linux-kernel - Re: [PATCH] drm/panthor: Fix NULL vs IS_ERR() bug in panthor_ioctl_tiler_heap

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Date: Tue, 2 Apr 2024 18:52:12 +0530
From: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
To: Boris Brezillon <boris.brezillon@...labora.com>
Cc: Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Heiko Stuebner <heiko@...ech.de>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dan.carpenter@...aro.org,
        kernel-janitors@...r.kernel.org, error27@...il.com
Subject: Re: [PATCH] drm/panthor: Fix NULL vs IS_ERR() bug in
 panthor_ioctl_tiler_heap_destroy()

Hello Boris,

On 02/04/24 18:03, Boris Brezillon wrote:
> Hello Harshit,
> 
> On Tue,  2 Apr 2024 03:33:58 -0700
> Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com> wrote:
> 
>> panthor_vm_get_heap_pool() returns ERR_PTR on failure.
>>
>> Fixes: 4bdca1150792 ("drm/panthor: Add the driver frontend block")
>> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
>> ---
>> This is spotted by smatch and the patch is only compile tested
>> ---
>>   drivers/gpu/drm/panthor/panthor_drv.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/panthor/panthor_drv.c b/drivers/gpu/drm/panthor/panthor_drv.c
>> index 11b3ccd58f85..050b905b0453 100644
>> --- a/drivers/gpu/drm/panthor/panthor_drv.c
>> +++ b/drivers/gpu/drm/panthor/panthor_drv.c
>> @@ -1090,8 +1090,8 @@ static int panthor_ioctl_tiler_heap_destroy(struct drm_device *ddev, void *data,
>>   		return -EINVAL;
>>   
>>   	pool = panthor_vm_get_heap_pool(vm, false);
>> -	if (!pool) {
>> -		ret = -EINVAL;
>> +	if (IS_ERR(pool)) {
>> +		ret = PTR_ERR(pool);
> 
> Actually, panthor_vm_get_heap_pool() will return NULL if there's no
> heap pool attached to this VM and create=false, so this was correct.
> This being said, I'm fine making that consistent by returning
> ERR_PTR(-ENOENT) instead of NULL in that case. This way we don't have
> two different semantics based on the 'create' value.
> 

Thanks for explaining. I missed the case where create is false and there 
is no heap pool attached, so panthor_vm_get_heap_pool() can return NULL.

1878  *
1879  * Return: A valid pointer on success, an ERR_PTR() otherwise.
1880  */
1881 struct panthor_heap_pool *panthor_vm_get_heap_pool(struct 
panthor_vm *vm, bool create)

The documentation says it returns ERR_PTR() on failure, so is it worth 
doing something like:

diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c 
b/drivers/gpu/drm/panthor/panthor_mmu.c
index fdd35249169f..e1285cdb09ff 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -1893,6 +1893,8 @@ struct panthor_heap_pool 
*panthor_vm_get_heap_pool(struct panthor_vm *vm, bool c
                         vm->heaps.pool = panthor_heap_pool_get(pool);
         } else {
                 pool = panthor_heap_pool_get(vm->heaps.pool);
+               if (!pool)
+                       pool = ERR_PTR(-ENOENT);
         }
         mutex_unlock(&vm->heaps.lock);


and change all callers of panthor_vm_get_heap_pool() to only check for 
IS_ERR() ?


> Oh, and please merge everything into a single patch instead of one patch
> per call-site.
> 

Sure, I noticed one after the other. I will fix them together in v2.

Thanks,
Harshit
> Regards,
> 
> Boris
> 
>>   		goto out_put_vm;
>>   	}
>>   
>