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Message-ID: <b090141f-0822-609f-429d-ea790d7fc828@arm.com>
Date:   Thu, 10 Feb 2022 10:43:17 +0000
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Subject: Re: [PATCH] iommu: explicitly check for NULL in
 iommu_dma_get_resv_regions()
Content-Language: en-GB
To:     Aleksandr Fedorov <sanekf@nxt.ru>, Joerg Roedel <joro@8bytes.org>,
        Will Deacon <will@kernel.org>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
References: <5481021644415780@myt5-a5512e99e394.qloud-c.yandex.net>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <5481021644415780@myt5-a5512e99e394.qloud-c.yandex.net>
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On 2022-02-09 14:09, Aleksandr Fedorov wrote:
> iommu_dma_get_resv_regions() assumes that iommu_fwspec field for
> corresponding device is set which is not always true.  Since
> iommu_dma_get_resv_regions() seems to be a future-proof generic API
> that can be used by any iommu driver, add an explicit check for NULL.

Except it's not a "generic" interface for drivers to call at random, 
it's a helper for retrieving common firmware-based information 
specifically for drivers already using the fwspec mechanism for common 
firmware bindings. If any driver calls this with a device *without* a 
valid fwnode, it deserves to crash because it's done something 
fundamentally wrong.

I concur that it's not exactly obvious that "non-IOMMU-specific" means 
"based on common firmware bindings, thus implying fwspec".

Robin.

> Currently it can work by accident since compiler can eliminate
> the 'iommu_fwspec' check altogether when CONFIG_ACPI_IORT=n, but
> code elimination from optimizations is not reliable.
> 
> Signed-off-by: Aleksandr Fedorov <halcien@gmail.com>
> ---
> A compilation failure has been observed on a gcc-compatible compiler based on EDG.
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index d85d54f2b549..474b1b7211d7 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -382,10 +382,10 @@ void iommu_put_dma_cookie(struct iommu_domain *domain)
>    */
>   void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list)
>   {
> +	struct iommu_fwspec *iommu_fwspec = dev_iommu_fwspec_get(dev);
>   
> -	if (!is_of_node(dev_iommu_fwspec_get(dev)->iommu_fwnode))
> +	if (iommu_fwspec && !is_of_node(iommu_fwspec->iommu_fwnode))
>   		iort_iommu_msi_get_resv_regions(dev, list);
> -
>   }
>   EXPORT_SYMBOL(iommu_dma_get_resv_regions);
>   
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