Re: [rtl] using RTLinux for video stream handling

[Corey Minyard <minyard@acm.org>]

Zdenek Kabelac <kabi@fi.muni.cz> writes:
> Corey Minyard wrote:

> > This is not true.  X is a user space processes, but it is possible for
> > user space processes to disable interrupts.  I have personally done
> > it.  You have to modify your iopl and you have to be a supervisor
> > process, but you can do it.  And some X servers do this, I'm pretty
> > sure.  I know they go directly to the hardware.
> > 
> > And since X directly calls the cli and sti instructions instead of
> > calling the Linux functions in the kernel, even RTLinux cannot stop
> > this.
> 
> Are you sure, the non-kernel-space process could use STI & CLI
> instructions ??? I've always though this is privileged instruction
> and there is no way, user-space program could use them.
> 
> (This would break whole multitasking, and we are not on Amiga)

I am 100% sure.  Try the following (as root):

  iopl(3)
  asm("cli");
  asm("sti");

It will work just fine.  You can try adding an infinite loop after
the "cli", but it's the reset button after that happens :-).  You
can also do direct port I/O (inb, outb, etc.).

It only breaks multitasking if you use it and only root is allowed to
do it, so it's not a security hazard.  Your average program shouldn't
be doing this.

-- 
Corey Minyard               Internet:  minyard@acm.org
  Work: minyard@nortel.ca       UUCP:  minyard@wf-rch.cirr.com
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