Re: [PATCH 13/16] drm/fb-helper: Avoid race with DRM userspace

[Daniel Vetter <daniel@ffwll.ch>]

On Sat, Mar 30, 2019 at 10:07:58PM +0100, Noralf Trønnes wrote:
> 
> 
> Den 28.03.2019 09.17, skrev Daniel Vetter:
> > On Tue, Mar 26, 2019 at 06:55:43PM +0100, Noralf Trønnes wrote:
> >> drm_fb_helper_is_bound() is used to check if DRM userspace is in control.
> >> This is done by looking at the fb on the primary plane. By the time
> >> fb-helper gets around to committing, it's possible that the facts have
> >> changed.
> >>
> >> Avoid this race by holding the drm_device->master_mutex lock while
> >> committing. When DRM userspace does its first open, it will now wait
> >> until fb-helper is done. The helper will stay away if there's a master.
> >>
> >> Locking rule: Always take the fb-helper lock first.
> >>
> >> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >> Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
> > 
> > I think it'd be good to reorder this earlier in the series. And I'm
> > wondering why you didn't replace all occurences of the _is_bound()
> > function. With the consistent master state check we're doing with this I
> > don't think any of the fbdev checking is still needed. Plus looking at
> > crtc->primary->fb without any locks is racy, so would be good to ditch
> > that hack.
> 
> _is_bound() is used in drm_fb_helper_hotplug_event() to decide on
> delayed hotplug. The master lock won't help here. I have studied
> drm_fb_helper for 2 years now, and I still think it is convoluted and
> difficult to grasp in places. So I kept this one call site. I'm vary of
> making unrelated changes in this series.

Hm, why would it not work for the hotplug case? _is_bound already checks
dev->master (but with races, unlike this here). And the other checks are
various levels of cargo-culting. dev->master is (well, has become through
retrofitting) the "who owns the display" tracking, I think fully
standardizing on that makes sense. The only case where there's a
difference is if:
- some compositor dropped master
- but didn't disable it's framebuffers
We actually want that for smooth vt switching, so that the new compositor
could take over in a transition. Having a special case with fbdev where
the compositor has to disable all its fb or fbcon won't take over feels a
bit irky.
-Daniel

