Re: nvidia machdep.c patch still useful ?

[David Cuthbert <dacut@xxxxxxxxx>]

Matthew Dillon wrote:
>     %fs cannot be set to anything useful by userland, at least I don't
>     think it can.  %gs can be set to a user-defined custom segment.

My kernel's not quite up-to-date, but the snippet below successfully 
queries and sets %fs.

I'm fairly certain that setting any i386 segment register (except %cs) 
is a non-privileged instruction.  Of course, the descriptor must support 
the appropriate access to actually *use* it...

---

#include <stdio.h>

unsigned short get_fs(void)
{
    unsigned short result;
    __asm__("movw %%fs, %0" : "=r" (result));
    return result;
}

void set_fs(unsigned short value)
{
    __asm__("movw %0, %%fs" : : "r" (value));
    return;
}


int main()
{
    printf("fs is %d\n", get_fs());
    set_fs(0);
    printf("fs is %d\n", get_fs());

    return 0;
}