c.c

//#!gcc -O0 -g3 -gdwarf-2 -Wall c.c -o /tmp/a.out && /tmp/a.out 1 2

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/mman.h>

uint8_t* memory[255];
uint8_t program_counter;

void incr(void) {
	program_counter++;
}

void decr(void) {
	program_counter--;
}

void template(void) {
	asm volatile ("nop");
}

void get_function_body(void* func, uint8_t** buf, uint8_t* len) {
	uint8_t* sub = (uint8_t*)func;
	int i;
	int start = 4, end;
	// skip push %rbp, mov    %rsp,%rbp
	for (i = start; i < 255; i++) {
		if (sub[i] == 0xc9) { // leaveq
			end = i;
			break;
		}
		printf("%x ", sub[i]);
	}
	printf("\n");
	printf("\n");
	printf("len=%d\n", end-start);
	*buf = sub + start;
	*len = end - start;
}

int main (int argc, char* argv[]) {
	int i;

	program_counter = 0;


	printf("pc=%d\n", program_counter);

	uint8_t* buf; uint8_t len;
	// get function body pointer and length
	get_function_body(incr, &buf, &len);

	uint8_t  code_len = len + 6;
	uint8_t* code = mmap(NULL, code_len, PROT_EXEC | PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	int x = 0;
	// push %rbp
	code[x++] = 0x55;
	// mov  %rsp,%rbp
	code[x++] = 0x48;
	code[x++] = 0x89;
	code[x++] = 0xe5;

	for (i = 0; i < len; i++) {
		code[x++] = buf[i];
	}

	// leaveq
	code[x++] = 0xc9;
	// retq
	code[x++] = 0xc3;

	printf("code=%x\n", code);
	for (i = 0; i < code_len; i++) {
		printf("%x ", code[i]);
	}
	printf("\n");

	// asm volatile ("callq %0;" : : "r"(code) );
	((void (*)(void))code )();

	munmap(code, code_len);

	printf("pc=%d\n", program_counter);

	return 0;
}