Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider: - [High] TOCTOU race between `drm_backlight_link()` and device-wide backlight inhibit/uninhibit iterations causes permanent count corruption. - [High] The LUMINANCE property is unconditionally exposed to all atomic clients, violating the DRM_CLIENT_CAP_LUMINANCE capability contract. - [Low] Clients cannot cleanly unset DRM_CLIENT_CAP_LUMINANCE if they have previously unset DRM_CLIENT_CAP_ATOMIC. --
commit 3e9d95a8d4fc1bbb26ab6124a33c012ee7b1f848 Author: Mario Limonciello <[email protected]> drm: add DRM_CLIENT_CAP_LUMINANCE This patch adds a DRIVER_CONNECTOR_LUMINANCE driver feature and a DRM_CLIENT_CAP_LUMINANCE client capability. When a client sets this capability, every DRM-connected backlight is marked as taken over, and legacy sysfs writes return -EBUSY. > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > index 9039a39c43243..de2c9d9ab5285 100644 > --- a/drivers/gpu/drm/drm_ioctl.c > +++ b/drivers/gpu/drm/drm_ioctl.c [ ... ] > @@ -392,6 +394,28 @@ drm_setclientcap(struct drm_device *dev, void *data, > struct drm_file *file_priv) > file_priv->plane_color_pipeline = req->value; > break; > } > + case DRM_CLIENT_CAP_LUMINANCE: > + if (!drm_core_check_feature(dev, DRIVER_CONNECTOR_LUMINANCE)) > + return -EOPNOTSUPP; > + if (!file_priv->atomic) > + return -EINVAL; [Severity: Low] Can this prevent clients from cleanly unsetting DRM_CLIENT_CAP_LUMINANCE if they have previously unset DRM_CLIENT_CAP_ATOMIC? If a client sets ATOMIC=1, LUMINANCE=1, ATOMIC=0, and then tries to unset LUMINANCE=0, this unconditional atomic check will cause the last call to return -EINVAL. > + if (req->value > 1) > + return -EINVAL; > + > + /* > + * Serialize the compare-and-act so concurrent ioctls on a > shared > + * fd cannot unbalance the device-wide inhibit count. > + */ > + mutex_lock(&dev->mode_config.mutex); > + if (req->value != file_priv->supports_luminance_control) { > + if (req->value) > + drm_backlight_inhibit_legacy_all(dev); [Severity: High] Does this call introduce a TOCTOU race between drm_backlight_link() and the device-wide backlight inhibit iteration? Looking at drm_backlight_inhibit_legacy_all() in drivers/gpu/drm/drm_backlight.c, it drops drm_backlight_lock before iterating connectors: scoped_guard(spinlock, &drm_backlight_lock) dev->mode_config.luminance_clients++; drm_connector_list_iter_begin(dev, &iter); If a concurrent hotplug/probe thread calls drm_backlight_link(), it reads the already-incremented dev->mode_config.luminance_clients and assigns it to the new connector's b->luminance_clients. Then the ioctl thread resumes, iterates the connector list, reaches this newly linked connector, and calls drm_backlight_inhibit_legacy(), which increments b->luminance_clients again. This causes the connector's count to get permanently stuck at a non-zero value, locking out legacy sysfs backlight control for the rest of the system uptime. > + else > + drm_backlight_uninhibit_legacy_all(dev); > + file_priv->supports_luminance_control = req->value; [Severity: High] Is the LUMINANCE property being unconditionally exposed to all atomic clients, violating the DRM_CLIENT_CAP_LUMINANCE capability contract? The UAPI behavior documented in include/uapi/drm/drm.h states: "The LUMINANCE property becomes visible on supported connectors". However, the capability tracked here (file_priv->supports_luminance_control) is never utilized by the DRM core (e.g., in drm_mode_object_get_properties) to filter property visibility. drm_mode_object_get_properties() only checks if the client is atomic, but fails to check if the property is luminance and if the client supports it: if ((prop->flags & DRM_MODE_PROP_ATOMIC) && !atomic) continue; As a result, any generic atomic client (like a compositor restoring state) will see and restore the LUMINANCE property without setting DRM_CLIENT_CAP_LUMINANCE, bypassing the sysfs inhibition. > + } > + mutex_unlock(&dev->mode_config.mutex); > + break; > default: > return -EINVAL; > } -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=5
