Pardon for the long introduction, but I haven't seen any other questions for this on SO.
I'm playing with DRM (Direct Rendering Manager, a wrapper for Linux kernel mode setting) and I'm having difficulty understanding a part of its design.
Basically, I can open a graphic card device in my virtual terminal, set up frame buffers, change connector and its CRTC just fine. This results in me being able to render to VT in a lightweight graphic mode without need for X server (that's what kms is about, and in fact X server uses it underneath).
Then I wanted to implement graceful VT switching, so when I hit ctrl+alt+f3 etc., I can see my other consoles. Turns out it's easy to do with calling ioctl()
with stuff from linux/vt.h
and handling some user signals.
But then I tried to switch from my graphic program to a running X server. Bzzt! didn't work at all. X server didn't draw anything at all. After some digging I found that in Linux kernel, only one program can do kernel mode setting. So what happens is this:
- I switch from X to a virtual terminal
- I run my program
- This program enters graphic mode with
drmOpen
,drmModeSetCRTC
etc. - I switch back to X
- X has no longer privileges to restore its own mode.
Then I found this in wayland source code: drmDropMaster()
and drmSetMaster()
. These functions are supposed to release and regain privileges to set modes so that X server can continue to work, and after switching back to my program, it can take it from there.
Finally the real question. These functions require root privileges. This is the part I don't understand. I can mess with kernel modes, but I can't say "okay X11, I'm done playing, I'm giving you the access now"? Why? Or should this work in theory, and I'm just doing something wrong in my code? (e.g. work with wrong file descriptors, or whatever.)
If I try to run my program as a normal user, I get "permission denied". If I run it as root, it works fine - I can switch from X to my program and vice versa.
Why?