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	Wed, 07 Aug 2024 13:44:53 +0000
Date: Wed, 7 Aug 2024 14:44:53 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathias Krause <minipli@grsecurity.net>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Ajay Kaher <ajay.kaher@broadcom.com>,
	linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ilkka =?iso-8859-1?Q?Naulap=E4=E4?= <digirigawa@gmail.com>,
	Brad Spengler <spender@grsecurity.net>
Subject: Re: [PATCH 2/2] tracefs: Don't overlay 'struct inode'
Message-ID: <20240807134453.GZ5334@ZenIV>
References: <20240807115143.45927-1-minipli@grsecurity.net>
 <20240807115143.45927-3-minipli@grsecurity.net>
 <20240807093545.4ec51d61@gandalf.local.home>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Wed, Aug 07, 2024 at 09:35:45AM -0400, Steven Rostedt wrote:

> Perhaps:
> 
> diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
> index f704d8348357..ab6d6c3d835d 100644
> --- a/fs/tracefs/internal.h
> +++ b/fs/tracefs/internal.h
> @@ -10,12 +10,12 @@ enum {
>  };
>  
>  struct tracefs_inode {
> +	struct inode            vfs_inode;
> +	/* The below gets initialized with memset_after(ti, 0, vfs_inode) */
>  	union {
> -		struct inode            vfs_inode;
> +		struct list_head	list;
>  		struct rcu_head		rcu;
>  	};
> -	/* The below gets initialized with memset_after(ti, 0, vfs_inode) */
> -	struct list_head	list;
>  	unsigned long           flags;
>  	void                    *private;
>  };

	Your current variant gives you an RCU-delayed call of
tracefs_free_inode(), which schedules an RCU-delayed call of
tracefs_free_inode_rcu().

	Do you really need that double RCU delay to start with?
Because if you do not, just do that list_del_rcu() in ->destroy_inode()
(which is called without an RCU delay) and have kmem_cache_free()
in ->free_inode() (which is called *with* RCU delay started after
the call of ->destroy_inode()).