If the DRM core fails to init for whatever reason, ensure that no driver ever calls drm_dev_register().
This is best done at drm_dev_init() as it covers drivers that call drm_dev_alloc() as well as drivers that prefer to embed struct drm_device into their own device struct and call drm_dev_init() themselves.
In my case I had so many dynamic device majors used that the major number for DRM (226) was stolen, causing DRM core init to fail after failing to register a chrdev, and ultimately calling debugfs_remove() on drm_debugfs_root in drm_core_exit().
After drm core failed to init, VGEM was still calling drm_dev_register(), ultimately leading to drm_debugfs_init(), with drm_debugfs_root passed as the root for the new debugfs dir at debugfs_create_dir().
This led to a kernel panic once we were either derefencing root->d_inode while it was NULL or calling root->d_inode->i_op->lookup() while it was NULL in debugfs at inode_lock() or lookup_*().
Signed-off-by: Alexandru Moise 00moses.alexander00@gmail.com --- drivers/gpu/drm/drm_drv.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)
diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index 37b8ad3e30d8..2ed2d919beae 100644 --- a/drivers/gpu/drm/drm_drv.c +++ b/drivers/gpu/drm/drm_drv.c @@ -63,6 +63,15 @@ module_param_named(debug, drm_debug, int, 0600); static DEFINE_SPINLOCK(drm_minor_lock); static struct idr drm_minors_idr;
+/* + * If the drm core fails to init for whatever reason, + * we should prevent any drivers from registering with it. + * It's best to check this at drm_dev_init(), as some drivers + * prefer to embed struct drm_device into their own device + * structure and call drm_dev_init() themselves. + */ +static bool drm_core_init_complete = false; + static struct dentry *drm_debugfs_root;
#define DRM_PRINTK_FMT "[" DRM_NAME ":%s]%s %pV" @@ -484,6 +493,11 @@ int drm_dev_init(struct drm_device *dev, { int ret;
+ if (!drm_core_init_complete) { + DRM_ERROR("DRM core is not initialized\n"); + return -ENODEV; + } + kref_init(&dev->ref); dev->dev = parent; dev->driver = driver; @@ -966,6 +980,8 @@ static int __init drm_core_init(void) if (ret < 0) goto error;
+ drm_core_init_complete = true; + DRM_DEBUG("Initialized\n"); return 0;