Write jump to memory - c++

Question

I'm trying to write a jump to memory, and I can't seem to find anywhere which can explain me how it works.
typedef UINT(WINAPI* tResetWriteWatch)(LPVOID lpBaseAddress, SIZE_T dwRegionSize);
UINT WINAPI ResetWriteWatchHook(LPVOID lpBaseAddress, SIZE_T dwRegionSize){
printf("Function called\n");
return 0;
}
void main(){
DWORD64 hookAddr = (DWORD64)&ResetWriteWatch;
WriteJump(hookAddr, ResetWriteWatchHook/*Let's say it's 0x7FE12345678*/);//Writes E9 XX XX XX XX to memory
}
My main issue is that I don't understand: how do I convert asm JMP 0x7FE12345678 to E9 XX XX XX XX so I can write it at hookAddr.
Process is 64 bits.

Answer 1

This is how it's commonly done on a 32bit program (not sure how much is different on 64 bit), but this should give you an idea on where to go. This is specific to windows because of VirtualProtect, but you can use mprotect if you are on linux.
#include <stdio.h>
#include <windows.h>
void foo() {
printf("Foo!");
}
void my_foo() {
printf("My foo!");
}
int setJMP(void *from, void *to) {
DWORD protection;
if (!VirtualProtect(from, 5, PAGE_EXECUTE_READWRITE, &protection)) { // We must be able to write to it (don't necessarily need execute and read)
return 0;
}
*(char *)from = 0xE9; // jmp opcode
*(int *)(from + 1) = (int)(to - from - 5); // relative addr
return VirtualProtect(from, 5, protection, &protection); // Restore original protection
}
int main() {
setJMP(foo, my_foo);
foo(); // outputs "My foo!"
return 0;
}

Answer 2

I would suggest using an assembler to generate the correct bytes. Simply create the following file and run it through NASM:
BITS 64
JUMP 0x7fe1234
The result is:
2f e9 fe 12 00 07 (due to the LE byte order). This is a relative jump, so it will be hard to generate from high level code. You might want to use the opcode EA instead, which performs an absolute jump. Then you can simply use the absolute address of the location to which you want to jump.