Re: [PATCH v8 3/7] drm: lock drm_global_mutex earlier in the ioctl handler

[Desmond Cheong Zhi Xi]

On 26/8/21 5:58 pm, Daniel Vetter wrote:
> On Thu, Aug 26, 2021 at 10:01:18AM +0800, Desmond Cheong Zhi Xi wrote:
> > In a future patch, a read lock on drm_device.master_rwsem is
> > held in the ioctl handler before the check for ioctl
> > permissions. However, this inverts the lock hierarchy of
> > drm_global_mutex --> master_rwsem.
> >
> > To avoid this, we do some prep work to grab the drm_global_mutex
> > before checking for ioctl permissions.
> >
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_ioctl.c | 18 +++++++++---------
> >   1 file changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> > index d25713b09b80..158629d88319 100644
> > --- a/drivers/gpu/drm/drm_ioctl.c
> > +++ b/drivers/gpu/drm/drm_ioctl.c
> > @@ -772,19 +772,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
> >   	if (drm_dev_is_unplugged(dev))
> >   		return -ENODEV;
> >   
> > +	/* Enforce sane locking for modern driver ioctls. */
> > +	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
>
> Maybe have a local bool locked_ioctl for this so it's extremely clear it's
> the same condition in both?
>
> Either way: Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

Thanks for the suggestion and review. Sounds good, I'll update and send 
out a new version.

(Sorry for delays, been busy with moving)

> > +		mutex_lock(&drm_global_mutex);
> > +
> >   	retcode = drm_ioctl_permit(flags, file_priv);
> >   	if (unlikely(retcode))
> > -		return retcode;
> > +		goto out;
> >   
> > -	/* Enforce sane locking for modern driver ioctls. */
> > -	if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
> > -	    (flags & DRM_UNLOCKED))
> > -		retcode = func(dev, kdata, file_priv);
> > -	else {
> > -		mutex_lock(&drm_global_mutex);
> > -		retcode = func(dev, kdata, file_priv);
> > +	retcode = func(dev, kdata, file_priv);
> > +
> > +out:
> > +	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
> >   		mutex_unlock(&drm_global_mutex);
> > -	}
> >   	return retcode;
> >   }
> >   EXPORT_SYMBOL(drm_ioctl_kernel);
> > --
> > 2.25.1