linux-kernel - Re: [PATCH] LinuxPPS - PPS support for Linux

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Date:	Wed, 24 Oct 2007 09:00:31 +0200
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] LinuxPPS - PPS support for Linux

On Tue, Oct 23, 2007 at 02:08:19PM -0700, Roland Dreier wrote:
> A few comments:
> 
>  > +		dev_err(port->dev, "PPS support disabled due port \"%s\" is "
>  > +					"in polling mode\n",
> 
> I think "because" instead of "due" is closer to standard English.

Fixed.

>  > +		printk(KERN_ERR "pps: %s: too much PPS sources in the system\n",
>  > +					info->name);
> 
> Similarly should be "many" instead of "much".

Fixed.

>  > +	/* Get new ID for the new PPS source */
>  > +	if (idr_pre_get(&pps_idr, GFP_KERNEL) == 0) {
>  > +		err = -ENOMEM;
>  > +		goto kfree_pps;
>  > +	}
>  > +
>  > +	spin_lock_irq(&idr_lock);
>  > +	err = idr_get_new(&pps_idr, pps, &id);
>  > +	spin_unlock_irq(&idr_lock);
>  > +
>  > +	if (err < 0)
>  > +		goto kfree_pps;
> 
> You usually can handle idr_get_new() returning -EAGAIN by jumping back
> to the idr_pre_get(), to handle someone else coming in and stealing
> the memory you just preallocated.  In this case it may not matter
> since it's pretty unlikely that a lot of contexts are using the idr at
> the same time.  But anyway...

I don't understand what you mean. Can you please submit an example
code?

>  > +void pps_unregister_source(int source)
>  > ...
>  > +	wait_event(pps->usage_queue, atomic_read(&pps->usage) == 0);
>  > +
>  > +	pps_sysfs_remove_source_entry(pps);
>  > +	pps_unregister_cdev(pps);
>  > +	kfree(pps);
> 
> This reference counting looks dubious to me... later on in the code
> you have:
> 
>  > +static int pps_cdev_open(struct inode *inode, struct file *file)
>  > +{
>  > +	struct pps_device *pps = container_of(inode->i_cdev,
>  > +						struct pps_device, cdev);
>  > +
>  > +	/* Lock the PPS source against (possible) deregistration */
>  > +	atomic_inc(&pps->usage);
> 
> with no locking, so I see no reason why the atomic_inc() couldn't
> happen right after the wait_event() sees a count of 0 and lets the
> deregistration continue.  Which would lead to use-after-free.

Mmm... you are right... can you please suggest to me how can I easily
fix this problem?

Ciao,

Rodolfo

-- 

GNU/Linux Solutions                  e-mail:    giometti@...eenne.com
Linux Device Driver                             giometti@...dd.com
Embedded Systems                     		giometti@...ux.it
UNIX programming                     phone:     +39 349 2432127
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