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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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