Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] dma-direct: Fix potential NULL pointer dereference
To:     Christoph Hellwig <hch@lst.de>
Cc:     konrad.wilk@oracle.com, m.szyprowski@samsung.com,
        robin.murphy@arm.com, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
References: <1600178594-22801-1-git-send-email-thomas.tai@oracle.com>
 <20200915140719.GA14831@lst.de>
From:   Thomas Tai <thomas.tai@oracle.com>
Message-ID: <f5cba632-421a-f375-3697-51a182a53a32@oracle.com>
Date:   Tue, 15 Sep 2020 10:11:51 -0400
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.12.0
MIME-Version: 1.0
In-Reply-To: <20200915140719.GA14831@lst.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2020-09-15 10:07 a.m., Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 08:03:14AM -0600, Thomas Tai wrote:
>> When booting the kernel v5.9-rc4 on a VM, the kernel would panic when
>> printing a warning message in swiotlb_map(). It is because dev->dma_mask
>> can potentially be a null pointer. Using the dma_get_mask() macro can
>> avoid the NULL pointer dereference.
> 
> dma_mask must not be zero.  This means drm is calling DMA API functions
> on something weird.  This needs to be fixed in the caller.
> 

Thanks, Christoph for your comment. The caller already fixed the null 
pointer in the latest v5.9-rc5. I am thinking that if we had used the 
dma_get_mask(), the kernel couldn't panic and could properly print out 
the warning message.

Thomas