Self modifying code always segmentation faults on Linux

Question

i found an article about self modifying code and tried to do some examples, but i get always segmentation faults. As fas as i can understand, there is a violation in memory

Answer 1

Modern CPUs have a feature called DEP which prevents execution of code on the stack. Previously, this was possible; now, it is not. The binary is loaded into read-only memory by default.

With that out of the way, you can use the mprotect system call to mark your binary's location in memory as executable - SO LONG AS YOUR CODE IS NOT DEP-PROTECTED. So don't try to put code and the stack and then jump into it.

Answer 2

You can also disable write-protection for the entire program by passing the switch -N to the linker. If you are invoking the linker from gcc, pass Wl,-N. If you invoke ld directly, pass -N.

Answer 3

You should to change memory access permissions in runtime.

#include <sys/mman.h>

void *addr  = get_address_of_instruction_pointer();
int  length = 4096;   /* size of a page */

if (mprotect(addr, length, PROT_READ | PROT_WRITE | PROT_EXEC) == 0) {
    /* current code page is now writable and code from it is allowed for execution */
}