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* [PATCH v10 0/4] drm: update locking for modesetting
@ 2021-08-31  7:24 Desmond Cheong Zhi Xi
  2021-08-31  7:24 ` [PATCH v10 1/4] drm: fix null ptr dereference in drm_master_release Desmond Cheong Zhi Xi
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-08-31  7:24 UTC (permalink / raw)
  To: maarten.lankhorst, mripard, tzimmermann, airlied, daniel,
	sumit.semwal, christian.koenig
  Cc: Desmond Cheong Zhi Xi, dri-devel, linux-kernel, intel-gfx, skhan,
	gregkh, linux-kernel-mentees, linux-media, linaro-mm-sig

Sorry for the noise, rebasing on top of drm-misc-next. Please ignore the
v9 series.

Hi,

I updated the patch set with some suggestions by Daniel Vetter, and
dropped the patches after patch 4 so that we can stick the landing for
avoiding races with modesetting rights before dealing with the tricky
spinlock.

Overall, this series fixes races with modesetting rights, and converts
drm_device.master_mutex into master_rwsem.

- Patch 1: Fix a potential null ptr dereference in drm_master_release

- Patch 2: Convert master_mutex into rwsem (avoids creating a new lock)

- Patch 3: Update global mutex locking in the ioctl handler (avoids
deadlock when grabbing read lock on master_rwsem in drm_ioctl_kernel)

- Patch 4: Plug races with drm modesetting rights

v9 -> v10:
- Rebase on top of drm-misc-next, caught by Intel-gfx CI

v8 -> v9 (suggested by Daniel Vetter):
- Drop patches 5-7 to handle it in another series
- Add the appropriate Fixes: tag for the null ptr dereference fix
(patch 1)
- Create a locked_ioctl bool to clarify locking/unlocking patterns in
the ioctl handler (patch 3)
- Clarify the kernel doc for master_rwsem (patch 4)

v7 -> v8:
- Avoid calling drm_lease_held in drm_mode_setcrtc and
drm_wait_vblank_ioctl, caught by Intel-gfx CI

v6 -> v7:
- Export __drm_mode_object_find for loadable modules, caught by the
Intel-gfx CI

v5 -> v6:
- Fix recursive locking on master_rwsem, caught by the Intel-gfx CI

v4 -> v5:
- Avoid calling drm_file_get_master while holding on to the modeset
mutex, caught by the Intel-gfx CI

v3 -> v4 (suggested by Daniel Vetter):
- Drop a patch that added an unnecessary master_lookup_lock in
drm_master_release
- Drop a patch that addressed a non-existent race in
drm_is_current_master_locked
- Remove fixes for non-existent null ptr dereferences
- Protect drm_master.magic_map,unique{_len} with master_rwsem instead of
master_lookup_lock
- Drop the patch that moved master_lookup_lock into struct drm_device
- Drop a patch to export task_work_add
- Revert the check for the global mutex in the ioctl handler to use
drm_core_check_feature instead of drm_dev_needs_global_mutex
- Push down master_rwsem locking for selected ioctls to avoid lock
hierarchy inversions, and to allow us to hold write locks on
master_rwsem instead of flushing readers
- Remove master_lookup_lock by replacing it with master_rwsem

v2 -> v3:
- Unexport drm_master_flush, as suggested by Daniel Vetter.
- Merge master_mutex and master_rwsem, as suggested by Daniel Vetter.
- Export task_work_add, reported by kernel test robot.
- Make master_flush static, reported by kernel test robot.
- Move master_lookup_lock into struct drm_device.
- Add a missing lock on master_lookup_lock in drm_master_release.
- Fix a potential race in drm_is_current_master_locked.
- Fix potential null ptr dereferences in drm_{auth, ioctl}.
- Protect magic_map,unique{_len} with  master_lookup_lock.
- Convert master_mutex into a rwsem.
- Update global mutex locking in the ioctl handler.

v1 -> v2 (suggested by Daniel Vetter):
- Address an additional race when drm_open runs.
- Switch from SRCU to rwsem to synchronise readers and writers.
- Implement drm_master_flush with task_work so that flushes can be
queued to run before returning to userspace without creating a new
DRM_MASTER_FLUSH ioctl flag.

