Re: [PATCH] drm/gem: Fix a GEM leak in drm_gem_get_unmapped_area()

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 8 Jan 2026 15:04:33 +0100
Loïc Molinari <loic.molinari@collabora.com> wrote:

> On 08/01/2026 14:43, Boris Brezillon wrote:
> > On Thu, 8 Jan 2026 14:25:01 +0100
> > Loïc Molinari <loic.molinari@collabora.com> wrote:
> >   
> >> On 07/01/2026 12:12, Boris Brezillon wrote:  
> >>> On Tue,  6 Jan 2026 17:49:35 +0100
> >>> Boris Brezillon <boris.brezillon@collabora.com> wrote:
> >>>      
> >>>> drm_gem_object_lookup_at_offset() can return a valid object with
> >>>> filp or filp->f_op->get_unmapped_area set to NULL. Make sure we still
> >>>> release the ref we acquired on such objects.
> >>>>
> >>>> Cc: Loïc Molinari <loic.molinari@collabora.com>
> >>>> Fixes: 99bda20d6d4c ("drm/gem: Introduce drm_gem_get_unmapped_area() fop")
> >>>> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> >>>> ---
> >>>>    drivers/gpu/drm/drm_gem.c | 10 ++++++----
> >>>>    1 file changed, 6 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> >>>> index 36c8af123877..f7cbf6e8d1e0 100644
> >>>> --- a/drivers/gpu/drm/drm_gem.c
> >>>> +++ b/drivers/gpu/drm/drm_gem.c
> >>>> @@ -1298,11 +1298,13 @@ unsigned long drm_gem_get_unmapped_area(struct file *filp, unsigned long uaddr,
> >>>>    	unsigned long ret;
> >>>>    
> >>>>    	obj = drm_gem_object_lookup_at_offset(filp, pgoff, len >> PAGE_SHIFT);
> >>>> -	if (IS_ERR(obj) || !obj->filp || !obj->filp->f_op->get_unmapped_area)
> >>>> -		return mm_get_unmapped_area(filp, uaddr, len, 0, flags);
> >>>> +	if (IS_ERR(obj))
> >>>> +		obj = NULL;
> >>>>    
> >>>> -	ret = obj->filp->f_op->get_unmapped_area(obj->filp, uaddr, len, 0,
> >>>> -						 flags);
> >>>> +	if (!obj || !obj->filp || !obj->filp->f_op->get_unmapped_area)
> >>>> +		ret = mm_get_unmapped_area(filp, uaddr, len, 0, flags);  
> >>>
> >>> Also, I'm wondering if we shouldn't pass pgoff instead of zero here to
> >>> have the exact same behavior that existed before
> >>> drm_gem_get_unmapped_area() was introduced.
> >>>      
> >>
> >> For mappings with a file (the DRM file in our case), if
> >> filp->f_op->get_unmapped_area isn't set then generic_get_unmapped_area()
> >> is used and it ignores the pgoff argument.  
> > 
> > That's true for architectures that rely on the default implementation
> > (Arm64 for instance), but other architectures might have their own
> > implementation.
> > 
> >   
> >> So it wasn't 0 before but was
> >> ignored anyway.  
> > 
> > Didn't check all of them but the Arm implementation does take this
> > pgoff into account if I read the code correctly [1]. The fact this
> > argument is passed around makes me think other architectures might take
> > this into account too. I know this pgoff is just a fake offset into the
> > /dev/dri pseudo-file, but if we want to err on the safe side, we'd
> > rather do exactly what was done before drm_gem_get_unmapped_area() was
> > introduced for the ->filp==NULL case. That's just my 2 cts, of course.  
> 
> You're right, I focused on arm64 here and there's obviously other archs 
> so we'd better pass pgoff for the fallback case.

Do you want me to send a patch doing that, or should I take care of it?