Re: [PATCH v6] drm/virtio: Add option to disable KMS support

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 3/3/23 02:35, Rob Clark wrote:
> @@ -223,12 +223,15 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
>  			num_scanouts, &num_scanouts);
>  	vgdev->num_scanouts = min_t(uint32_t, num_scanouts,
>  				    VIRTIO_GPU_MAX_SCANOUTS);
> -	if (!vgdev->num_scanouts) {
> -		DRM_ERROR("num_scanouts is zero\n");
> -		ret = -EINVAL;
> -		goto err_scanouts;
> +
> +	if (IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) || !vgdev->num_scanouts) {

Unfortunately the IS_ENABLED check needs to be inverted here.

Secondly, with the IS_ENABLED check fixed and CONFIG_DRM_VIRTIO_GPU_KMS
disabled, kernel crashes with a NULL deref on boot after getting the
VIRTIO_GPU_EVENT_DISPLAY from host.

 ==================================================================
[    0.870144] BUG: KASAN: null-ptr-deref in
drm_kms_helper_hotplug_event+0x2b/0x50
[    0.870588] Read of size 8 at addr 0000000000000010 by task
kworker/0:1/14
[    0.870588]
[    0.870588] CPU: 0 PID: 14 Comm: kworker/0:1 Not tainted
6.2.0-next-20230303+ #387
[    0.871441] scsi host0: ahci
[    0.870588] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
[    0.870588] Workqueue: events virtio_gpu_dequeue_ctrl_func
[    0.870588] Call Trace:
[    0.870588]  <TASK>
[    0.872841] scsi host1: ahci
[    0.870588]  dump_stack_lvl+0x46/0x70
[    0.870588]  kasan_report+0xbb/0xf0
[    0.870588]  ? drm_kms_helper_hotplug_event+0x2b/0x50
[    0.870588]  drm_kms_helper_hotplug_event+0x2b/0x50
[    0.870588]  virtio_gpu_dequeue_ctrl_func+0x143/0x500
[    0.870588]  ? lock_is_held_type+0xd8/0x130
[    0.870588]  ? virtio_gpu_free_vbufs+0x40/0x40
[    0.875105] scsi host2: ahci
[    0.870588]  process_one_work+0x4f5/0x9a0
[    0.870588]  ? pwq_dec_nr_in_flight+0x100/0x100
[    0.870588]  ? spin_bug+0xe0/0xe0
[    0.870588]  worker_thread+0x8c/0x610
[    0.870588]  ? process_one_work+0x9a0/0x9a0
[    0.870588]  kthread+0x15a/0x190
[    0.870588]  ? kthread_complete_and_exit+0x20/0x20
[    0.870588]  ret_from_fork+0x1f/0x30
[    0.877201] scsi host3: ahci
[    0.870588]  </TASK>
[    0.870588]
==================================================================

I'll send a follow up fix.

-- 
Best regards,
Dmitry