Best wishes,
Desmond

Desmond Cheong Zhi Xi (4):
  drm: fix null ptr dereference in drm_master_release
  drm: convert drm_device.master_mutex into a rwsem
  drm: lock drm_global_mutex earlier in the ioctl handler
  drm: avoid races with modesetting rights

 drivers/gpu/drm/drm_auth.c    | 39 ++++++++++++++++------------
 drivers/gpu/drm/drm_debugfs.c |  4 +--
 drivers/gpu/drm/drm_drv.c     |  3 +--
 drivers/gpu/drm/drm_file.c    |  6 ++---
 drivers/gpu/drm/drm_ioctl.c   | 49 ++++++++++++++++++++++-------------
 drivers/gpu/drm/drm_lease.c   | 35 +++++++++++++++++--------
 include/drm/drm_auth.h        |  6 ++---
 include/drm/drm_device.h      | 16 +++++++++---
 include/drm/drm_file.h        | 12 ++++-----
 9 files changed, 104 insertions(+), 66 deletions(-)

-- 
2.25.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v10 1/4] drm: fix null ptr dereference in drm_master_release
  2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
@ 2021-08-31  7:24 ` Desmond Cheong Zhi Xi
  2021-08-31  7:24 ` [PATCH v10 2/4] drm: convert drm_device.master_mutex into a rwsem Desmond Cheong Zhi Xi
                   ` (3 subsequent siblings)
  4 siblings, 0 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-08-31  7:24 UTC (permalink / raw)
  To: maarten.lankhorst, mripard, tzimmermann, airlied, daniel,
	sumit.semwal, christian.koenig
  Cc: Desmond Cheong Zhi Xi, dri-devel, linux-kernel, intel-gfx, skhan,
	gregkh, linux-kernel-mentees, linux-media, linaro-mm-sig, stable,
	Daniel Vetter

drm_master_release can be called on a drm_file without a master, which
results in a null ptr dereference of file_priv->master->magic_map. The
three cases are:

1. Error path in drm_open_helper
  drm_open():
    drm_open_helper():
      drm_master_open():
        drm_new_set_master(); <--- returns -ENOMEM,
                                   drm_file.master not set
      drm_file_free():
        drm_master_release(); <--- NULL ptr dereference
                                   (file_priv->master->magic_map)

2. Error path in mock_drm_getfile
  mock_drm_getfile():
    anon_inode_getfile(); <--- returns error, drm_file.master not set
    drm_file_free():
      drm_master_release(); <--- NULL ptr dereference
                                 (file_priv->master->magic_map)

3. In drm_client_close, as drm_client_open doesn't set up a master

drm_file.master is set up in drm_open_helper through the call to
drm_master_open, so we mirror it with a call to drm_master_release in
drm_close_helper, and remove drm_master_release from drm_file_free to
avoid the null ptr dereference.

Fixes: 7eeaeb90a6a5 ("drm/file: Don't set master on in-kernel clients")
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_file.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index ed25168619fc..90b62f360da1 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -282,9 +282,6 @@ void drm_file_free(struct drm_file *file)
 
 	drm_legacy_ctxbitmap_flush(dev, file);
 
-	if (drm_is_primary_client(file))
-		drm_master_release(file);
-
 	if (dev->driver->postclose)
 		dev->driver->postclose(dev, file);
 
@@ -305,6 +302,9 @@ static void drm_close_helper(struct file *filp)
 	list_del(&file_priv->lhead);
 	mutex_unlock(&dev->filelist_mutex);
 
+	if (drm_is_primary_client(file_priv))
+		drm_master_release(file_priv);
+
 	drm_file_free(file_priv);
 }
 
-- 
2.25.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v10 2/4] drm: convert drm_device.master_mutex into a rwsem
  2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
  2021-08-31  7:24 ` [PATCH v10 1/4] drm: fix null ptr dereference in drm_master_release Desmond Cheong Zhi Xi
@ 2021-08-31  7:24 ` Desmond Cheong Zhi Xi
  2021-08-31  7:25 ` [PATCH v10 3/4] drm: lock drm_global_mutex earlier in the ioctl handler Desmond Cheong Zhi Xi
                   ` (2 subsequent siblings)
  4 siblings, 0 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-08-31  7:24 UTC (permalink / raw)
  To: maarten.lankhorst, mripard, tzimmermann, airlied, daniel,
	sumit.semwal, christian.koenig
  Cc: Desmond Cheong Zhi Xi, dri-devel, linux-kernel, intel-gfx, skhan,
	gregkh, linux-kernel-mentees, linux-media, linaro-mm-sig,
	Daniel Vetter

drm_device.master_mutex currently protects the following:
- drm_device.master
- drm_file.master
- drm_file.was_master
- drm_file.is_master
- drm_master.unique
- drm_master.unique_len
- drm_master.magic_map

There is a clear separation between functions that read or change
these attributes. Hence, convert master_mutex into a rwsem to enable
concurrent readers.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_auth.c    | 35 ++++++++++++++++++-----------------
 drivers/gpu/drm/drm_debugfs.c |  4 ++--
 drivers/gpu/drm/drm_drv.c     |  3 +--
 drivers/gpu/drm/drm_ioctl.c   | 10 +++++-----
 include/drm/drm_auth.h        |  6 +++---
 include/drm/drm_device.h      | 10 ++++++----
 include/drm/drm_file.h        | 12 ++++++------
 7 files changed, 41 insertions(+), 39 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 60a6b21474b1..73ade0513ccb 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -64,7 +64,7 @@
 static bool drm_is_current_master_locked(struct drm_file *fpriv)
 {
 	lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
-			    lockdep_is_held(&fpriv->minor->dev->master_mutex));
+			    lockdep_is_held(&fpriv->minor->dev->master_rwsem));
 
