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

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Date:   Thu, 2 Nov 2017 08:15:36 -0500
From:   Dave Kleikamp <dave.kleikamp@...cle.com>
To:     Juerg Haefliger <juerg.haefliger@...onical.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 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.

> 
> ...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;
>>>>
>