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:

quoted

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 <redacted>
---
  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 [off-list ref]

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

(Sorry for delays, been busy with moving)

quoted

+		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