 	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
 }
@@ -96,7 +96,7 @@ int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 	struct drm_auth *auth = data;
 	int ret = 0;
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 	if (!file_priv->magic) {
 		ret = idr_alloc(&file_priv->master->magic_map, file_priv,
 				1, 0, GFP_KERNEL);
@@ -104,7 +104,7 @@ int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 			file_priv->magic = ret;
 	}
 	auth->magic = file_priv->magic;
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 
 	DRM_DEBUG("%u\n", auth->magic);
 
@@ -119,13 +119,13 @@ int drm_authmagic(struct drm_device *dev, void *data,
 
 	DRM_DEBUG("%u\n", auth->magic);
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 	file = idr_find(&file_priv->master->magic_map, auth->magic);
 	if (file) {
 		file->authenticated = 1;
 		idr_replace(&file_priv->master->magic_map, NULL, auth->magic);
 	}
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 
 	return file ? 0 : -EINVAL;
 }
@@ -167,7 +167,7 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
 	struct drm_master *old_master;
 	struct drm_master *new_master;
 
-	lockdep_assert_held_once(&dev->master_mutex);
+	lockdep_assert_held_once(&dev->master_rwsem);
 
 	WARN_ON(fpriv->is_master);
 	old_master = fpriv->master;
@@ -249,7 +249,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 {
 	int ret;
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 
 	ret = drm_master_check_perm(dev, file_priv);
 	if (ret)
@@ -281,7 +281,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 
 	drm_set_master(dev, file_priv, false);
 out_unlock:
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 	return ret;
 }
 
@@ -298,7 +298,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 {
 	int ret;
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 
 	ret = drm_master_check_perm(dev, file_priv);
 	if (ret)
@@ -321,8 +321,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 	}
 
 	drm_drop_master(dev, file_priv);
+
 out_unlock:
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 	return ret;
 }
 
@@ -334,7 +335,7 @@ int drm_master_open(struct drm_file *file_priv)
 	/* if there is no current master make this fd it, but do not create
 	 * any master object for render clients
 	 */
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 	if (!dev->master) {
 		ret = drm_new_set_master(dev, file_priv);
 	} else {
@@ -342,7 +343,7 @@ int drm_master_open(struct drm_file *file_priv)
 		file_priv->master = drm_master_get(dev->master);
 		spin_unlock(&file_priv->master_lookup_lock);
 	}
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 
 	return ret;
 }
@@ -352,7 +353,7 @@ void drm_master_release(struct drm_file *file_priv)
 	struct drm_device *dev = file_priv->minor->dev;
 	struct drm_master *master;
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 	master = file_priv->master;
 	if (file_priv->magic)
 		idr_remove(&file_priv->master->magic_map, file_priv->magic);
@@ -375,7 +376,7 @@ void drm_master_release(struct drm_file *file_priv)
 	/* drop the master reference held by the file priv */
 	if (file_priv->master)
 		drm_master_put(&file_priv->master);
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 }
 
 /**
@@ -450,9 +451,9 @@ 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);
+	down_read(&dev->master_rwsem);
 	if (dev->master) {
-		mutex_unlock(&dev->master_mutex);
+		up_read(&dev->master_rwsem);
 		return false;
 	}
 
@@ -463,6 +464,6 @@ 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);
+	up_read(&dev->master_rwsem);
 }
 EXPORT_SYMBOL(drm_master_internal_release);
diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index b0a826489488..b34c9c263188 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -55,7 +55,7 @@ static int drm_name_info(struct seq_file *m, void *data)
 	struct drm_device *dev = minor->dev;
 	struct drm_master *master;
 
-	mutex_lock(&dev->master_mutex);
+	down_read(&dev->master_rwsem);
 	master = dev->master;
 	seq_printf(m, "%s", dev->driver->name);
 	if (dev->dev)
@@ -65,7 +65,7 @@ static int drm_name_info(struct seq_file *m, void *data)
 	if (dev->unique)
 		seq_printf(m, " unique=%s", dev->unique);
 	seq_printf(m, "\n");
-	mutex_unlock(&dev->master_mutex);
+	up_read(&dev->master_rwsem);
 
