siglol.c

#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/ucontext.h>

#include <stdint.h>

void interpret(char op) {
    printf("interpreting %02x\n", op);
}

void done() {
    printf("all done!\n");
}

void handler(int signal, siginfo_t *info, void *context) {
    ucontext_t *uctx = (ucontext_t*)context;
    // /!\ OSX!! /!\.
//    uint64_t curr_instr = uctx->uc_mcontext->__ss.__rip;
    uint64_t curr_instr = uctx->uc_mcontext.gregs[16];
    char op = *(char*)(curr_instr + 1);
    uint64_t next_instr = curr_instr + 2;
    interpret(op);
    uctx->uc_mcontext.gregs[16] = (*(char*)next_instr == 0x00 ? done : next_instr);
}

struct sigaction act = {
    .sa_sigaction = handler,
    .sa_mask = 0,
    .sa_flags = SA_SIGINFO
};

// get this to sit in executable space :)
// for macho targets:
// char foo[] __attribute__ ((section ("__TEXT,_foo"))) =

// gcc/elf target: fix warning like
// > /tmp/ccDOtPro.s:113: Warning: ignoring changed section attributes for .text 
// emitted because section(X) writes `.section X,"aw",@progbits`
// so ".text" writes `.section .text,"aw",@progbits, where the extra elements are invalid for ELF.
// instead, use ".text#", so the gas listing is `section .text#,"aw",@progbits`, commenting out the warning-causing junk
char foo[] __attribute__ ((section (".text#"))) =
  { 0x06, 0x01, 0x06, 0x0a, 0x06, 0x0f, 0x06, 0x01, 0x06, 0x00, 0x00 };

int main(int argc, char** argv) {
    sigaction(SIGILL, &act, (void*)0);
    ((void (*)())foo)();
}