Spawning processes with display output from Linux kernel module

yaronshr · 03-31-2017, 10:56 AM

I am trying to spawn a process from a kernel module, that has visual output - for now, I'm just trying to spawn xeyes. I am trying to do this using call_usermodehelper. I am getting a return value of 256.

I am specifying these environmental variables in envp:
HOME=/
DISPLAY=:0 (this is what I get when I echo $DISPLAY in any of my terminals)
PATH=/sbin:/bin:/usr/sbin:/usr/bin

Here is my module code:

Code:

#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>

int spawnProcess(void) {

    char *argv[] = { "/usr/bin/xeyes", NULL };
    static char *envp[] = { "HOME=/","DISPLAY=:0", "PATH=/sbin:/bin:/usr/sbin:/usr/bin", NULL };

    printk(KERN_DEBUG "Inside spawnProcess\n");

    return call_usermodehelper( argv[0], argv, envp, UMH_WAIT_PROC );
}

static int __init moduleLoader(void) {
    int i;
    printk(KERN_INFO "My module is loading...\n");
    i = spawnProcess();
    printk(KERN_DEBUG "return value %d\n",i);
    printk(KERN_INFO "My module is done loading.\n");
    return 0;
}

static void __exit moduleUnloader(void) {
    printk(KERN_INFO "My module was unloaded.\n");
    return;
}

module_init(moduleLoader);
module_exit(moduleUnloader);

MODULE_AUTHOR("Yaron Shragai");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Lorem ipsum dolor sit amet");

Here is the output in the system log:

Mar 29 17:38:16 yaron-VirtualBox kernel: [84070.533179] My module is loading...
Mar 29 17:38:16 yaron-VirtualBox kernel: [84070.533180] Inside spawnProcess
Mar 29 17:38:16 yaron-VirtualBox kernel: [84070.534297] return value 256
Mar 29 17:38:16 yaron-VirtualBox kernel: [84070.534299] My module is done loading.

I have been able to successfully spawn a process that does not generate display output (namely, /usr/bin/logger), as per the example here: https://www.ibm.com/developerworks/l...er-space-apps/

I am running 64-bit Ubuntu 16.04 LTS on a VirtualBox VM.
Output from uname -a:
Linux yaron-VirtualBox 4.4.0-57-generic #78-Ubuntu SMP Fri Dec 9 23:50:32 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Thanks for any help!

MensaWater · 04-05-2017, 07:20 AM

You might want to have a look at Xvfb (X virtual frame buffer) to see how it does its thing. It may give you ideas on what to do in your module.

Xvfb is something that allows one to start X-GUI items in the background. We've used it for years because many Oracle products spawn GUIs even when started in background.

yaronshr · 04-05-2017, 09:21 AM

Thank you @MensaWater, I'll take a look.
By the way, a bit of added detail: When I try to spawn x programs (e.g., xterm, xeyes), I get a failure of 256, as shown in the OP. When I try to spawn a program that has stdoutput (e.g., the ls command, or, a program that just has one printf), I get a failure of -2. (If the spawned program has no screen output - e.g., just accesses files - it works.)

pan64 · 04-05-2017, 09:27 AM

I guess only: root has no right to use that X server. You may try to open a root shell and try xeyes or similar first (with the same DISPLAY settings).
by the way I think HOME is /root, not / usually.

yaronshr · 04-05-2017, 09:46 AM

pan64, tested that out as you suggest. A root terminal (at least on my system) can indeed spawn x applications - so that's not the issue.

pan64 · 04-06-2017, 01:10 AM

so if you can execute a binary, construct a new one which will start xterm and catch all the stdout and stderr and probably you will see some usable info about it