linux-kernel - Re: [PATCH] char: misc: make misc_open() and misc

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Date:   Mon, 4 Jul 2022 13:57:56 +0000
From:   Wedson Almeida Filho <wedsonaf@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] char: misc: make misc_open() and misc_register() killable

On Mon, Jul 04, 2022 at 10:48:32PM +0900, Tetsuo Handa wrote:
> On 2022/07/04 21:59, Wedson Almeida Filho wrote:
> >> @@ -139,6 +139,10 @@ static int misc_open(struct inode *inode, struct file *file)
> >>  
> >>  	err = 0;
> >>  	replace_fops(file, new_fops);
> >> +	if (iter->unlocked_open && file->f_op->open) {
> >> +		mutex_unlock(&misc_mtx);
> >> +		return file->f_op->open(inode, file);
> >> +	}
> > 
> > One of the invariants of miscdev is that once misc_deregister() returns,
> > no new calls to f_op->open() are made. (Although, of course, you can
> > still have open files but that's a whole different problem.)
> 
> The point of this change is that file->f_op after mutex_unlock(&misc_mtx) is
>  from new_fops which is guaranteed to hold a ref on "struct file_operations"
> via new_fops = fops_get("struct miscdevice"->fops).
> That is, a module ref remains valid even after mutex_unlock(&misc_mtx).
> 
> And as with major_names_lock case quoted below, this change assumes that
> misc_deregister() is called from module's __exit function, and fops_get()
> is preventing the module owning new_fops from calling __exit function.

Your assumption is not sound. misc_deregister() can be (and is)
legitimately called from other places, for example, a driver's remove()
callback. In fact, when I grep for misc_deregister(), the second
instance is such a case.

Anyway, I agree that something should be done about the issue you
report, I'm just arguing that this proposal isn't a good one because it
breaks an invariant that makes it really hard for drivers to properly
cleanup.

> 
> +		if ((*n)->major != major || !(*n)->probe)
> +			continue;
> +		if (!try_module_get((*n)->owner))
> +			break;
> +		/*
> +		 * Calling probe function with major_names_lock held causes
> +		 * circular locking dependency problem. Thus, call it after
> +		 * releasing major_names_lock.
> +		 */
> +		probe_fn = (*n)->probe;
> +		mutex_unlock(&major_names_lock);
> +		/*
> +		 * Assuming that unregister_blkdev() is called from module's
> +		 * __exit function, a module refcount taken above allows us
> +		 * to safely call probe function without major_names_lock held.
> +		 */
> +		probe_fn(devt);
> +		module_put((*n)->owner);
> +		return;
>