Linux – How Do Desktop Environments Call ACPI Functions

Found the solution here.

What you want to do is write a trusted I/O enabling program in C that allows access to only the desired ports, then uses execvp() to execute your script at the caller's address space. You'll then setuid root to the compiled I/O enabler.

Here's some sample code adapted from the above source (be sure to use an address block you don't mind writing to):

#include <stdio.h>
#include <stdlib.h>
#include <sys/io.h>

#define DESIRED_PORT    0x300
#define NUM_BYTES       8

int main(int argc, char*argv[])
{
    if (argc < 2) {
        printf("Error: no target program specified.\n");
        exit(1);
    }

    if (ioperm(DESIRED_PORT, NUM_BYTES, 1)) {
        printf("Error: couldn't set port permissions.\n");
        exit(1);
    }

    // Set uid to current user's id before executing the script
    setgid(getgid());
    setuid(getuid());

    if (execvp(argv[1], &argv[1]) < 0) {
        printf("Error: target program execution error.\n");
        exit(1);
    }
}

Let's call it io_enable.c, then compile and setuid root:

$ gcc io_enable.c -o io_enable
$ sudo chown root io_enable
$ sudo chmod u+s io_enable

Next, we can test it with the following python script:

#!/usr/bin/python
import portio

ADDR = 0x300

fd = open('/tmp/portio.log', 'w')

for i in range(10):
    portio.outb(i, ADDR)
    fd.write('Wrote %d, read %d.\n' % (i, portio.inb(ADDR)))

fd.close()

I'm calling it io_test.py and then running it like so:

$ ./io_enable python io_test.py

Looks like it works:

$ cat /tmp/portio.log
Wrote 0, read 0.
Wrote 1, read 1.
Wrote 2, read 2.
Wrote 3, read 3.
Wrote 4, read 4.
Wrote 5, read 5.
Wrote 6, read 6.
Wrote 7, read 7.
Wrote 8, read 8.
Wrote 9, read 9.

After having a look at the source code, it seems that consolekit(ck) uses a short script to do shutdown and reboot. These two scripts are installed as

/usr/lib/ConsoleKit/scripts/ck-system-{stop,restart}

in Arch Linux and they simply find and exec {,/usr}/sbin/shutdown to do that.

Therefore, there seems no way to configure ck to do that now (by normal I just mean simply edit some file(s) in /etc), and the work around is straightforward.

1. simply edit those two scripts to exec systemctl {shutdown,reboot}. (But at least on Arch Linux, it will probably be overwrite after each upgrading.)

2. simply create a wrapper script at /sbin/shutdown that do the right thing. (This will cause conflict if you want to install sysvinit later, but hopefully not a problem.)

I will also look for (and create if there haven't been one) a bug report on the problem and I hope it can be done in a better way in the future (really don't like to do configuration outside /etc).

NOTE: Arch Linux now have the systemd-sysvcompat package which provide these (init/halt/shutdown etc.) as symlink to systemctl/systemd.