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Date:   Tue, 15 Sep 2020 17:09:29 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Thomas Tai <thomas.tai@oracle.com>
Cc:     Christoph Hellwig <hch@lst.de>, konrad.wilk@oracle.com,
        m.szyprowski@samsung.com, robin.murphy@arm.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] dma-direct: Fix potential NULL pointer dereference
Message-ID: <20200915150929.GA19770@lst.de>
References: <1600178594-22801-1-git-send-email-thomas.tai@oracle.com> <20200915140719.GA14831@lst.de> <f5cba632-421a-f375-3697-51a182a53a32@oracle.com> <20200915142624.GA16005@lst.de> <da9ec51d-aab5-695d-e388-5ae7c0bb30ea@oracle.com>
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On Tue, Sep 15, 2020 at 10:40:39AM -0400, Thomas Tai wrote:
>> +++ b/include/linux/dma-direct.h
>> @@ -62,9 +62,6 @@ 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;
>> -
>
> I am concerned that some drivers may rely on this NULL checking. Would you 
> think we can keep this checking and use the following WARN_ON_ONCE()?

dma_capable is not a helper for drivers, but just for dma-direct
and related code.  And this patch adds the checks for the three
places how we call into the ->map* methods.