linux-kernel - Re: [PATCH 2/2] tracefs: Don't overlay 'struct inode'

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Message-ID: <e10e465e-98d9-430f-bb71-cf4b8e046a71@grsecurity.net>
Date: Wed, 7 Aug 2024 22:24:09 +0200
From: Mathias Krause <minipli@...ecurity.net>
To: Al Viro <viro@...iv.linux.org.uk>, Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Ajay Kaher <ajay.kaher@...adcom.com>, linux-trace-kernel@...r.kernel.org,
 linux-kernel@...r.kernel.org, Ilkka Naulapää
 <digirigawa@...il.com>, Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH 2/2] tracefs: Don't overlay 'struct inode'

On 07.08.24 15:44, Al Viro wrote:
> 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()).

Jepp, sounds much better indeed and doesn't require 'struct
tracefs_inode' to have its own 'struct rcu_head' member.

Thanks,
Mathias