Fork and Exec a process within the kernel

Hi all,

I know it's not the best approach to the problem I'm attempting to solve, but I'm attempting to fork a thread and exec a process from inside the kernel. I tried creating a kernel thread and attempted to exec from there. I actually make it into my child thread, but the exec doesn't appear to do anything. Any ideas how to do this.

Eric

Try grep'ing for call_usermodehelper()

                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <[email protected]> 1.650.933.1373

Thanks for the pointer! My user mode program is running. Any idea how to control which console it shows up on?

Eric

Good.

No clue.

                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <[email protected]> 1.650.933.1373

On Tue, 2004-08-10 11:22:10 -0400, Eric Masson <[email protected]> wrote in message <[email protected]>:

The best idea for a usermode helper is to first close stdin/stdout/stderr and re-open it with /dev/null...

If you need to communicate, write into some logfile.

MfG, JBG

Jan-Benedict Glaw       [email protected]    . +49-172-7608481             _ O _
"Eine Freie Meinung in  einem Freien Kopf    | Gegen Zensur | Gegen Krieg  _ _ O
 fuer einen Freien Staat voll Freier Bürger" | im Internet! |   im Irak!   O O O
ret = do_actions((curr | FREE_SPEECH) & ~(NEW_COPYRIGHT_LAW | DRM | TCPA));

OK, ignore the peanut gallery, this is the real answer:

./foo > /dev/tty1

See /etc/syslog.conf on Debian for another example. Anywhere you could write to a file you can write to /dev/tty$FOO and it will do the right thing.

UNIX is full of stuff like this. Pretty cool, huh?

Lee

/dev/console is a symlink to /dev/tty0.

Alt-F1 = /dev/tty0
Alt-F2 = /dev/tty1
Alt-F3 = /dev/tty2

... etc.

    struct termios term;

    tcgetattr(0, &term);	// Get old terminal characteristics
    (void)close(0);		// Close old terminal(s)
    (void)close(1);
    (void)close(2);
    fd = open("/dev/console", O_RDWR);
    tcsetattr(fd, &term);	// Restore characteristics
    dup2(fd, 0);
    dup2(fd, 1);
    dup2(fd, 2);

Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.

Please don't mislead newbies, Richard. /dev/console is NOT a link to /dev/tty0, it's a completely different device:

erik@abra2:~ >ls -l /dev/console
crw------- 1 root tty 5, 1 Apr 7 09:13 /dev/console
erik@abra2:~ >ls -l /dev/tty0
crw------- 1 root tty 4, 0 Feb 10 2000 /dev/tty0

On x86 desktop systems console output usually comes on the virtual terminals, but you can also use serial console. My embedded StrongARM board only has serial console.

And what happens when you have console on a device that's not a serial port like a line printer?

Erik

+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands

Bullshit. I know how to use file.

Script started on Wed Aug 11 07:21:39 2004

# file /dev/console
/dev/console: symbolic link to /dev/tty0
# exit
Script done on Wed Aug 11 07:21:51 2004

Then that's your problem.

It will still work but there is no input and it can't be turned into a "controlling terminal".

Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.

It might be a symlink on your machine, but that doesn't mean it's the right way. For almost 7 years, /dev/console is a separate device, not a symlink. Here's the relevant section from Documentation/devices.txt:

The console device, /dev/console, is the device to which system messages should be sent, and on which logins should be permitted in single-user mode. Starting with Linux 2.1.71, /dev/console is managed by the kernel; for previous versions it should be a symbolic link to either /dev/tty0, a specific virtual console such as /dev/tty1, or to a serial port primary (tty*, not cu*) device, depending on the configuration of the system.

Linux-2.1.71 was released on December 4, 1997. In your signature you claim to run linux-2.4.26. Please update your system.

Erik

+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands

RedHat is NOT Linux. The MAJOR-MINOR 5.1 used in RedHat for the console has a very serious problem for anybody doing development work. If there are any kernel messages, they go to __all__ open terminals. This means that the only "quiet" terminals that may be available to kill off a runaway process are available iff you can log in over the network.

Most everybody I know, who does serious development work, and certainly those who want to control where the %*)&$#!@ kernel messages go, will make sure they go to the "ALT-F1" as I have shown. The original query was about how to make kernel messages go to where, i.e., what VT do they come from. I have shown how you can control where they go.

Now, if you look at the kernel source, you will note that just prior to attempting to exec /sbin/init, /dev/console is opened. This means that you can properly use whatever you want if you continue to use a sym-link.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.

If you don't want console messages, you just disable console at all, or you use a serial console (using the "console=" on the kernel command line), or you adjust the kernel console log level. Please have a look at the "dmesg" manual page, especially the "-n" parameter. In emergency situations, Alt-Sysrq-[0-8] will allow you to do the same (on VGA consoles for PCs, that is. for serial consoles use Break-[0-8]).

Not all the world is a PC, there is a lot of linux beyond x86. Looks to me like you haven't done development on embedded systems that don't have any VGA terminal at all.

Getting kernel messages on a certain pseudo tty is just a matter of supplying the correct "console=" paramater to the kernel. For example:
"console=tty1" will get your console output on pseudo tty 1. Alternatively, you can configure syslog to write all kernel messages to a pseudo tty.

Here is the original question:

Thanks for the pointer! My user mode program is running. Any idea how to control which console it shows up on?

No reference on how to get kernel messages somewhere.

That was true for linux < 2.1.71, which was released almost 7 years ago. Things have changed. Newer kernels use the "console=" kernel command line to select the console. With multiple the "console=" parameter, you can even have mutiple consoles.

Erik

+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands

Actually you might want to

a) man sysklogd
b) check devices.txt distributed with the kernel.

The former will explain why you are wrong and how to channel log messages, the latter will explain how you are further wrong.