Re: [PATCH] drm/managed: Fix off-by-one in warning

[Sam Ravnborg <sam@ravnborg.org>]

On Sat, Mar 28, 2020 at 11:02:26PM +0100, Daniel Vetter wrote:
> On Sat, Mar 28, 2020 at 7:49 PM Sam Ravnborg <sam@ravnborg.org> wrote:
> >
> > Hi Daniel.
> >
> > On Sat, Mar 28, 2020 at 05:23:58PM +0100, Daniel Vetter wrote:
> > > I'm thinking this is the warning that fired in the 0day report, but I
> > > can't double-check yet since 0day didn't upload its source tree
> > > anywhere I can check. And all the drivers I can easily test don't use
> > > drm_dev_alloc anymore ...
> > >
> > > Also if I'm correct supreme amounts of bad luck because usually kslap
> > > (for bigger structures) gives us something quite a bit bigger than
> > > what we asked for.
> > >
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Fixes: c6603c740e0e ("drm: add managed resources tied to drm_device")
> > > Cc: Sam Ravnborg <sam@ravnborg.org>
> > > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > > Cc: Dan Carpenter <dan.carpenter@oracle.com>
> > > Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > Cc: Neil Armstrong <narmstrong@baylibre.com
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > ---
> > >  drivers/gpu/drm/drm_managed.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_managed.c b/drivers/gpu/drm/drm_managed.c
> > > index 4955241ceb4c..9cebfe370a65 100644
> > > --- a/drivers/gpu/drm/drm_managed.c
> > > +++ b/drivers/gpu/drm/drm_managed.c
> > > @@ -139,8 +139,7 @@ void drmm_add_final_kfree(struct drm_device *dev, void *container)
> > >  {
> > >       WARN_ON(dev->managed.final_kfree);
> > >       WARN_ON(dev < (struct drm_device *) container);
> > > -     WARN_ON(dev + 1 >=
> > > -             (struct drm_device *) (container + ksize(container)));
> > > +     WARN_ON(dev + 1 > (struct drm_device *) (container + ksize(container)));
> >
> > I do not think this is the right fix...
> > The original code would trigger if
> > 1) the container only had a drm_device - and nothing else
> > 2) and the allocated size was the same
> 
> Yup, which apparently happens for all the drivers calling
> drm_dev_alloc(). At least on the unlucky architecture that 0day tested
> on (or build settings, or whatever). The issue was hit with drm/bochs,
> which is still using drm_dev_alloc (like most older-ish drivers).

That explains it and then the checks makes sense.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>


	Sam
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