Re: [PATCH] arm: dma-mapping: fix off-by-one check in arm_setup_iommu_dma_ops

[Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>]

Hi Marek,

On 28/04/15 10:08, Marek Szyprowski wrote:
> Patch 22b3c181c6c324a46f71aae806d8ddbe61d25761 ("arm: dma-mapping: limit
> IOMMU mapping size") added a check for IO address space size. However
> this patch broke IOMMU initialization for typical platforms initialized
> from device tree, which get the default IO address space size of 4GiB.
> This value doesn't fit into size_t and fails a check introduced by that
> commit resulting in failed dma-mapping/iommu initialization. This patch
> fixes this issue by adding proper support for full 4GiB address space
> size.
>
> Signed-off-by: Marek Szyprowski <m.szyprowski-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>

Much nicer than my hack of just passing in size-1, thanks! I'd offer a 
tested-by for the default 32-bit case, however...

> ---
>   arch/arm/include/asm/dma-iommu.h | 2 +-
>   arch/arm/mm/dma-mapping.c        | 9 +++------
>   2 files changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm/include/asm/dma-iommu.h b/arch/arm/include/asm/dma-iommu.h
> index 8e3fcb924db6..2ef282f96651 100644
> --- a/arch/arm/include/asm/dma-iommu.h
> +++ b/arch/arm/include/asm/dma-iommu.h
> @@ -25,7 +25,7 @@ struct dma_iommu_mapping {
>   };
>
>   struct dma_iommu_mapping *
> -arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, size_t size);
> +arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, u64 size);
>
>   void arm_iommu_release_mapping(struct dma_iommu_mapping *mapping);
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 09c5fe3d30c2..b43b762eebc1 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1878,7 +1878,7 @@ struct dma_map_ops iommu_coherent_ops = {
>    * arm_iommu_attach_device function.
>    */
>   struct dma_iommu_mapping *
> -arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, size_t size)
> +arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, u64 size)
>   {
>   	unsigned int bits = size >> PAGE_SHIFT;

...doesn't this u64->int conversion now have the potential to truncate 
(if the device has a large enough DMA mask specified) and end up 
generating a weirdly small mapping rather than failing outright? 
Admittedly you'd have to have a 44-bit or larger DMA mask, and I'm not 
sure what the practical likelihood of seeing that even on LPAE systems 
is, but still I'm a little uneasy about it.

Robin.

>   	unsigned int bitmap_size = BITS_TO_LONGS(bits) * sizeof(long);
> @@ -2057,11 +2057,8 @@ static bool arm_setup_iommu_dma_ops(struct device *dev, u64 dma_base, u64 size,
>   	if (!iommu)
>   		return false;
>
> -	/*
> -	 * currently arm_iommu_create_mapping() takes a max of size_t
> -	 * for size param. So check this limit for now.
> -	 */
> -	if (size > SIZE_MAX)
> +	/* Only 32-bit DMA address space is supported for now */
> +	if (size > DMA_BIT_MASK(32) + 1)
>   		return false;
>
>   	mapping = arm_iommu_create_mapping(dev->bus, dma_base, size);
>