linux-kernel - Re: [PATCH v2 1/2] dma-direct: check for overflows on 32 bit DMA addresses

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Date:   Wed, 30 Oct 2019 14:41:40 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     rubini@...dd.com, hch@...radead.org, linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, helgaas@...nel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/2] dma-direct: check for overflows on 32 bit DMA
 addresses

On Fri, Oct 18, 2019 at 01:00:43PM +0200, Nicolas Saenz Julienne wrote:
> +#ifndef CONFIG_ARCH_DMA_ADDR_T_64BIT
> +	/* Check if DMA address overflowed */
> +	if (min(addr, addr + size - 1) <
> +		__phys_to_dma(dev, (phys_addr_t)(min_low_pfn << PAGE_SHIFT)))
> +		return false;
> +#endif

Would be nice to use IS_ENABLED and PFN_PHYS here, and I also think we
need to use phys_to_dma to take care of the encryption bit.  If you then
also introduce an end variable we can make the whole thing actually look
nice:

static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
{
	dma_addr_t end = addr + size - 1;

        if (!dev->dma_mask)
                return false;

	if (!IS_ENABLED(CONFIG_ARCH_DMA_ADDR_T_64BIT) &&
	    min(addr, end) < phys_to_dma(dev, PFN_PHYS(min_low_pfn)))
		return false;

        return end <= min_not_zero(*dev->dma_mask, dev->bus_dma_mask);
}

Otherwise this looks sensible to me.