 	return 0;
 }
diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 7a5097467ba5..4556bf42954c 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -570,7 +570,6 @@ static void drm_dev_init_release(struct drm_device *dev, void *res)
 	/* Prevent use-after-free in drm_managed_release when debugging is
 	 * enabled. Slightly awkward, but can't really be helped. */
 	dev->dev = NULL;
-	mutex_destroy(&dev->master_mutex);
 	mutex_destroy(&dev->clientlist_mutex);
 	mutex_destroy(&dev->filelist_mutex);
 	mutex_destroy(&dev->struct_mutex);
@@ -611,7 +610,7 @@ static int drm_dev_init(struct drm_device *dev,
 	mutex_init(&dev->struct_mutex);
 	mutex_init(&dev->filelist_mutex);
 	mutex_init(&dev->clientlist_mutex);
-	mutex_init(&dev->master_mutex);
+	init_rwsem(&dev->master_rwsem);
 
 	ret = drmm_add_action(dev, drm_dev_init_release, NULL);
 	if (ret)
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 8b8744dcf691..9fc00e36c5d6 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -119,16 +119,16 @@ int drm_getunique(struct drm_device *dev, void *data,
 	struct drm_unique *u = data;
 	struct drm_master *master;
 
-	mutex_lock(&dev->master_mutex);
+	down_read(&dev->master_rwsem);
 	master = file_priv->master;
 	if (u->unique_len >= master->unique_len) {
 		if (copy_to_user(u->unique, master->unique, master->unique_len)) {
-			mutex_unlock(&dev->master_mutex);
+			up_read(&dev->master_rwsem);
 			return -EFAULT;
 		}
 	}
 	u->unique_len = master->unique_len;
-	mutex_unlock(&dev->master_mutex);
+	up_read(&dev->master_rwsem);
 
 	return 0;
 }
@@ -385,7 +385,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
 	struct drm_set_version *sv = data;
 	int if_version, retcode = 0;
 
-	mutex_lock(&dev->master_mutex);
+	down_write(&dev->master_rwsem);
 	if (sv->drm_di_major != -1) {
 		if (sv->drm_di_major != DRM_IF_MAJOR ||
 		    sv->drm_di_minor < 0 || sv->drm_di_minor > DRM_IF_MINOR) {
@@ -420,7 +420,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
 	sv->drm_di_minor = DRM_IF_MINOR;
 	sv->drm_dd_major = dev->driver->major;
 	sv->drm_dd_minor = dev->driver->minor;
-	mutex_unlock(&dev->master_mutex);
+	up_write(&dev->master_rwsem);
 
 	return retcode;
 }
diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
index ba248ca8866f..f0a89e5fcaad 100644
--- a/include/drm/drm_auth.h
+++ b/include/drm/drm_auth.h
@@ -67,17 +67,17 @@ struct drm_master {
 	struct drm_device *dev;
 	/**
 	 * @unique: Unique identifier: e.g. busid. Protected by
-	 * &drm_device.master_mutex.
+	 * &drm_device.master_rwsem.
 	 */
 	char *unique;
 	/**
 	 * @unique_len: Length of unique field. Protected by
-	 * &drm_device.master_mutex.
+	 * &drm_device.master_rwsem.
 	 */
 	int unique_len;
 	/**
 	 * @magic_map: Map of used authentication tokens. Protected by
-	 * &drm_device.master_mutex.
+	 * &drm_device.master_rwsem.
 	 */
 	struct idr magic_map;
 	void *driver_priv;
diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 604b1d1b2d72..142fb2f6e74d 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -107,7 +107,7 @@ struct drm_device {
 	 * @master:
 	 *
 	 * Currently active master for this device.
-	 * Protected by &master_mutex
+	 * Protected by &master_rwsem
 	 */
 	struct drm_master *master;
 
@@ -146,11 +146,13 @@ struct drm_device {
 	struct mutex struct_mutex;
 
 	/**
-	 * @master_mutex:
+	 * @master_rwsem:
 	 *
-	 * Lock for &drm_minor.master and &drm_file.is_master
+	 * Lock for &drm_device.master, &drm_file.was_master,
+	 * &drm_file.is_master, &drm_file.master, &drm_master.unique,
+	 * &drm_master.unique_len, and &drm_master.magic_map.
 	 */
-	struct mutex master_mutex;
+	struct rw_semaphore master_rwsem;
 
