linux-kernel - Re: [PATCH] iommu/rockchip: check return value of of_find_device_by_node() in rk_iommu_of

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Date:   Thu, 29 Oct 2020 13:51:23 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     "yukuai (C)" <yukuai3@...wei.com>, joro@...tes.org,
        heiko@...ech.de, jeffy.chen@...k-chips.com
Cc:     iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH] iommu/rockchip: check return value of
 of_find_device_by_node() in rk_iommu_of_xlate()

On 2020-10-29 13:19, yukuai (C) wrote:
> 
> On 2020/10/29 18:08, Robin Murphy wrote:
>> On 2020-10-29 09:22, Yu Kuai wrote:
>>> If of_find_device_by_node() failed in rk_iommu_of_xlate(), null pointer
>>> dereference will be triggered. Thus return error code if
>>> of_find_device_by_node() failed.
>>
>> How can that happen? (Given that ".suppress_bind_attrs = true")
>>
>> Robin.
> 
> I'm not sure if that could happen...
> 
> My thought is that it's better to do such checking to aviod any possible
> problem.

->of_xlate() is only invoked on the specific set of ops returned by 
iommu_ops_from_fwnode(). In turn, iommu_ops_from_fwnode() will only 
return those ops if the driver has successfully probed and called 
iommu_register_device() with the relevant DT node. For the driver to 
have been able to probe at all, a platform device associated with that 
DT node must have been created, and therefore of_find_device_by_node() 
cannot fail.

If there ever were some problem serious enough to break that fundamental 
assumption, then I *want* these drivers to crash right here, with a nice 
clear stack trace to start debugging from. So no, I firmly disagree that 
adding redundant code, which will never do anything except attempt to 
paper over catastrophic memory corruption, is "better". Sorry :)

Robin.