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SubjectRe: Fetching a value of control registers from user space
On 4 Jan 1999, Josip Gracin wrote:

> Hi!
> When I try to use the following program I get a SEGV fault. Why is this?
> I thought reading a value from control registers isn't supposed to
> generate an exception.
> int
> main ()
> {
> unsigned long fa;
> asm ("movl %%cr2, %0": "=r" (fa));
> return 0;
> }
> Thanks in advance.

Quote: (Intel 26-212)
MOV to/from Special Registers.
#GP(0) if current privilege level is not 0.

This is a general protection violation. No user-mode program would
(should) ever use these instructions.

In principle, a user-mode program should not even be able to do

main ()
register unsigned long eax asm("%eax");
asm ("movl %ebx, %eax\n");
printf("%08lX\n", eax);
return 0;

... reading the value from an uninitialized register. But you can.
The CPU can't protect everything. This came up in a discussion about
having the kernel zero, or predefine, general purpose registers upon
process creation. The problem(s) being screwing up fork(), et al.

If you want to mess with priv level 0 stuff, make a module.

Dick Johnson
Penguin : Linux version 2.1.131 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.
Wisdom : It's not a Y2K problem. It's a Y2Day problem.

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