linux-kernel - Re: [PATCH v5 1/8] debugfs: prevent access to possibly dead file

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Date:	Thu, 10 Mar 2016 22:59:02 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/8] debugfs: prevent access to possibly dead file_operations at file open

On Sun, Mar 06 2016, Nicolai Stange <nicstange@...il.com> wrote:

> + *
> + * Allow any ongoing concurrent call into debugfs_remove() or
> + * debugfs_remove_recursive() blocked by a former call to
> + * debugfs_use_file_start() to proceed and return to its caller.
> + */
> +static void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu)
> +{
> +	srcu_read_unlock(&debugfs_srcu, srcu_idx);
> +}
> +
> +#define F_DENTRY(filp) ((filp)->f_path.dentry)
> +
> +#define REAL_FOPS_DEREF(dentry)					\
> +	((const struct file_operations *)(dentry)->d_fsdata)
> +
> +static int open_proxy_open(struct inode *inode, struct file *filp)
> +{
> +	const struct dentry *dentry = F_DENTRY(filp);
> +	const struct file_operations *real_fops = NULL;
> +	int srcu_idx, r;
> +
> +	r = debugfs_use_file_start(dentry, &srcu_idx);
> +	if (r) {
> +		debugfs_use_file_finish(srcu_idx);
> +		r = -ENOENT;
> +		goto out;

this...

> +out:
> +	fops_put(real_fops);
> +	debugfs_use_file_finish(srcu_idx);

... and that seems like an obvious double unlock?

Rasmus