[UNSOVABLE] Screenshoting Another X Display

Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

Is there any way to get a screenshot of another X server instance, running at another $DISPLAY?

If I try:

import -window root -display :2.0 ./test.png

I get just a black image.
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eccerr0r · Posted: Tue Nov 08, 2016 12:00 am Post subject:

Yes it should work just fine, anything that has proper -display (DISPLAY=) handling theoretically should work.

However I've had problems with applications that use Xvideo that sends video data straight to the screen bypassing the normal X video buffer.

And also had problems with screensaver kick-in...
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Roman_Gruber · Posted: Tue Nov 08, 2016 12:26 pm Post subject:

I think it depends on your window manager

I'm kinda certain that scrot works in i3wm on any attached screen with my custom key binding.

Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

scrot also doesn't work here. Maybe my problem is not really clear.

Imagine I have two X sessions started (I've tried with Enlightenment and KDE), one at display :1.0 and another at display :2.0

In display :1.0, I open a terminal and type "sleep 5 && scrot test.png". Then I quickly switch to display :2.0 by pressing ALT+CTRL+F2 and wait. Once I look at test.png, it is completely black.

I am imagining that X stops rendering the screen at :1.0 as soon as I switch to another session, because supposedly it is not needed... is that correct?
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Roman_Gruber · Posted: Tue Nov 08, 2016 2:54 pm Post subject:

I think it depends on how you have set it up your box, your environment

I'm not sure what your gpu driver or the x-server does.
You may be correct that the gpu driver or the x-server stops rendering that second "inactive" x session.

Here we are back to the question, how you have set it up. And whats the impact on that configuration.

Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

dufeu · l33t Joined: 30 Aug 2002 Posts: 924 Location: US-FL-EST

Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

Zucca · Posted: Thu Nov 10, 2016 1:24 pm Post subject:

Interesting... However being able to do this poses a security risk. Especially in places like libraries (I still hope no-one logs into bank from library PC) where they may have set up a multiseated setup to save money by having just one PC for multiple users. So another user could take screenshots from another seat's screen.
But I think it's already solved when you set the second seat and assign a GPU for it. Then the first seat should not have any accees to it... Finally SELinux should, at least, be able to deny screenshotting another seat's screen.

So... Just a thought.

eccerr0r · Posted: Thu Nov 10, 2016 4:25 pm Post subject:

This should not be a privacy risk if things are set up properly. You need to have root (why not just dump all video and main memory to look for anything) or have access to the session cookie anyway (you just need to find it...).

Ideally each seat should have their own login of course, all bets are off if all share the same login.

I don't see this as an impossible task, this should be possible to do, just that there may not be a tool to easily do it. And if that screen were blanked due to screensaver, yeah that'd be blank.
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russK · l33t Joined: 27 Jun 2006 Posts: 665

I saw this method on Google+

Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

Juippisi · Developer Joined: 30 Sep 2005 Posts: 724 Location: /home

Zucca · Posted: Thu Dec 01, 2016 9:23 pm Post subject:

russK · l33t Joined: 27 Jun 2006 Posts: 665

russK · l33t Joined: 27 Jun 2006 Posts: 665

eccerr0r · Posted: Fri Dec 02, 2016 4:31 am Post subject:

As said earlier, if you can find the xauth cookie file and point to it, then you can easily do it.

If that display you don't care about security and have full control of it otherwise, you could just give up all control to that display by using xhost to do hostwide authentication.

But if you're getting black screens, this isn't the problem...
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Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

I still don't get the black magic you guys are doing to achieve it!
I only get black screens.

I wrote this little C+Python script that is supposed to get a rectangle of the screen and convert it to a PIL image.
get_screen.c
pyGetScreen.py

Then after:

eccerr0r · Posted: Sun Dec 25, 2016 4:18 pm Post subject:

Each display has its own session cookie so you can't mess with the other display.

Actually, can you do this experiment: can you start 'xterm' or some other application on the other display?

If that works, your xauth/x security/MIT-session-cookie is working, and now I have no clue...
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Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

eccerr0r · Posted: Sun Dec 25, 2016 11:12 pm Post subject:

Ok, I think I see the same behavior when starting a second X server on the same GPU versus running a separate screen via Xnest or Xvnc on :1, :2, etc.

(gdm running on :0, my normal on :1, and a test X :2)

When use xwd on -display :2 from :1, it is black. It seems that indeed the X server does not keep state when not active on the VT.

I guess it makes a difference what X server, and to "fix" would require a server hack.
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dufeu · l33t Joined: 30 Aug 2002 Posts: 924 Location: US-FL-EST

eccerr0r · Posted: Mon Dec 26, 2016 7:13 am Post subject:

Well, that's the thing. Depending on the hardware/software I don't see why it couldn't maintain state for a second context with the memory on board, especially if the GPU can do GPGPU operations - and thus the GPU processors can dedicate to its own section of GPU memory. Then it would be easier to switch context displayed on the output. But I guess it's clear now - they do not share.

The strange thing is that if a X client changes its state on a screen that's not the current VT, it would also end up discarding all events until it becomes foreground again and a redraw is issued. That was a bit confusing that the warning should have stopped all clients and not AIGLX only.

But still it was quite interesting by experimentation this was the case for GPU/graphics card video memory, and I was thinking that the "other" server was actually Xvnc or Xnest display which have a completely virtual frame buffer (or perhaps another video card with its own fb) which can be snooped upon, and not the same video hardware with different context.
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kernelOfTruth · Posted: Mon Dec 26, 2016 7:00 pm Post subject:

*subscribing*,

interesting topic
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Holysword · l33t Joined: 19 Nov 2006 Posts: 946 Location: Greece

Okay, so long story short, this is a limitation of X itself, by design?
If so, are there any hopes that this will be simpler with Wayland instead?
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