From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Date: Mon, 20 Jun 2011 00:52:55 +0000 Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of Message-Id: <20110620005255.GF19693@parisc-linux.org> List-Id: References: <201106171038.25988.ptesarik@suse.cz> <20110617093032.GA19235@elte.hu> <4DFE7FF9.9070406@gmail.com> <20110620004252.GE19693@parisc-linux.org> <4DFE9850.7030400@gmail.com> In-Reply-To: <4DFE9850.7030400@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-arm-kernel@lists.infradead.org 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."