linux-kernel - Re: /dev/mem "unbounded?"

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Message-ID: <4EBB16BE.9050308@zytor.com>
Date:	Wed, 09 Nov 2011 16:11:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrei Warkentin <awarkentin@...are.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: /dev/mem "unbounded?"

On 11/09/2011 02:38 PM, Andrei Warkentin wrote:
> Hi Peter,
> 
> ----- Original Message -----
>>
>> That would be incorrect behavior, though, except perhaps for the
>> range
>> that cannot be addressed by the processor.  It is explicitly
>> permitted
>> to address ranges that does not have addresses mapped to it.
> 
> There is a current mechanism for restricting access to a subset
> of addresses, and it is used to enforce < 1MB accesses on x86
> if CONFIG_STRICT_DEVMEM. This could be extended.
> 

Well, that mechanism is broken.  The way it *should* work is that any
region which is system RAM should be denied access, and the rest is
accessible.  The current behavior is a hack due to the behavior of some
old versions of Xorg, but that has long been fixed.

> Do you think there is any value in specifying something like
> CONFIG_DEV_MEM_ONLY_CLAIMED, which would only allow accesses
> until the end of the last range claimed with request_region (but
> could, of course, well be unmapped). This will allow accesses to
> any unclaimed "holes" in between. I.e., if you have 0-100m claimed,
> followed by 300-700m claimed, then reading /dev/mem will work up
> until you reach 700m.

No.  If you don't know what you're accessing, you should not be touching
/dev/mem under any circumstances.  Odds are that even if you're only
reading, there are registers with side effects in there somewhere.

	-hpa

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