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Date: Sat, 21 Jul 2012 09:15:23 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Colin Cross <ccross@android.com>,
        Richard Purdie <richard.purdie@linuxfoundation.org>,
        lkml <linux-kernel@vger.kernel.org>, Bryan Wu <bryan.wu@canonical.com>
Subject: Re: sysfs permissions on dynamic attributes (led delay_on and
 delay_off)
Message-ID: <20120721161523.GB22896@kroah.com>
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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.

greg k-h