Re: [PATCH] drm/gem: Fix and enable CHANGE_HANDLE

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] Missing idr_preload_end() in the idr_alloc error path leaves preemption permanently disabled.
--

--- Patch [1]: [PATCH] drm/gem: Fix and enable CHANGE_HANDLE ---
commit 4926fe37208872dd7c9115003f10226cea7a200a
Author: David Francis <David.Francis@amd.com>
drm/gem: Fix and enable CHANGE_HANDLE

This patch enables the CHANGE_HANDLE ioctl for GEM and adds validation
for the handle inputs. It also adds idr_preload before idr_alloc to
ensure memory allocation can be fulfilled without sleeping while holding
a spinlock.

> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 018df97d590dc..00693626d7683 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
[ ... ]
> @@ -1038,14 +1025,22 @@ int drm_gem_change_handle_ioctl(struct drm_device *dev, void *data,
[ ... ]
>  	mutex_lock(&file_priv->prime.lock);
> +	idr_preload(GFP_KERNEL);
>  	spin_lock(&file_priv->table_lock);
>  	ret = idr_alloc(&file_priv->object_idr, NULL, new_handle, new_handle + 1,
>  			GFP_NOWAIT);
> @@ -1059,10 +1054,12 @@ int drm_gem_change_handle_ioctl(struct drm_device *dev, void *data,

[Severity: Critical]
Does this error path leave preemption permanently disabled?

Looking at the code just after idr_alloc() in drm_gem_change_handle_ioctl(),
there is an error path that skips idr_preload_end():

	ret = idr_alloc(&file_priv->object_idr, NULL, new_handle, new_handle + 1,
			GFP_NOWAIT);

	if (ret < 0) {
		spin_unlock(&file_priv->table_lock);
		goto out_unlock;
	}

If idr_alloc() fails (for example, returning -ENOSPC when new_handle is
already in use), the code jumps to out_unlock which only releases prime.lock
and returns. Since idr_preload() disables preemption, wouldn't we need an
idr_preload_end() call before returning to avoid a scheduling while atomic
system panic?

>  	if (IS_ERR_OR_NULL(obj)) {
>  		idr_remove(&file_priv->object_idr, new_handle);
>  		spin_unlock(&file_priv->table_lock);
> +		idr_preload_end();
>  		ret = -ENOENT;
>  		goto out_unlock;
>  	}
>  	spin_unlock(&file_priv->table_lock);
> +	idr_preload_end();
>  
>  	if (obj->dma_buf) {
>  		ret = drm_prime_add_buf_handle(&file_priv->prime, obj->dma_buf,

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260807183735.1019040-1-David.Francis@amd.com?part=1