linux-kernel - Re: [PATCH] dma-direct: Fix potential NULL pointer dereference

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Date:   Wed, 16 Sep 2020 08:19:49 +0200
From:   Christoph Hellwig <hch@....de>
To:     Thomas Tai <thomas.tai@...cle.com>
Cc:     Christoph Hellwig <hch@....de>, konrad.wilk@...cle.com,
        m.szyprowski@...sung.com, robin.murphy@....com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Fix potential NULL pointer dereference

On Tue, Sep 15, 2020 at 04:46:17PM -0400, Thomas Tai wrote:
> I tried out the suggested changes, and it successfully warned the null 
> pointer without panic. I notice that there are some places outside the 
> dma-direct, which calls dma_capable().
>
> https://elixir.bootlin.com/linux/v5.9-rc5/source/arch/x86/kernel/amd_gart_64.c#L187
>
> https://elixir.bootlin.com/linux/v5.9-rc5/source/drivers/xen/swiotlb-xen.c#L387

All of these still come in throught the wrappers in kernel/dma/mapping.c.

> Given that the WARN_ON_ONCE already did the intended warning, would you be 
> ok that I keep the null checking in dma_capable()?

No, the generic dma mapping layer is the right place.