Re: [PATCH v2 2/3] drm/i915: Document our internal limit on object size

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 17/10/2016 09:00, Chris Wilson wrote:
> In many places, we try to count pages using a 32 bit integer. That
> implies if we are asked to create an object larger than 43bits, we will
> subtly crash much later. Catch this on the boundary, and add a warning
> to remind ourselves later on our exabyte systems.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_drv.h |  2 +-
>   drivers/gpu/drm/i915/i915_gem.c | 17 +++++++++++++++--
>   2 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 092c5a0a44f0..a2b5fc72fdd9 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3105,7 +3105,7 @@ void i915_gem_object_free(struct drm_i915_gem_object *obj);
>   void i915_gem_object_init(struct drm_i915_gem_object *obj,
>   			 const struct drm_i915_gem_object_ops *ops);
>   struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
> -						  size_t size);
> +						   u64 size);
>   struct drm_i915_gem_object *i915_gem_object_create_from_data(
>   		struct drm_device *dev, const void *data, size_t size);
>   void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file);
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 838dc159a2d1..181bda2db587 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4131,14 +4131,27 @@ static const struct drm_i915_gem_object_ops i915_gem_object_ops = {
>   	.put_pages = i915_gem_object_put_pages_gtt,
>   };
>   
> -struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
> -						  size_t size)
> +#define overflows_type(x, T) \
> +	(sizeof(x) > sizeof(T) && (x) >> (sizeof(T) * BITS_PER_BYTE))
> +

Looks like it wouldn't detect storing unsigned int in a signed int but I 
guess we don't care that much as long as this is local use only. Just 
slightly relevant because of the int page_count situation we mention below.

> +struct drm_i915_gem_object *
> +i915_gem_object_create(struct drm_device *dev, u64 size)
>   {
>   	struct drm_i915_gem_object *obj;
>   	struct address_space *mapping;
>   	gfp_t mask;
>   	int ret;
>   
> +	/* There is a prevalence of the assumption that we fit the object's
> +	 * page count inside a 32bit variable. Let's document this and catch

_Signed_ 32-bit integer as you have explained to justify the INT_MAX below.

> +	 * if we ever need to fix it.
> +	 */
> +	if (WARN_ON(size >> PAGE_SHIFT > INT_MAX))
> +		return ERR_PTR(-E2BIG);
> +
> +	if (overflows_type(size, obj->base.size))
> +		return ERR_PTR(-E2BIG);
> +
>   	obj = i915_gem_object_alloc(dev);
>   	if (obj == NULL)
>   		return ERR_PTR(-ENOMEM);

With the comment clarification,

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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