linux-kernel - Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

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Date:	Sun, 17 Mar 2013 15:29:39 +0100
From:	Kay Sievers <kay@...y.org>
To:	Myron Stowe <mstowe@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Myron Stowe <myron.stowe@...hat.com>,
	linux-hotplug@...r.kernel.org, linux-pci@...r.kernel.org,
	yuxiangl@...vell.com, yxlraid@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

On Sun, Mar 17, 2013 at 3:20 PM, Myron Stowe <mstowe@...hat.com> wrote:
> On Sun, 2013-03-17 at 15:00 +0100, Kay Sievers wrote:
>> On Sun, Mar 17, 2013 at 2:38 PM, Alex Williamson
>> <alex.williamson@...hat.com> wrote:
>> > I'm assuming that the device only breaks because udevadm is dumping the
>> > full I/O port register space of the device and that if an actual driver
>> > was interacting with it through this interface that it would work.  Who
>> > knows how many devices will have read side-effects by udevadm blindly
>> > dumping these files.  Thanks,
>>
>> Sysfs is a too public interface to export things there which make
>> devices/driver choke on a simple read() of an attribute.
>>
>> This is nothing specific to udevadm, any tool can do that. Udevadm
>> will never read any of the files during normal operation. The admin
>> explicitly asked udevadm with a specific command to dump all the stuff
>> the device offers.
>>
>> The kernel driver needs to be fixed to allow that, in the worst case,
>> the attributes not exported at all. People should take more care what
>> they export in /sys, it's not a hidden and private ioctl what's
>> exported there, stuff is very visible and will be looked at.
>>
>> Telling userspace not to use specific stuff in /sys I would not expect
>> to work as a strategy; there is too much weird stuff out there that
>> will always try to do that ...
>
> Kay - could you comment on Foot Note 3 in
> https://lkml.org/lkml/2013/3/16/168
>
> With respect to 'udev', you are working on the assumption that all files
> in sysfs must be readable with no consequences which may be implied by
> the Documentation's sysfs.txt file's mentioning ASCII.  If we are to
> interpret that as strictly as you seem to want to then why is there
> sysfs support for creating binary files?

They cannot be distinguished from outside, so there is nothing I know
that could make a difference to userspace tools.

Tools -- no matter how useful they are not not, it's that they do that
for many years already -- need to be able to read() the stuff in
there, without causing any damage to the system.

Kay
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