Messages in this thread | | | Date | Tue, 5 Jan 1999 09:09:10 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: Fetching a value of control registers from user space |
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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 this:
int 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.
Cheers, Dick Johnson ***** FILE SYSTEM WAS MODIFIED ***** 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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