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Date: Sat, 21 Jul 2012 09:23:58 -0700
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Subject: Re: sysfs permissions on dynamic attributes (led delay_on and delay_off)
From: Colin Cross <ccross@android.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Richard Purdie <richard.purdie@linuxfoundation.org>,
        lkml <linux-kernel@vger.kernel.org>, Bryan Wu <bryan.wu@canonical.com>
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On Sat, Jul 21, 2012 at 9:15 AM, Greg KH <gregkh@linuxfoundation.org> wrote:
> On Sat, Jul 21, 2012 at 01:08:55PM -0300, Henrique de Moraes Holschuh wrote:
>> On Sat, 21 Jul 2012, Colin Cross wrote:
>> > The delay_on and delay_off files could easily override the values from
>> > the trigger.
>> >
>> > Sending a KOBJ_CHANGE uevent is not a great solution, it's still
>> > horribly racy in userspace.  This script would never work reliably:
>> > echo timer > trigger
>> > echo 1000 > delay_on
>> > echo 1000 > delay_off
>> > echo 255 > brightness
>>
>> Yes, and the proper fix is to instead use a fully async userspace based on
>> uevent callbacks.  Nasty as all hell.  Or the quick fix, which is to wait
>> for the system to settle after every sysfs operation that could create new
>> sysfs nodes.
>>
>> You could make sure that (1) no sysfs operation will return control to
>> userspace until it is complete, so you'd have all new sysfs nodes available
>> at the time the first echo returns [I believe it already works like that],
>
> Yes it does, what's the problem here?
>
>> and (2) either enhance sysfs to create nodes with the desired ownership and
>> permissions
>
> From the kernel, you can also do this today, if you know it's "safe" for
> users to read/write them.

You can set the permissions, but I don't know of any way to set the
owner.  For my case, I need them to be writable by the "system" user
but not by all users.