linux-kernel - Re: [PATCH v3 4/7] iommu: Switch gather->end to unsigned long long

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Date:   Wed, 16 Dec 2020 12:10:22 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Yong Wu <yong.wu@...iatek.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>
Cc:     youlin.pei@...iatek.com, anan.sun@...iatek.com,
        Nicolas Boichat <drinkcat@...omium.org>,
        srv_heupstream@...iatek.com, chao.hao@...iatek.com,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Tomasz Figa <tfiga@...gle.com>,
        iommu@...ts.linux-foundation.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        Greg Kroah-Hartman <gregkh@...gle.com>,
        kernel-team@...roid.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 4/7] iommu: Switch gather->end to unsigned long long

On 2020-12-16 10:36, Yong Wu wrote:
> Currently gather->end is "unsigned long" which may be overflow in
> arch32 in the corner case: 0xfff00000 + 0x100000(iova + size).
> Although it doesn't affect the size(end - start), it affects the checking
> "gather->end < end"

This won't help the same situation at the top of a 64-bit address space, 
though, and now that we have TTBR1 support for AArch64 format that is 
definitely a thing. Better to just encode the end address as the actual 
end address, i.e. iova + size - 1. We don't lose anything other than the 
ability to encode zero-sized invalidations that don't make sense anyway.

Robin.

> Fixes: a7d20dc19d9e ("iommu: Introduce struct iommu_iotlb_gather for batching TLB flushes")
> Signed-off-by: Yong Wu <yong.wu@...iatek.com>
> ---
>   include/linux/iommu.h | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 794d4085edd3..6e907a95d981 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -178,7 +178,7 @@ enum iommu_dev_features {
>    */
>   struct iommu_iotlb_gather {
>   	unsigned long		start;
> -	unsigned long		end;
> +	unsigned long long	end;
>   	size_t			pgsize;
>   };
>   
> @@ -537,7 +537,8 @@ static inline void iommu_iotlb_gather_add_page(struct iommu_domain *domain,
>   					       struct iommu_iotlb_gather *gather,
>   					       unsigned long iova, size_t size)
>   {
> -	unsigned long start = iova, end = start + size;
> +	unsigned long start = iova;
> +	unsigned long long end = start + size;
>   
>   	/*
>   	 * If the new page is disjoint from the current range or is mapped at
>