Re: [PATCH] iommu/rockchip: check return value of of_find_device_by_node() in rk_iommu_of_xlate()

[Robin Murphy]

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.