linux-kernel - Re: [PATCH] staging: bcm2835-audio: Fix memory corruption

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Date:   Thu, 10 Aug 2017 11:52:42 +0100
From:   Phil Elwell <phil@...pberrypi.org>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Eric Anholt <eric@...olt.net>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Aishwarya Pant <aishpant@...il.com>,
        linux-rpi-kernel@...ts.infradead.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: bcm2835-audio: Fix memory corruption

On 10/08/2017 11:21, Dan Carpenter wrote:
> The original patch did not go through the normal review process...
> 
> On Tue, Aug 08, 2017 at 01:05:02PM +0100, Phil Elwell wrote:
>> I'm all for fixing memory leaks, but freeing a block while it is still
>> being used is a recipe for hard-to-debug kernel exceptions.
>>
> 
> This bug completely breaks the driver doesn't it?  It's not very subtle
> so it should be easy to diagnose with git bisect.

It's more subtle than that - the failure isn't consistent, sometimes crashing
and sometimes not depending on how and when memory is reused.

>> 1) There is already a vchi method for freeing the instance, so use it.
>> 2) Only call it on error, and then only before initted is false.
>>
>> Signed-off-by: Phil Elwell <phil@...pberrypi.org>
>> Fixes: 0adbfd4694c2 ("staging: bcm2835-audio: fix memory leak in bcm2835_audio_open_connection()")
>> ---
>>  drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c
>> index 5f3d8f2..89f96f3 100644
>> --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c
>> +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c
>> @@ -409,6 +409,7 @@ static int bcm2835_audio_open_connection(struct bcm2835_alsa_stream *alsa_stream
>>  			LOG_ERR("%s: failed to connect VCHI instance (ret=%d)\n",
>>  				__func__, ret);
>>  
>> +			vchi_disconnect(vchi_instance);
> 
> This is ugly because why are we calling disconnect() if connect() fails?
> These functions should be symetric so disconnect only disconnects and
> we call a different function to undo vchi_initialise().

Agreed - I'm not going to change the API, but I can add a comment.

>>  			ret = -EIO;
>>  			goto err_free_mem;
> 
> This label name is out of date.  There is a later error path where
> vc_vchi_audio_init() fails and we leak on that path.

Also agreed. I'll rework it.

> Also why is
> vchi_instance a static variable?  Arglebargleblargleblah...

You may well think that. I couldn't possibly comment.

Phil