fsi: Aspeed: Fix a potential double free

'aspeed' is a devm_alloc'ed, so there is no need to free it explicitly or
there will be a double free().

Remove the 'release' function that is wrong and unneeded.

Fixes: 606397d67f41 ("fsi: Add ast2600 master driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
This patch is completely theoretical. It looks good to me, but there is a
little too much indirections for me. I'm also not that familiar with
fixing issue related to 'release' function...

... So review with care :)
---
 drivers/fsi/fsi-master-aspeed.c | 9 ---------
 1 file changed, 9 deletions(-)

Le 27/12/2021 à 07:29, Greg KH a écrit :
> On Sun, Dec 26, 2021 at 05:56:02PM +0100, Christophe JAILLET wrote:
>> 'aspeed' is a devm_alloc'ed, so there is no need to free it explicitly or
>> there will be a double free().
> 
> A struct device can never be devm_alloced for obvious reasons.  Perhaps
> that is the real problem here?

Thanks for the feed-back.

This goes beyond my knowledge of how this should work.
As I can not test myself, I won't be of any help.
I'll let you or anyone else check if something needs to be fixed, and 
how to fix it properly.

Just take my patch as a "Hey! Looks strange to have a kfree() in a 
driver that only call devm_kzalloc() to allocate memory. S.o. should 
give a deeper look at it". :)

CJ

> 
>> Remove the 'release' function that is wrong and unneeded.
>>
>> Fixes: 606397d67f41 ("fsi: Add ast2600 master driver")
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
>> ---
>> This patch is completely theoretical. It looks good to me, but there is a
>> little too much indirections for me. I'm also not that familiar with
>> fixing issue related to 'release' function...
>>
>> ... So review with care :)
>> ---
>>   drivers/fsi/fsi-master-aspeed.c | 9 ---------
>>   1 file changed, 9 deletions(-)
>>
>> diff --git a/drivers/fsi/fsi-master-aspeed.c b/drivers/fsi/fsi-master-aspeed.c
>> index 8606e55c1721..4a745ccb60cf 100644
>> --- a/drivers/fsi/fsi-master-aspeed.c
>> +++ b/drivers/fsi/fsi-master-aspeed.c
>> @@ -373,14 +373,6 @@ static int aspeed_master_break(struct fsi_master *master, int link)
>>   	return aspeed_master_write(master, link, 0, addr, &cmd, 4);
>>   }
>>   
>> -static void aspeed_master_release(struct device *dev)
>> -{
>> -	struct fsi_master_aspeed *aspeed =
>> -		to_fsi_master_aspeed(dev_to_fsi_master(dev));
>> -
>> -	kfree(aspeed);
>> -}
>> -
>>   /* mmode encoders */
>>   static inline u32 fsi_mmode_crs0(u32 x)
>>   {
>> @@ -603,7 +595,6 @@ static int fsi_master_aspeed_probe(struct platform_device *pdev)
>>   	dev_info(&pdev->dev, "hub version %08x (%d links)\n", reg, links);
>>   
>>   	aspeed->master.dev.parent = &pdev->dev;
>> -	aspeed->master.dev.release = aspeed_master_release;
> 
> Odd, then what deletes this device structure when the release function
> wants to be called?  You should have gotten a big warning from the
> kernel when removing the device from the system at runtime, did you test
> this somehow?
> 
> This does not look correct at all.
> 
> greg k-h
>

Le 06/01/2022 à 18:25, Guenter Roeck a écrit :
> On 1/6/22 12:14 AM, Dan Carpenter wrote:
>> On Mon, Dec 27, 2021 at 07:29:07AM +0100, Greg KH wrote:
>>> On Sun, Dec 26, 2021 at 05:56:02PM +0100, Christophe JAILLET wrote:
>>>> 'aspeed' is a devm_alloc'ed, so there is no need to free it 
>>>> explicitly or
>>>> there will be a double free().
>>>
>>> A struct device can never be devm_alloced for obvious reasons.  Perhaps
>>> that is the real problem here?
>>>
>>
>> I don't understand how "aspeed" is a struct device.
>>
> 
> -static void aspeed_master_release(struct device *dev)
> -{
> -    struct fsi_master_aspeed *aspeed =
> -        to_fsi_master_aspeed(dev_to_fsi_master(dev));
> -
> -    kfree(aspeed);
> -}
> 
> So "dev" is embedded in struct fsi_master, and struct fsi_master is 
> embedded
> in struct fsi_master_aspeed. Since "struct device" is embedded, the data
> structure embedding it must be released with the release function, as is 
> done
> here. The problem is indeed that the data structure is allocated with
> devm_kzalloc(), which as Greg points out must not be devm_ allocated
> (because its lifetime does not match the lifetime of devm_ allocated
> memory).

Thanks a lot for the detailed explanation.
Crystal clear for me now.

Do you want me to send a patch to remove the devm_ or will you?

CJ

> 
>> I've been working on understanding device managed memory recently for
>> Smatch.  It's really complicated.  There are a bunch of rules/heuristics
>> that I'm slowly creating to generate new warnings but I'm a long way
>> from understanding it well myself.
>>
> 
> A data structure embedding struct device must not be devm_ allocated,
> and it must be released with the release callback. Maybe there is
> a means to flag that somehow ?
> 
> Guenter
>