 	/**
 	 * @open_count:
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index a3acb7ac3550..d12bb2ba7814 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -205,7 +205,7 @@ struct drm_file {
 	 * @was_master:
 	 *
 	 * This client has or had, master capability. Protected by struct
-	 * &drm_device.master_mutex.
+	 * &drm_device.master_rwsem.
 	 *
 	 * This is used to ensure that CAP_SYS_ADMIN is not enforced, if the
 	 * client is or was master in the past.
@@ -216,7 +216,7 @@ struct drm_file {
 	 * @is_master:
 	 *
 	 * This client is the creator of @master. Protected by struct
-	 * &drm_device.master_mutex.
+	 * &drm_device.master_rwsem.
 	 *
 	 * See also the :ref:`section on primary nodes and authentication
 	 * <drm_primary_node>`.
@@ -227,19 +227,19 @@ struct drm_file {
 	 * @master:
 	 *
 	 * Master this node is currently associated with. Protected by struct
-	 * &drm_device.master_mutex, and serialized by @master_lookup_lock.
+	 * &drm_device.master_rwsem, and serialized by @master_lookup_lock.
 	 *
 	 * Only relevant if drm_is_primary_client() returns true. Note that
 	 * this only matches &drm_device.master if the master is the currently
 	 * active one.
 	 *
-	 * To update @master, both &drm_device.master_mutex and
+	 * To update @master, both &drm_device.master_rwsem and
 	 * @master_lookup_lock need to be held, therefore holding either of
 	 * them is safe and enough for the read side.
 	 *
 	 * When dereferencing this pointer, either hold struct
-	 * &drm_device.master_mutex for the duration of the pointer's use, or
-	 * use drm_file_get_master() if struct &drm_device.master_mutex is not
+	 * &drm_device.master_rwsem for the duration of the pointer's use, or
+	 * use drm_file_get_master() if struct &drm_device.master_rwsem is not
 	 * currently held and there is no other need to hold it. This prevents
 	 * @master from being freed during use.
 	 *
-- 
2.25.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v10 3/4] drm: lock drm_global_mutex earlier in the ioctl handler
  2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
  2021-08-31  7:24 ` [PATCH v10 1/4] drm: fix null ptr dereference in drm_master_release Desmond Cheong Zhi Xi
  2021-08-31  7:24 ` [PATCH v10 2/4] drm: convert drm_device.master_mutex into a rwsem Desmond Cheong Zhi Xi
@ 2021-08-31  7:25 ` Desmond Cheong Zhi Xi
  2021-08-31  7:25 ` [PATCH v10 4/4] drm: avoid races with modesetting rights Desmond Cheong Zhi Xi
  2021-09-07 19:50 ` [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
  4 siblings, 0 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-08-31  7:25 UTC (permalink / raw)
  To: maarten.lankhorst, mripard, tzimmermann, airlied, daniel,
	sumit.semwal, christian.koenig
  Cc: Desmond Cheong Zhi Xi, dri-devel, linux-kernel, intel-gfx, skhan,
	gregkh, linux-kernel-mentees, linux-media, linaro-mm-sig,
	Daniel Vetter

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@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_ioctl.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 9fc00e36c5d6..fe9c4c0264a9 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -767,24 +767,27 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 {
 	struct drm_file *file_priv = file->private_data;
 	struct drm_device *dev = file_priv->minor->dev;
+	bool locked_ioctl;
 	int retcode;
 
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
+	/* Enforce sane locking for modern driver ioctls. */
+	locked_ioctl = (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) &&
+			!(flags & DRM_UNLOCKED));
+	if (locked_ioctl)
+		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 (locked_ioctl)
 		mutex_unlock(&drm_global_mutex);
-	}
 	return retcode;
 }
 EXPORT_SYMBOL(drm_ioctl_kernel);
-- 
2.25.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v10 4/4] drm: avoid races with modesetting rights
  2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
                   ` (2 preceding siblings ...)
  2021-08-31  7:25 ` [PATCH v10 3/4] drm: lock drm_global_mutex earlier in the ioctl handler Desmond Cheong Zhi Xi
@ 2021-08-31  7:25 ` Desmond Cheong Zhi Xi
  2021-09-07 19:50 ` [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
  4 siblings, 0 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-08-31  7:25 UTC (permalink / raw)
  To: maarten.lankhorst, mripard, tzimmermann, airlied, daniel,
	sumit.semwal, christian.koenig
  Cc: Desmond Cheong Zhi Xi, dri-devel, linux-kernel, intel-gfx, skhan,
	gregkh, linux-kernel-mentees, linux-media, linaro-mm-sig,
	Daniel Vetter

In drm_client_modeset.c and drm_fb_helper.c,
drm_master_internal_{acquire,release} are used to avoid races with DRM
userspace. These functions hold onto drm_device.master_rwsem while
committing, and bail if there's already a master.

However, there are other places where modesetting rights can race. A
time-of-check-to-time-of-use error can occur if an ioctl that changes
the modeset has its rights revoked after it validates its permissions,
but before it completes.

There are four places where modesetting permissions can change:

- DROP_MASTER ioctl removes rights for a master and its leases

- REVOKE_LEASE ioctl revokes rights for a specific lease

- SET_MASTER ioctl sets the device master if the master role hasn't
been acquired yet

- drm_open which can create a new master for a device if one does not
currently exist

These races can be avoided using drm_device.master_rwsem: users that
perform modesetting should hold a read lock on the new
drm_device.master_rwsem, and users that change these permissions
should hold a write lock.