> The reason I included this patch is because it's used with the
> bootsplash example support code. I can move the patch earlier in the series.
> 
> Noralf.
> 
> > -Daniel
> > 
> >> ---
> >>  drivers/gpu/drm/drm_auth.c      | 20 ++++++++++++++
> >>  drivers/gpu/drm/drm_fb_helper.c | 49 ++++++++++++++++++++++++---------
> >>  drivers/gpu/drm/drm_internal.h  |  2 ++
> >>  3 files changed, 58 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> >> index 1669c42c40ed..db199807b7dc 100644
> >> --- a/drivers/gpu/drm/drm_auth.c
> >> +++ b/drivers/gpu/drm/drm_auth.c
> >> @@ -368,3 +368,23 @@ void drm_master_put(struct drm_master **master)
> >>  	*master = NULL;
> >>  }
> >>  EXPORT_SYMBOL(drm_master_put);
> >> +
> >> +/* Used by drm_client and drm_fb_helper */
> >> +bool drm_master_internal_acquire(struct drm_device *dev)
> >> +{
> >> +	mutex_lock(&dev->master_mutex);
> >> +	if (dev->master) {
> >> +		mutex_unlock(&dev->master_mutex);
> >> +		return false;
> >> +	}
> >> +
> >> +	return true;
> >> +}
> >> +EXPORT_SYMBOL(drm_master_internal_acquire);
> >> +
> >> +/* Used by drm_client and drm_fb_helper */
> >> +void drm_master_internal_release(struct drm_device *dev)
> >> +{
> >> +	mutex_unlock(&dev->master_mutex);
> >> +}
> >> +EXPORT_SYMBOL(drm_master_internal_release);
> >> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> >> index 4a073cd4e423..9f253fcf3f79 100644
> >> --- a/drivers/gpu/drm/drm_fb_helper.c
> >> +++ b/drivers/gpu/drm/drm_fb_helper.c
> >> @@ -41,6 +41,8 @@
> >>  #include <drm/drm_crtc_helper.h>
> >>  #include <drm/drm_atomic.h>
> >>  
> >> +#include "drm_internal.h"
> >> +
> >>  static bool drm_fbdev_emulation = true;
> >>  module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
> >>  MODULE_PARM_DESC(fbdev_emulation,
> >> @@ -235,7 +237,12 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
> >>  		return 0;
> >>  
> >>  	mutex_lock(&fb_helper->lock);
> >> -	ret = drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
> >> +	if (drm_master_internal_acquire(fb_helper->dev)) {
> >> +		ret = drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
> >> +		drm_master_internal_release(fb_helper->dev);
> >> +	} else {
> >> +		ret = -EBUSY;
> >> +	}
> >>  
> >>  	do_delayed = fb_helper->delayed_hotplug;
> >>  	if (do_delayed)
> >> @@ -332,13 +339,16 @@ static struct sysrq_key_op sysrq_drm_fb_helper_restore_op = { };
> >>  static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
> >>  {
> >>  	struct drm_fb_helper *fb_helper = info->par;
> >> +	struct drm_device *dev = fb_helper->dev;
> >>  
> >>  	/*
> >>  	 * For each CRTC in this fb, turn the connectors on/off.
> >>  	 */
> >>  	mutex_lock(&fb_helper->lock);
> >> -	if (drm_fb_helper_is_bound(fb_helper))
> >> -		drm_client_modesets_dpms(fb_helper->dev, fb_helper->modesets, dpms_mode);
> >> +	if (drm_master_internal_acquire(dev)) {
> >> +		drm_client_modesets_dpms(dev, fb_helper->modesets, dpms_mode);
> >> +		drm_master_internal_release(dev);
> >> +	}
> >>  	mutex_unlock(&fb_helper->lock);
> >>  }
> >>  
> >> @@ -1097,6 +1107,7 @@ static int setcmap_atomic(struct fb_cmap *cmap, struct fb_info *info)
> >>  int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
> >>  {
> >>  	struct drm_fb_helper *fb_helper = info->par;
> >> +	struct drm_device *dev = fb_helper->dev;
> >>  	int ret;
> >>  
> >>  	if (oops_in_progress)
> >> @@ -1104,9 +1115,9 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
> >>  
> >>  	mutex_lock(&fb_helper->lock);
> >>  
> >> -	if (!drm_fb_helper_is_bound(fb_helper)) {
> >> +	if (!drm_master_internal_acquire(dev)) {
> >>  		ret = -EBUSY;
> >> -		goto out;
> >> +		goto unlock;
> >>  	}
> >>  
> >>  	if (info->fix.visual == FB_VISUAL_TRUECOLOR)
> >> @@ -1116,7 +1127,8 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
> >>  	else
> >>  		ret = setcmap_legacy(cmap, info);
> >>  
> >> -out:
> >> +	drm_master_internal_release(dev);
> >> +unlock:
> >>  	mutex_unlock(&fb_helper->lock);
> >>  
> >>  	return ret;
> >> @@ -1136,11 +1148,13 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
> >>  			unsigned long arg)
> >>  {
> >>  	struct drm_fb_helper *fb_helper = info->par;
> >> +	struct drm_device *dev = fb_helper->dev;
> >>  	struct drm_crtc *crtc;
> >>  	int ret = 0;
> >>  
> >>  	mutex_lock(&fb_helper->lock);
> >> -	if (!drm_fb_helper_is_bound(fb_helper)) {
> >> +
> >> +	if (!drm_master_internal_acquire(dev)) {
> >>  		ret = -EBUSY;
> >>  		goto unlock;
> >>  	}
> >> @@ -1177,13 +1191,15 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
> >>  		}
> >>  
> >>  		ret = 0;
> >> -		goto unlock;
> >> +		break;
> >>  	default:
> >>  		ret = -ENOTTY;
> >>  	}
> >>  
> >> +	drm_master_internal_release(dev);
> >>  unlock:
> >>  	mutex_unlock(&fb_helper->lock);
> >> +
> >>  	return ret;
> >>  }
> >>  EXPORT_SYMBOL(drm_fb_helper_ioctl);
> >> @@ -1426,15 +1442,19 @@ int drm_fb_helper_pan_display(struct fb_var_screeninfo *var,
> >>  		return -EBUSY;
> >>  
> >>  	mutex_lock(&fb_helper->lock);
> >> -	if (!drm_fb_helper_is_bound(fb_helper)) {
> >> -		mutex_unlock(&fb_helper->lock);
> >> -		return -EBUSY;
> >> +
> >> +	if (!drm_master_internal_acquire(dev)) {
> >> +		ret = -EBUSY;
> >> +		goto unlock;
> >>  	}
> >>  
> >>  	if (drm_drv_uses_atomic_modeset(dev))
> >>  		ret = pan_display_atomic(var, info);
> >>  	else
> >>  		ret = pan_display_legacy(var, info);
> >> +
> >> +	drm_master_internal_release(dev);
> >> +unlock:
> >>  	mutex_unlock(&fb_helper->lock);
> >>  
> >>  	return ret;
> >> @@ -1451,6 +1471,7 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
> >>  	int ret = 0;
> >>  	int crtc_count = 0;
> >>  	struct drm_connector_list_iter conn_iter;
> >> +	struct drm_device *dev = fb_helper->dev;
> >>  	struct drm_fb_helper_surface_size sizes;
> >>  	struct drm_connector *connector;
> >>  	struct drm_mode_set *mode_set;
> >> @@ -1593,8 +1614,10 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
> >>  		DRM_INFO("Cannot find any crtc or sizes\n");
> >>  
> >>  		/* First time: disable all crtc's.. */
> >> -		if (!fb_helper->deferred_setup && !READ_ONCE(fb_helper->dev->master))
> >> -			drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
> >> +		if (!fb_helper->deferred_setup && drm_master_internal_acquire(dev)) {
> >> +			drm_client_modesets_commit(dev, fb_helper->modesets);
> >> +			drm_master_internal_release(dev);
> >> +		}
> >>  		return -EAGAIN;
> >>  	}
> >>  
> >> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> >> index 251d67e04c2d..6d5d1184c084 100644
> >> --- a/drivers/gpu/drm/drm_internal.h
> >> +++ b/drivers/gpu/drm/drm_internal.h
> >> @@ -91,6 +91,8 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
> >>  			 struct drm_file *file_priv);
> >>  int drm_master_open(struct drm_file *file_priv);
> >>  void drm_master_release(struct drm_file *file_priv);
> >> +bool drm_master_internal_acquire(struct drm_device *dev);
> >> +void drm_master_internal_release(struct drm_device *dev);
> >>  
> >>  /* drm_sysfs.c */
> >>  extern struct class *drm_class;
> >> -- 
> >> 2.20.1
> >>
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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