linux-kernel - Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses

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Message-ID: <20110620005255.GF19693@parisc-linux.org>
Date:	Sun, 19 Jun 2011 18:52:55 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Petr Tesarik <ptesarik@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of
	arbitrary physical addresses

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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