I have written a program which tries to read from and write to the control registers.
The program compiles fine, but when the inline assembly is about to be executed, it produces a segmentation fault.
Code:
void instructions(int val)
{
int i;
int value;
for(i = 0; i < val; i++)
__asm__("mov %cr0, %eax");
}
I used GDB and stepped through each assembly line and it is on the mov %cr0,%eax
that the segmentation fault is occurring.
Anyone who knows what is wrong?
Quoting from Intel® 64 and IA-32 Architectures Software Developer Manuals 3-650 Vol. 2A on moving to and from control registers:
This instruction can be executed only when the current privilege level is 0.
Which means the instruction can only be executed in kernel mode.
A minimal kernel module, that logs the contents of cr0, cr2 and cr3 could look something like this (32-bit code path untested):
/* hello.c */
#include <linux/module.h>
#include <linux/kernel.h>
int init_module(void)
{
#ifdef __x86_64__
u64 cr0, cr2, cr3;
__asm__ __volatile__ (
"mov %%cr0, %%rax\n\t"
"mov %%eax, %0\n\t"
"mov %%cr2, %%rax\n\t"
"mov %%eax, %1\n\t"
"mov %%cr3, %%rax\n\t"
"mov %%eax, %2\n\t"
: "=m" (cr0), "=m" (cr2), "=m" (cr3)
: /* no input */
: "%rax"
);
#elif defined(__i386__)
u32 cr0, cr2, cr3;
__asm__ __volatile__ (
"mov %%cr0, %%eax\n\t"
"mov %%eax, %0\n\t"
"mov %%cr2, %%eax\n\t"
"mov %%eax, %1\n\t"
"mov %%cr3, %%eax\n\t"
"mov %%eax, %2\n\t"
: "=m" (cr0), "=m" (cr2), "=m" (cr3)
: /* no input */
: "%eax"
);
#endif
printk(KERN_INFO "cr0 = 0x%8.8X\n", cr0);
printk(KERN_INFO "cr2 = 0x%8.8X\n", cr2);
printk(KERN_INFO "cr3 = 0x%8.8X\n", cr3);
return 0;
}
void cleanup_module(void)
{
}
# Makefile
obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
test: all
sudo insmod ./hello.ko
sudo rmmod hello
dmesg | tail