linux-kernel - Re: [PATCH v2] procfs: fix numbering in /proc/locks

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Date:	Fri, 08 Oct 2010 10:29:10 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Matthew Wilcox <matthew@....cx>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	xiyou.wangcong@...il.com
Subject: Re: [PATCH v2] procfs: fix numbering in /proc/locks

On 10/08/2010 01:27 AM, Andrew Morton wrote:
> On Tue, 05 Oct 2010 14:14:11 +0200
> Jerome Marchand <jmarchan@...hat.com> wrote:
> 
>> On 09/30/2010 02:38 PM, Jerome Marchand wrote:
>>>
>>> The lock number in /proc/locks (first field) is implemented by a counter
>>> (private field of struct seq_file) which is incremented at each call of
>>> locks_show() and reset to 1 in locks_start() whatever the offset is. It
>>> should be reset according to the actual position in the list.
>>>
>>> Moreover, locks_show() can be called twice to print a single line thus
>>> skipping a number. The counter should be incremented in locks_next().
>>>
>>> Signed-off-by: Jerome Marchand <jmarchan@...hat.com>
>>> ---
>>>  locks.c |    4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/locks.c b/fs/locks.c
>>> index ab24d49..49d7343 100644
>>> --- a/fs/locks.c
>>> +++ b/fs/locks.c
>>> @@ -2166,19 +2166,19 @@ static int locks_show(struct seq_file *f, void *v)
>>>  	list_for_each_entry(bfl, &fl->fl_block, fl_block)
>>>  		lock_get_status(f, bfl, (long)f->private, " ->");
>>>  
>>> -	f->private++;
>>>  	return 0;
>>>  }
>>>  
>>>  static void *locks_start(struct seq_file *f, loff_t *pos)
>>>  {
>>>  	lock_kernel();
>>> -	f->private = (void *)1;
>>> +	f->private = (void *) (*pos + 1);
>>
>> That cast trigger a warning on some arch:
>> "warning: cast to pointer from integer of different size"
>>
>> There is no real risk here. At worst /proc/locks will show wrong number
>> if there is more than 2^32 locks, but should I mute the warning it with
>> something like:
>>         f->private = (void *) (size_t) (*pos + 1);
>> ?
> 
> Putting a loff_t into a void* is a pretty alarming thing to do.  If
> we're really going to do that then use a (long) cast and put a very
> good comment at the code site explaining why the bug doesn't matter, so
> people aren't misled.
> 
> But really, why sweat it?  kmalloc the eight bytes, make ->private
> point at that and we never have to think about it again.  Bonus points
> for doing this without any typecasts ;)
> 

That's definitely cleaner. I'll do that.

Thanks,
Jerome
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