linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Fix undefined behavior in GBPA

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Message-ID: <20220701143401.GA28408@willie-the-truck>
Date:   Fri, 1 Jul 2022 15:34:02 +0100
From:   Will Deacon <will@...nel.org>
To:     Xenia Ragiadakou <burzalodowa@...il.com>
Cc:     joro@...tes.org, robin.murphy@....com,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix undefined behavior in GBPA_UPDATE

On Thu, Jun 30, 2022 at 09:39:59AM +0300, Xenia Ragiadakou wrote:
> The expression 1 << 31 results in undefined behaviour because the type of
> integer constant 1 is (signed) int and the result of shifting 1 by 31 bits
> is not representable in the (signed) int type.
> 
> Change the type of 1 to unsigned int by adding the U suffix.
> 
> Signed-off-by: Xenia Ragiadakou <burzalodowa@...il.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> index cd48590ada30..44fbd499edea 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> @@ -96,7 +96,7 @@
>  #define CR2_E2H				(1 << 0)
>  
>  #define ARM_SMMU_GBPA			0x44
> -#define GBPA_UPDATE			(1 << 31)
> +#define GBPA_UPDATE			(1U << 31)

There are loads of these kicking around in the kernel sources and we compile
with -fno-strict-overflow.

If you really want to change these, then let's use the BIT() macro instead,
but I think it's really just churn.

Will