linux-kernel - Re: [PATCH 6/6] eventfs: clean up dentry ops and add revalidate function

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Date: Wed, 31 Jan 2024 01:12:46 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] eventfs: clean up dentry ops and add revalidate
 function

On Tue, Jan 30, 2024 at 11:03:55AM -0800, Linus Torvalds wrote:

> +void eventfs_d_release(struct dentry *dentry)
>  {
> -	struct eventfs_inode *ei;
> -
> -	mutex_lock(&eventfs_mutex);
> -	ei = dentry->d_fsdata;
> -	if (ei) {
> -		dentry->d_fsdata = NULL;
> -		put_ei(ei);
> -	}
> -	mutex_unlock(&eventfs_mutex);
> +	put_ei(dentry->d_fsdata);
>  }

I'd rather pass ->d_fsdata to that sucker (or exposed put_ei(),
for that matter).

> @@ -857,6 +847,5 @@ void eventfs_remove_events_dir(struct eventfs_inode *ei)
>  	 * sticks around while the other ei->dentry are created
>  	 * and destroyed dynamically.
>  	 */
> -	simple_recursive_removal(dentry, NULL);

That also needs to move earlier in the series - bisect hazard.

> + *
> + * Note that d_revalidate is called potentially under RCU,
> + * so it can't take the eventfs mutex etc. It's fine - if
> + * we open a file just as it's marked dead, things will
> + * still work just fine, and just see the old stale case.

Looks like use after free, unless freeing ei is RCU-delayed...

> +	return !(ei && ei->is_freed);