Re: [RFC PATCH] restrict /dev/mem to idle io memory ranges

From: Dan Williams
Date: Fri Nov 20 2015 - 15:26:53 EST

On Fri, Nov 20, 2015 at 12:12 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Fri, Nov 20, 2015 at 09:31:33AM -0800, Dan Williams wrote:
>> This effectively promotes IORESOURCE_BUSY to IORESOURCE_EXCLUSIVE
>> semantics by default. If userspace really believes it is safe to access
>> the memory region it can also perform the extra step of disabling an
>> active driver. This protects device address ranges with read side
>> effects and otherwise directs userspace to use the driver.
> I'm happy with this as long as we retain the option to disable this
> new behaviour.
> The reason being, when developing a driver, it is _very_ useful to
> be able to poke around in the device's (and system memory) address
> spaces with tools like devmem2 to work out what's going on when
> things go wrong.
> To put it another way, I think it's a good idea to disable access to
> these regions on production systems, but for driver development, we
> want to retain the ability to poke around in physical address space
> in any way we so desire.

Sounds ok to me, but I do think it's a good idea to default it to the
same value as STRICT_DEVMEM. Perhaps:

bool "Filter I/O access to /dev/mem" if EXPERT

When this in do we even need IORESOURCE_EXCLUSIVE? It's barely used.
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