LinuxFB access control idea

[David H. <dho...@ca...>]

I don't know whether anyone on this list has considered this idea, but I
thought I'd put it up anyway:

I've recently been having a look at writing an FB driver, and it occurs to
me that it might be possible to control access to multiple virtual
framebuffers by making use of the Linux VM system.

Basically, when a framebuffer is active, the processes that use it would
have the relevant virtual pages marked as "present" in the page
table. When not active, the attached processes would have the same pages
marked as "not present".

This means that the "nopage" function on the VMA for the framebuffer would
be called to load the page. This could send a SIGTTOU signal to indicate
that the framebuffer is not available for use at the moment (rather than
sending a SIGBUS signal, since the page can't be made available). A
SIGCONT signal would then be automatically sent when the framebuffer
becomes available again.

Unless the SIGTTOU signal is trapped or ignored, this would suspend the
process, and the SIGCONT signal would resume it.

Cheers,
David Howells