/*
 * How to dereference a null pointer and get away with it.
 *
 * G. Back, CS 3214
 */
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/signal.h>
#define __USE_GNU 1
#include <ucontext.h>

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_EIP]);

/* The assembly code for main (below) is:
        
 8048507:       b8 00 00 00 00          mov    $0x0,%eax
----> This is where it faults, eip has value 804850c on fault 
 804850c:       8b 00                   mov    (%eax),%eax
----> If it hadn't faulted, result would be in %eax
 804850e:       89 44 24 04             mov    %eax,0x4(%esp)
*/
    ctxt->uc_mcontext.gregs[REG_EAX] = 42;      // put '42' in eax
    ctxt->uc_mcontext.gregs[REG_EIP] += 2;      // skip '8b 00' above

    // upon return, OS will read *ctxt and restore CPU's registers
}

int
main()
{
    int status;
    struct sigaction act;

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

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