Comment 4 for bug 914348

Indeed user processes cannot, and should not be able to, change sysctls. First, they do not have the necessary privileges, and second, if you have multiple users logged in at the same time, they would all try to set the value concurrently, possibly to different values.

The usual approach to this kind of problem is to add an API to a system service such as upower, i. e. an API to get and set the touchpad LED. This API then gets a policykit restriction that only the currently active user session can call it, similar to e. g. triggering a suspend. For an example, you can look at org.freedesktop.upower.suspend in /usr/share/polkit-1/actions/org.freedesktop.upower.policy. Programs like gnome-settings-daemon can then call this API as normal user, and the system daemon is able to write the value to sysfs/the kernel, providing proper separation of privileges and avoiding the race condition for multiple users.

However, I would suggest to reconsider implementing this in gnome-settings-daemon. It seems like a hardware design wart that this needs to be controlled by software, but if it has to be done, I think it would be a lot more robust and efficient to do it right in the kernel's touchpad driver. Efficient because it avoids the whole "enable device in the kernel, send notification to userspace, react to it in g-s-d, send to upower, write to sysfs, trigger it in the kernel" loop, and robust because it will also work for other desktop environments which do not have gnome-settings-daemon (XFCE, KDE), in the lightdm greeter, gdm, and so on.

If you decide for an implementation in upower anyway, I suggest talking to Richard Hughes (hughsie) in #udev on freenode about it, as we do not change the API of widely known system daemons in Ubuntu only (this would only introduce long-term incompatibility and a large maintenance overhead), so that this work can get accepted upstream. If you want to go for an in-kernel implementation (which also should be quite a bit smaller), I recommend talking to the kernel team how to get this upstream, as I'm not familiar with that procedure.

Thanks!