To avoid deadlocks with master_rwsem, for ioctls that need to check
for modesetting permissions, but also need to hold a write lock on
master_rwsem to protect some other attribute (or recurses to some
function that holds a write lock, like drm_mode_create_lease_ioctl
which eventually calls drm_master_open), we remove the DRM_MASTER flag
and push the master_rwsem lock and permissions check into the ioctl.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_auth.c  |  4 ++++
 drivers/gpu/drm/drm_ioctl.c | 20 +++++++++++++++-----
 drivers/gpu/drm/drm_lease.c | 35 ++++++++++++++++++++++++-----------
 include/drm/drm_device.h    |  6 ++++++
 4 files changed, 49 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 73ade0513ccb..65065f7e1499 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -120,6 +120,10 @@ int drm_authmagic(struct drm_device *dev, void *data,
 	DRM_DEBUG("%u\n", auth->magic);
 
 	down_write(&dev->master_rwsem);
+	if (unlikely(!drm_is_current_master(file_priv))) {
+		up_write(&dev->master_rwsem);
+		return -EACCES;
+	}
 	file = idr_find(&file_priv->master->magic_map, auth->magic);
 	if (file) {
 		file->authenticated = 1;
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index fe9c4c0264a9..f6a8aa6c53b3 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -386,6 +386,10 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
 	int if_version, retcode = 0;
 
 	down_write(&dev->master_rwsem);
+	if (unlikely(!drm_is_current_master(file_priv))) {
+		retcode = -EACCES;
+		goto unlock;
+	}
 	if (sv->drm_di_major != -1) {
 		if (sv->drm_di_major != DRM_IF_MAJOR ||
 		    sv->drm_di_minor < 0 || sv->drm_di_minor > DRM_IF_MINOR) {
@@ -420,8 +424,9 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
 	sv->drm_di_minor = DRM_IF_MINOR;
 	sv->drm_dd_major = dev->driver->major;
 	sv->drm_dd_minor = dev->driver->minor;
-	up_write(&dev->master_rwsem);
 
+unlock:
+	up_write(&dev->master_rwsem);
 	return retcode;
 }
 
@@ -574,12 +579,12 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
 	DRM_IOCTL_DEF(DRM_IOCTL_GET_STATS, drm_getstats, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_GET_CAP, drm_getcap, DRM_RENDER_ALLOW),
 	DRM_IOCTL_DEF(DRM_IOCTL_SET_CLIENT_CAP, drm_setclientcap, 0),
-	DRM_IOCTL_DEF(DRM_IOCTL_SET_VERSION, drm_setversion, DRM_MASTER),
+	DRM_IOCTL_DEF(DRM_IOCTL_SET_VERSION, drm_setversion, 0),
 
 	DRM_IOCTL_DEF(DRM_IOCTL_SET_UNIQUE, drm_invalid_op, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 	DRM_IOCTL_DEF(DRM_IOCTL_BLOCK, drm_noop, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 	DRM_IOCTL_DEF(DRM_IOCTL_UNBLOCK, drm_noop, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
-	DRM_IOCTL_DEF(DRM_IOCTL_AUTH_MAGIC, drm_authmagic, DRM_MASTER),
+	DRM_IOCTL_DEF(DRM_IOCTL_AUTH_MAGIC, drm_authmagic, 0),
 
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_MAP, drm_legacy_addmap_ioctl, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_MAP, drm_legacy_rmmap_ioctl, DRM_AUTH),
@@ -706,10 +711,10 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
 		      DRM_RENDER_ALLOW),
 	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0),
-	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER),
-	DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, 0),
 };
 
 #define DRM_CORE_IOCTL_COUNT	ARRAY_SIZE(drm_ioctls)
@@ -779,6 +784,9 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 	if (locked_ioctl)
 		mutex_lock(&drm_global_mutex);
 
+	if (unlikely(flags & DRM_MASTER))
+		down_read(&dev->master_rwsem);
+
 	retcode = drm_ioctl_permit(flags, file_priv);
 	if (unlikely(retcode))
 		goto out;
@@ -786,6 +794,8 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 	retcode = func(dev, kdata, file_priv);
 
 out:
+	if (unlikely(flags & DRM_MASTER))
+		up_read(&dev->master_rwsem);
 	if (locked_ioctl)
 		mutex_unlock(&drm_global_mutex);
 	return retcode;
diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index dee4f24a1808..bed6f7636cbe 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -500,6 +500,18 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 		return -EINVAL;
 	}
 
