, 1);
> +		rcu_read_unlock();
>  		set_thread_flag(TIF_SIGPENDING);
>  		return -ERESTARTSYS;
>  	}
>  	spin_unlock_irq(&tty->ctrl_lock);
> +	rcu_read_unlock();
>  	return 0;
>  }
>  
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 57fc6ee..6bdfb98 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -388,33 +388,40 @@ 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;
>  
> +	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;
>  	}
> -	if (task_pgrp(current) == tty->pgrp)
> +	if (pgrp == tty->pgrp)
>  		goto out_unlock;
>  	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:
>  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
> +out_rcuunlock:
> +	rcu_read_unlock();
>  	return ret;
>  }
>  
> 

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