[clug] /dev/port

[Martijn van Oosterhout]

On Mon, May 30, 2005 at 09:13:32PM +1000, Adrian Blake wrote:
> Using strace :
> 
> open("/dev/port", O_RDWR)               = -1 EPERM (Operation not permitted)
> 
> 
> and for the exercise here are the permissions:
> 
> [adrian at mensa reva]$ ls -l /dev/port
> crw-rw-rw-  1 root root 1, 4 Jan  1  1970 /dev/port

Well, that would be caused by the following (Linux 2.4.27):

drivers/char/mem.c:
static int open_port(struct inode * inode, struct file * filp)
{
        return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}

I guess it was decided that handing out raw access to io ports is so
risky that it's only allowed for root, even if you give global wirte
access to /dev/port. In other words /dev/port is a way of accessing IO
ports from user space, but it doesn't override the protections in
place. You'll still need ioperm or root I guess...

Hope this helps,
-- 
Martijn van Oosterhout   <kleptog at svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.
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