Re: sysfs permissions on dynamic attributes (led delay_on and delay_off)

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Jul 21, 2012 at 08:42:12AM -0700, Colin Cross wrote:
> On Sat, Jul 21, 2012 at 4:21 AM, Richard Purdie
> <richard.purdie@linuxfoundation.org> wrote:
> > On Sat, 2012-07-21 at 01:26 -0700, Colin Cross wrote:
> >> On Sat, Jul 21, 2012 at 12:33 AM, Richard Purdie
> >> <richard.purdie@linuxfoundation.org> wrote:
> >> > On Fri, 2012-07-20 at 21:08 -0700, Greg KH wrote:
> >> >> On Fri, Jul 20, 2012 at 05:46:14PM -0700, Colin Cross wrote:
> >> >> > I'm trying to use the standard ledtrig-timer.c code to handle led
> >> >> > blinking for notifications on an Android device, and I'm hitting some
> >> >> > issues with setting permissions on the dynamically created delay_on
> >> >> > and delay_off attributes.  For most sysfs files, we have userspace
> >> >> > uevent parser that watches for device add notifications and
> >> >> > chowns/chmods attributes.  This doesn't work for delay_on and
> >> >> > delay_off, because they are created later, when "timer" is written to
> >> >> > the trigger attribute.  There is no uevent when the new files are
> >> >> > created, and sysfs doesn't support inotify, so I don't see any way to
> >> >> > receive an event to set the permissions.  This issue exists any time
> >> >> > that device_create_file is called after device_add.
> >> >> >
> >> >> > What is the appropriate way to get an event to set the permissions?
> >> >> > Add inotify support for sysfs file creation?  Send a KOBJ_CHANGE
> >> >> > uevent in device_create_file?
> >> >>
> >> >> No.
> >> >>
> >> >> > Send a KOBJ_CHANGE uevent from the driver after calling
> >> >> > device_create_file?
> >> >>
> >> >> Yes.
> >> >>
> >> >> > Dynamically create a timer device under /sys/class/leds/<led> so a new
> >> >> > add uevent gets sent?
> >> >>
> >> >> Ick.
> >> >>
> >> >> > Promote blinking to be a core led feature instead of a trigger, so the
> >> >> > files are always present?
> >> >>
> >> >> That's the best thing, why not just do that?
> >> >
> >> > This implies we should make every trigger a core led feature and
> >> > effectively do away with triggers. I'm not sure that makes sense.
> >>
> >> Blinking is already effectively a core feature.  It is implemented in
> >> led-core.c so it can be used by other triggers besides timer, it's
> >> state is stored in the led_classdev structure, not in the trigger
> >> data, and the only thing left in ledtrig-timer.c is the sysfs files.
> >
> > Having the attributes present all the time leads to some nasty questions
> > like how the on/off delays interact with things like say a network
> > activity trigger. Not all triggers are going to respect these delay
> > values and I can imagine a whole new set of nasty bug reports with no
> > easy solutions if this change is made...
> 
> 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

When this returned, the sysfs files would then be there, right?

> echo 1000 > delay_on
> echo 1000 > delay_off
> echo 255 > brightness

So this would work.

What is racy here?

greg k-h