[Short Tip] Workaround MIT-SHM error when starting QT/KDE apps with SUDO

Starting GUI programs as root usually is not a problem. In worst case, sudo inside a terminal should do the trick.

However, recently I had to start a QT application as sudo from within GNOME. It was the yubikey configuration GUI, a third party tool thus not part of any desktop environment. Executing the app failed, it only showed a gray window and multiple errors in the command line:

$ sudo /usr/bin/yubikey-personalization-gui 
X Error: BadAccess (attempt to access private resource denied) 10
  Extension:    130 (MIT-SHM)
  Minor opcode: 1 (X_ShmAttach)
  Resource id:  0x142
X Error: BadShmSeg (invalid shared segment parameter) 128
  Extension:    130 (MIT-SHM)
  Minor opcode: 5 (X_ShmCreatePixmap)
  Resource id:  0xfa
X Error: BadDrawable (invalid Pixmap or Window parameter) 9
  Major opcode: 62 (X_CopyArea)
  Resource id:  0x2800015

Workarounds like pkexec and adding a policykit rule didn’t help, either. The error indicates that there is a problem with the MIT Shared Memory Extension of X.

A good workaround is to deactivate the usage of the extension on command line:

$ sudo QT_X11_NO_MITSHM=1 /usr/bin/yubikey-personalization-gui

It works like a charm.

NVIDIA partially opening up their GPU specification

Yesterday NVIDIA announced that they are supporting Nouveau development by providing documentation on certain aspects of their GPUs. This is good for the Open Source community – but their competitors still provide much more.

If you look on Linux on the desktop the out-of-the-box graphics experience is still grubby and highly depends on the hardware. Most Intel cards are very well supported right out of the box, the default drivers are the best. But AMD and NVIDIA both do have proprietary drivers which are much better than the open source ones. AMD though improved the situation years ago by releasing many technical specs to the public and thus many developers had a chance to improve the drivers. NVIDIA however in the end did nothing to improve the situation on the open source side. In the meantime the pretty well working driver Nouveau came up, but they didn’t even support the development there.

Until yesterday: NVIDIA’s Andy Ritger offered to help the development by actively monitoring the Nouveau discussion lists, by providing an e-mail address to ask questions about the GPUs and, which is most important, by

releasing public documentation on certain aspects of our GPUs, with the intent to address areas that impact the out-of-the-box usability of NVIDIA GPUs with Nouveau. We intend to provide more documentation over time, and guidance in additional areas as we are able.

That is good news! Finally the developers of the open source driver have at least some support from the company they help anyway. As a result the out-of-the-box experience of NVIDIA backed machines might improve over time. But for modest 3D graphics performance they would have to release more technical details, probably on level with what AMD released.

In any way, NVIDIA’s commitment is a good step in the right direction. But there are still huge problems and dark spots in the Linux graphics world: the OpenGL support is outdated, and hybrid graphics support is far, far away from working seamlessly on Linux.

But there is also hope for rapid improvement on the situation due to suddenly many more users: with the new Steam Box building on top of Linux, Linux gaming might get quite some momentum – and thus much better drivers.

[Howto] Remapping buttons with xbindkeys and xte

Sometimes you buy devices like a mouse or a keyboard which provide additional buttons for special functions. Or which have buttons which do not behave as expected. In such cases the button actions can be mapped to other functions, or even to other buttons.

I recently bought a Logitech M705 mouse which works almost perfectly. However, there is one nagging bug: the middle mouse button does not trigger the usual event “mouseclick two” which is interpreted as pressing the middle button, the scroll wheel which is crucial for example browsing the web. Nothing happens.

In such cases the first step is to see if the operating system receives any input at all. Fire up a shell and start the program xev. It opens a small, white window where you can move your cursor to. As soon as the cursor enters the window, you will see a lot of log data on the shell: xev shows you all X events, thus all data you enter via keyboard or mouse.

In my case I pressed the middle button, and saw:

ButtonPress event, serial 40, synthetic NO, window 0x5800001,
    root 0xc7, subw 0x5800002, time 19610234, (46,38), root:(1784,61),
    state 0x10, button 6, same_screen YES
[...]
ButtonRelease event, serial 40, synthetic NO, window 0x5800001,
    root 0xc7, subw 0x5800002, time 19610234, (46,38), root:(1784,61),
    state 0x210, button 6, same_screen YES

The interesting part is that the button was not interpreted as “button 2” as I would have expected, but as “button 6”. That’s not what I expected, and thus the button must be remapped to the event “button 2”.

Mapping of keys and buttons can be done via xbindkeys: in case of KDE I created a symlink to start the program at each startup:

ls -la ~/.kde/env/xbindkeys 
lrwxrwxrwx 1 liquidat users 18 Aug  1 10:56 .kde/env/xbindkeys -> /usr/bin/xbindkeys

xbindkeys reads its configuration from ~/.xbindkeysrc, so that’s the place where we need to configure the actual mapping. The syntax is:

#    "command to start"
#       associated key

The most interesting part of the mapping is: how to trigger the action “button 2”? That is done by the program xte which generates fake input. Thus the final configuration is:

"xte 'mouseclick 2'"                                                                                                                                                                                           
  b:6  

And you are done. Pressing the mouse button 6 on the Logitech M705 now launches the mouseclick 2.

However, as stated correctly by the comments below, this is just an intermediate solution! A long time solution is to fix the mapping in evdev, the Linux input handling.