Re: [PATCH 00/10] Enhance /dev/mem to allow read/write ofarbitrary physical addresses

From: Matthew Wilcox
Date: Sun Jun 19 2011 - 20:53:04 EST

On Mon, Jun 20, 2011 at 10:46:08AM +1000, Ryan Mallon wrote:
> On 20/06/11 10:42, Matthew Wilcox wrote:
>> On Mon, Jun 20, 2011 at 09:02:17AM +1000, Ryan Mallon wrote:
>>> There are drivers where this makes sense. For example an FPGA device
>>> with a proprietary register layout on the memory bus can be done this
>>> way. The FPGA can simply be mapped in user-space via /dev/mem and
>>> handled there. If the device requires no access other than memory bus
>>> reads and writes then writing a custom char device driver just to get an
>>> mmap function seems a bit overkill.
>> Calling a 30 line device driver "overkill" might in itself be overkill?
> I mean overkill in the sense of having to write the driver at all. Why
> write a 30 line driver just to re-implement some functionality of
> /dev/mem?

Because it pushes the tradeoff in the right direction. Somebody wants
to do something weird is a little inconvenienced vs protecting the vast
majority of users from some security escalation problems.

Besides, if you have a real bus with discoverable regions
(like PCI BARs), the bus should have sysfs entries like
/sys/bus/pci/devices/0000\:06\:06.0/resource0 that can be mmaped.
Then there's no need for a device driver at all, *and* the privilege
escalation isn't achievable.

Of course, most embedded architectures have crap discoverability.

Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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