Re: [PATCH 2/2] iommu: apple-dart: Support specifying the DMA aperture in the DT

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Undefined behavior via invalid bit shift when parsing a 1-byte DMA window starting at address 0.
--

--- Patch [2]: [PATCH 2/2] iommu: apple-dart: Support specifying the DMA aperture in the DT ---
commit ce65194c53df34be71115a91f5d6328ad1a90109
Author: Janne Grunau <j@jannau.net>

iommu: apple-dart: Support specifying the DMA aperture in the DT

This commit adds support for restricting the DMA aperture via the Device Tree
using the iommus args. Devices can specify the start and end of the aperture
if the DART device node specifies #iommu-cells = <5>.

> diff --git a/drivers/iommu/apple-dart.c b/drivers/iommu/apple-dart.c
> --- a/drivers/iommu/apple-dart.c
> +++ b/drivers/iommu/apple-dart.c
> @@ -590,6 +600,7 @@ static int apple_dart_finalize_domain(struct apple_dart_domain *dart_domain,
>  {
>  	struct apple_dart *dart = cfg->stream_maps[0].dart;
>  	struct io_pgtable_cfg pgtbl_cfg;
> +	u32 ias = min_t(u32, dart->ias, fls64(cfg->dma_max));
>  	int ret = 0;
>  	int i, j;

[ ... ]

> @@ -623,10 +634,10 @@ static int apple_dart_finalize_domain(struct apple_dart_domain *dart_domain,
>  		goto done;
>  	}
>  
> +	dart_domain->mask = DMA_BIT_MASK(pgtbl_cfg.ias);

[Severity: Low]
Does this code trigger undefined behavior if a 1-byte DMA window starting at
address 0 is provided in the device tree?

If a device tree is configured with length = 1 and dma_min = 0,
apple_dart_of_xlate() calculates dma_max as 0:

apple_dart_of_xlate()
    dma_max = dma_min + length - 1; /* 0 + 1 - 1 = 0 */

This means cfg->dma_max will be 0 here, making fls64(cfg->dma_max) 0,
and resulting in ias = 0.

If ias is 0, this expands to DMA_BIT_MASK(0), which performs an invalid bit
shift (~0ULL >> 64).

Since size 0 (!length) is explicitly rejected in apple_dart_of_xlate(),
should size 1 also be handled to avoid this edge case?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260819-iommu-apple-dart-aperture-v1-0-252703f381aa@jannau.net?part=2