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 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.

Regards,

Boris

[1]https://elixir.bootlin.com/linux/v6.18.3/source/arch/arm/mm/mmap.c#L30