linux-kernel - Re: [Jfs-discussion] [PATCH] jfs: Add missing NULL pointer check in __get

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Date:   Thu, 2 Nov 2017 14:43:42 +0100
From:   Juerg Haefliger <juerg.haefliger@...onical.com>
To:     Dave Kleikamp <dave.kleikamp@...cle.com>,
        jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [Jfs-discussion] [PATCH] jfs: Add missing NULL pointer check in
 __get_metapage



On 11/02/2017 02:15 PM, Dave Kleikamp wrote:
> On 11/02/2017 01:59 AM, Juerg Haefliger wrote:
>>
>>
>> On 10/30/2017 11:13 PM, Dave Kleikamp wrote:
>>> On 10/25/2017 02:50 AM, Juerg Haefliger wrote:
>>>> Is this a patch you might consider?
>>>
>>> Sorry it's taken me so long to respond.
>>>
>>> I don't think this is the right fix. A failed allocation will still
>>> result in a null pointer dereference by the caller, __get_metapage(). I
>>> think the check needs to be put there. Like this:
>>>
>>> --- a/fs/jfs/jfs_metapage.c
>>> +++ b/fs/jfs/jfs_metapage.c
>>> @@ -663,6 +663,8 @@ struct metapage *__get_metapage(struct inode *inode,
>>> unsigned long lblock,
>>>  	} else {
>>>  		INCREMENT(mpStat.pagealloc);
>>>  		mp = alloc_metapage(GFP_NOFS);
>>> +		if (!mp)
>>> +			goto unlock;
>>>  		mp->page = page;
>>>  		mp->sb = inode->i_sb;
>>>  		mp->flag = 0;
>>
>> I don't understand. This is part of the patch that I sent.
> 
> Doh! How'd I miss that?

:-)


>>
>>
>>>
>>> Furthermore, it looks like all the callers of __get_metapage() check for
>>> a null return, so I'm not sure we need to handle the error at this
>>> point. I might have to look a bit harder at that, since there are many
>>> callers.
>>
>> I don't understand this either :-) Yes, the callers do check for a null
>> pointer but things blow up (in __get_metapage) before that check without
>> the above fix.
> 
> Yeah, the fix to __get_metapage() is necessary. I'm not convinced the
> first part of the patch, to alloc_metapage(), is necessary.

It's not. I just thought it'd be nice to get some sort of notification
in the log when the alloc fails. But if the callers log it then that's fine.

...Juerg


>>
>> ...Juerg
>>
>>
>>>
>>> Thanks,
>>> Shaggy
>>>
>>>>
>>>> Thanks
>>>> ...Juerg
>>>>
>>>>
>>>> On 10/04/2017 10:24 AM, Juerg Haefliger wrote:
>>>>> alloc_metapage can return a NULL pointer so check for that. And also emit
>>>>> an error message if that happens.
>>>>>
>>>>> Signed-off-by: Juerg Haefliger <juerg.haefliger@...onical.com>
>>>>> ---
>>>>>  fs/jfs/jfs_metapage.c | 20 +++++++++++++-------
>>>>>  1 file changed, 13 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/fs/jfs/jfs_metapage.c b/fs/jfs/jfs_metapage.c
>>>>> index 1c4b9ad4d7ab..00f21af66872 100644
>>>>> --- a/fs/jfs/jfs_metapage.c
>>>>> +++ b/fs/jfs/jfs_metapage.c
>>>>> @@ -187,14 +187,18 @@ static inline struct metapage *alloc_metapage(gfp_t gfp_mask)
>>>>>  {
>>>>>  	struct metapage *mp = mempool_alloc(metapage_mempool, gfp_mask);
>>>>>  
>>>>> -	if (mp) {
>>>>> -		mp->lid = 0;
>>>>> -		mp->lsn = 0;
>>>>> -		mp->data = NULL;
>>>>> -		mp->clsn = 0;
>>>>> -		mp->log = NULL;
>>>>> -		init_waitqueue_head(&mp->wait);
>>>>> +	if (!mp) {
>>>>> +		jfs_err("mempool_alloc failed!\n");
>>>>> +		return NULL;
>>>>>  	}
>>>>> +
>>>>> +	mp->lid = 0;
>>>>> +	mp->lsn = 0;
>>>>> +	mp->data = NULL;
>>>>> +	mp->clsn = 0;
>>>>> +	mp->log = NULL;
>>>>> +	init_waitqueue_head(&mp->wait);
>>>>> +
>>>>>  	return mp;
>>>>>  }
>>>>>  
>>>>> @@ -663,6 +667,8 @@ struct metapage *__get_metapage(struct inode *inode, unsigned long lblock,
>>>>>  	} else {
>>>>>  		INCREMENT(mpStat.pagealloc);
>>>>>  		mp = alloc_metapage(GFP_NOFS);
>>>>> +		if (!mp)
>>>>> +			goto unlock;
>>>>>  		mp->page = page;
>>>>>  		mp->sb = inode->i_sb;
>>>>>  		mp->flag = 0;
>>>>>
>>



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