linux-kernel - Re: [PATCH v2] remove no longer use of pdflush interface

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Date:	Wed, 6 Jun 2012 11:21:27 +0800
From:	Wanpeng Li <liwp.linux@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Rob Landley <rob@...dley.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	David Howells <dhowells@...hat.com>,
	James Morris <james.l.morris@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Michel Lespinasse <walken@...gle.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Gavin Shan <shangw@...ux.vnet.ibm.com>,
	Wanpeng Li <liwp.linux@...il.com>
Subject: Re: [PATCH v2] remove no longer use of pdflush interface

On Tue, Jun 05, 2012 at 03:28:45PM -0700, Andrew Morton wrote:


Hi Andrew,
>I do agree with the intent of the patch and the creation of
>proc_deprecated() seems a good idea - something we can use in the
>future.

Yes, actually I think many interfaces in /proc which will be removed in
the near future can take advantage of this function.

>
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1095,11 +1095,9 @@ static struct ctl_table vm_table[] = {
>>  		.extra1		= &zero,
>>  	},
>>  	{
>> -		.procname	= "nr_pdflush_threads",
>> -		.data		= &nr_pdflush_threads,
>> -		.maxlen		= sizeof nr_pdflush_threads,
>> -		.mode		= 0444 /* read-only*/,
>> -		.proc_handler	= proc_dointvec,
>> +		.procname       = "nr_pdflush_threads",
>> +		.mode           = 0444 /* read-only */,
>> +		.proc_handler   = proc_deprecated,
>>  	},
>>  	{
>>  		.procname	= "swappiness",
>> @@ -2505,6 +2503,15 @@ int proc_doulongvec_ms_jiffies_minmax(struct ctl_table *table, int write,
>>  
>>  #endif /* CONFIG_PROC_SYSCTL */
>>  
>> +/* notice associated proc deprecated */
>> +int proc_deprecated(struct ctl_table *table, int write,
>> +		    void __user *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> +	printk(KERN_WARNING "%s exported in /proc is deprecated\n",
>> +			table->procname);
>> +	return -ENOSYS;
>> +}
>
>I see a couple of things here.
>
>Firstly, I'd change the text from "deprecated" to "is scheduled for
>removal".  Which implies that the function should be called
>proc_obsolete().
>
>
>Secondly, this code will permit unprivileged users to flood the logs,
>by repeatedly reading /proc/sys/vm/nr_pdflush_threads.  We try to avoid
>this, as it is a form of denial-of-service attack.
>
>This is a bit hard to fix.  The typical way of addressing this is to
>use printk_once(), so the message only appears once per boot.  But that
>doesn't work for a generic function - we'd need to add one bit of state
>to the ctl_table to do this.  We can of course do that, but it's not
>obvious that it's _worth_ doing that just for handling obsolete
>entries.
>
>So perhaps the solution is to give up on the generic proc_obsolete()
>idea, and just add a handler specifically for nr_pdflush_threads, whcih
>uses printk_once().

What about modify the generic proc_obsolete just to put the warning message into
the buffer, then transfer to userspace, in order to users can see this warning.Do
you think this is a better idea?

Regards,
Wanpeng

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