/*
 * How to dereference a null pointer and get away with it.
 *
 * G. Back, CS 3214, updated Fall 2014
 *
 * Must compile with -O2
 */

/* required so that REG_RIP etc. are defined the first time sys/ucontext.h is included */
#define _GNU_SOURCE 1

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

static void 
install_signal_handler(int signal, void (*handler)(int, siginfo_t *, void *))
{
    struct sigaction act;

    act.sa_sigaction = handler;
    act.sa_flags = SA_SIGINFO;
    sigemptyset (&act.sa_mask);
    int status = sigaction (signal, &act, NULL);
    if (status) {
        perror ("sigaction");
        exit (1); 
    }
}

/*
 *   400668:       8b 34 25 00 00 00 00    mov    0x0,%esi
 *   40066f:       bf e2 07 40 00          mov    $0x4007e2,%edi
 *   400674:       31 c0                   xor    %eax,%eax
 *   400676:       e8 2d fe ff ff          callq  4004a8 <printf@plt>
 */
static void
catch_segfault(int signo, siginfo_t *info, void * _ctxt)
{
    ucontext_t * ctxt = _ctxt;

    printf("Catching Segfault at sig=%d fault addr is %p eip was at %x\n", 
            signo, info->si_addr, ctxt->uc_mcontext.gregs[REG_RIP]);

    // skip offending instruction
    ctxt->uc_mcontext.gregs[REG_RIP] += 7;
    // put 42 into %rsi 
    ctxt->uc_mcontext.gregs[REG_RSI] = 42;
    // upon return, OS will apply any changes to ctxt
    // to process's saved state, the restore the state
    // execution will continue at 400668 + 7 = 40066f
}

int
main()
{
    install_signal_handler(SIGSEGV, catch_segfault);
    printf("Dereferencing NULL -> %d ...\n", *(int *)NULL);
    return 0;
}