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 15:25:19 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.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()

On Tue, 2 Apr 2024 18:52:12 +0530
Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com> wrote:

> 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() ?

Yep, that's what I had in mind.

> 
> 
> > 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.

Great!

Thanks,

Boris