+	/* Clone the lessor file to create a new file for us */
+	DRM_DEBUG_LEASE("Allocating lease file\n");
+	lessee_file = file_clone_open(lessor_file);
+	if (IS_ERR(lessee_file))
+		return PTR_ERR(lessee_file);
+
+	down_read(&dev->master_rwsem);
+	if (unlikely(!drm_is_current_master(lessor_priv))) {
+		ret = -EACCES;
+		goto out_file;
+	}
+
 	lessor = drm_file_get_master(lessor_priv);
 	/* Do not allow sub-leases */
 	if (lessor->lessor) {
@@ -547,14 +559,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 		goto out_leases;
 	}
 
-	/* Clone the lessor file to create a new file for us */
-	DRM_DEBUG_LEASE("Allocating lease file\n");
-	lessee_file = file_clone_open(lessor_file);
-	if (IS_ERR(lessee_file)) {
-		ret = PTR_ERR(lessee_file);
-		goto out_lessee;
-	}
-
 	lessee_priv = lessee_file->private_data;
 	/* Change the file to a master one */
 	drm_master_put(&lessee_priv->master);
@@ -571,17 +575,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	fd_install(fd, lessee_file);
 
 	drm_master_put(&lessor);
+	up_read(&dev->master_rwsem);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
 	return 0;
 
-out_lessee:
-	drm_master_put(&lessee);
-
 out_leases:
 	put_unused_fd(fd);
 
 out_lessor:
 	drm_master_put(&lessor);
+
+out_file:
+	up_read(&dev->master_rwsem);
+	fput(lessee_file);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
 	return ret;
 }
@@ -705,6 +711,11 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	down_write(&dev->master_rwsem);
+	if (unlikely(!drm_is_current_master(lessor_priv))) {
+		ret = -EACCES;
+		goto unlock;
+	}
 	lessor = drm_file_get_master(lessor_priv);
 	mutex_lock(&dev->mode_config.idr_mutex);
 
@@ -728,5 +739,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 	mutex_unlock(&dev->mode_config.idr_mutex);
 	drm_master_put(&lessor);
 
+unlock:
+	up_write(&dev->master_rwsem);
 	return ret;
 }
diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 142fb2f6e74d..5504f9192408 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -151,6 +151,12 @@ struct drm_device {
 	 * Lock for &drm_device.master, &drm_file.was_master,
 	 * &drm_file.is_master, &drm_file.master, &drm_master.unique,
 	 * &drm_master.unique_len, and &drm_master.magic_map.
+	 *
+	 * Additionally, synchronizes access rights to exclusive resources like
+	 * modesetting access between multiple users. For example, users that
+	 * can change the modeset or display state must hold a read lock on
+	 * @master_rwsem, and users that change modesetting rights should hold
+	 * a write lock.
 	 */
 	struct rw_semaphore master_rwsem;
 
-- 
2.25.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH v10 0/4] drm: update locking for modesetting
  2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
                   ` (3 preceding siblings ...)
  2021-08-31  7:25 ` [PATCH v10 4/4] drm: avoid races with modesetting rights Desmond Cheong Zhi Xi
@ 2021-09-07 19:50 ` Desmond Cheong Zhi Xi
  4 siblings, 0 replies; 6+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-09-07 19:50 UTC (permalink / raw)
  To: daniel
  Cc: dri-devel, linux-kernel, intel-gfx, skhan, gregkh, tzimmermann,
	linux-kernel-mentees, linux-media, linaro-mm-sig, sumit.semwal,
	maarten.lankhorst, mripard, airlied, christian.koenig

