On Mon, Jun 20, 2011 at 10:46:08AM +1000, Ryan Mallon wrote:

On 20/06/11 10:42, Matthew Wilcox wrote:

quoted

On Mon, Jun 20, 2011 at 09:02:17AM +1000, Ryan Mallon wrote:

quoted

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."