On 31/8/21 3:24 am, Desmond Cheong Zhi Xi wrote:
> Sorry for the noise, rebasing on top of drm-misc-next. Please ignore the
> v9 series.
> 
> Hi,
> 
> I updated the patch set with some suggestions by Daniel Vetter, and
> dropped the patches after patch 4 so that we can stick the landing for
> avoiding races with modesetting rights before dealing with the tricky
> spinlock.
> 
> Overall, this series fixes races with modesetting rights, and converts
> drm_device.master_mutex into master_rwsem.
> 
> - Patch 1: Fix a potential null ptr dereference in drm_master_release
> 
> - Patch 2: Convert master_mutex into rwsem (avoids creating a new lock)
> 
> - Patch 3: Update global mutex locking in the ioctl handler (avoids
> deadlock when grabbing read lock on master_rwsem in drm_ioctl_kernel)
> 
> - Patch 4: Plug races with drm modesetting rights
> 
> v9 -> v10:
> - Rebase on top of drm-misc-next, caught by Intel-gfx CI
> 
> v8 -> v9 (suggested by Daniel Vetter):
> - Drop patches 5-7 to handle it in another series
> - Add the appropriate Fixes: tag for the null ptr dereference fix
> (patch 1)
> - Create a locked_ioctl bool to clarify locking/unlocking patterns in
> the ioctl handler (patch 3)
> - Clarify the kernel doc for master_rwsem (patch 4)
> 
> v7 -> v8:
> - Avoid calling drm_lease_held in drm_mode_setcrtc and
> drm_wait_vblank_ioctl, caught by Intel-gfx CI
> 
> v6 -> v7:
> - Export __drm_mode_object_find for loadable modules, caught by the
> Intel-gfx CI
> 
> v5 -> v6:
> - Fix recursive locking on master_rwsem, caught by the Intel-gfx CI
> 
> v4 -> v5:
> - Avoid calling drm_file_get_master while holding on to the modeset
> mutex, caught by the Intel-gfx CI
> 
> v3 -> v4 (suggested by Daniel Vetter):
> - Drop a patch that added an unnecessary master_lookup_lock in
> drm_master_release
> - Drop a patch that addressed a non-existent race in
> drm_is_current_master_locked
> - Remove fixes for non-existent null ptr dereferences
> - Protect drm_master.magic_map,unique{_len} with master_rwsem instead of
> master_lookup_lock
> - Drop the patch that moved master_lookup_lock into struct drm_device
> - Drop a patch to export task_work_add
> - Revert the check for the global mutex in the ioctl handler to use
> drm_core_check_feature instead of drm_dev_needs_global_mutex
> - Push down master_rwsem locking for selected ioctls to avoid lock
> hierarchy inversions, and to allow us to hold write locks on
> master_rwsem instead of flushing readers
> - Remove master_lookup_lock by replacing it with master_rwsem
> 
> v2 -> v3:
> - Unexport drm_master_flush, as suggested by Daniel Vetter.
> - Merge master_mutex and master_rwsem, as suggested by Daniel Vetter.
> - Export task_work_add, reported by kernel test robot.
> - Make master_flush static, reported by kernel test robot.
> - Move master_lookup_lock into struct drm_device.
> - Add a missing lock on master_lookup_lock in drm_master_release.
> - Fix a potential race in drm_is_current_master_locked.
> - Fix potential null ptr dereferences in drm_{auth, ioctl}.
> - Protect magic_map,unique{_len} with  master_lookup_lock.
> - Convert master_mutex into a rwsem.
> - Update global mutex locking in the ioctl handler.
> 
> v1 -> v2 (suggested by Daniel Vetter):
> - Address an additional race when drm_open runs.
> - Switch from SRCU to rwsem to synchronise readers and writers.
> - Implement drm_master_flush with task_work so that flushes can be
> queued to run before returning to userspace without creating a new
> DRM_MASTER_FLUSH ioctl flag.
> 
> Best wishes,
> Desmond
> 
> Desmond Cheong Zhi Xi (4):
>    drm: fix null ptr dereference in drm_master_release
>    drm: convert drm_device.master_mutex into a rwsem
>    drm: lock drm_global_mutex earlier in the ioctl handler
>    drm: avoid races with modesetting rights
> 
>   drivers/gpu/drm/drm_auth.c    | 39 ++++++++++++++++------------
>   drivers/gpu/drm/drm_debugfs.c |  4 +--
>   drivers/gpu/drm/drm_drv.c     |  3 +--
>   drivers/gpu/drm/drm_file.c    |  6 ++---
>   drivers/gpu/drm/drm_ioctl.c   | 49 ++++++++++++++++++++++-------------
>   drivers/gpu/drm/drm_lease.c   | 35 +++++++++++++++++--------
>   include/drm/drm_auth.h        |  6 ++---
>   include/drm/drm_device.h      | 16 +++++++++---
>   include/drm/drm_file.h        | 12 ++++-----
>   9 files changed, 104 insertions(+), 66 deletions(-)
> 

Hi Daniel,

Just pinging so this doesn't get buried, though I guess it's also a busy 
merge window. Any thoughts on the series as it is? Tests seemed to have 
passed with the Intel-gfx CI [1].

Not sure if I can set up the CI to do otherwise, but I think this series 
has to go in before I can test new patches to remove the 
drm_file.master_lookup_lock spinlock.

As always, thank you for your time.

Link: https://patchwork.freedesktop.org/series/93864/ [1]

Best wishes,
Desmond

^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2021-09-07 19:50 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-31  7:24 [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi
2021-08-31  7:24 ` [PATCH v10 1/4] drm: fix null ptr dereference in drm_master_release Desmond Cheong Zhi Xi
2021-08-31  7:24 ` [PATCH v10 2/4] drm: convert drm_device.master_mutex into a rwsem Desmond Cheong Zhi Xi
2021-08-31  7:25 ` [PATCH v10 3/4] drm: lock drm_global_mutex earlier in the ioctl handler Desmond Cheong Zhi Xi
2021-08-31  7:25 ` [PATCH v10 4/4] drm: avoid races with modesetting rights Desmond Cheong Zhi Xi
2021-09-07 19:50 ` [PATCH v10 0/4] drm: update locking for modesetting Desmond Cheong Zhi Xi

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