catch.c

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <sys/mman.h>

/**
 * Stack layout as proposed in http://www.cs.indiana.edu/~dyb/pubs/stack.pdf
 * [rbp + ...] = pointer to base of stack segment?
 * [rbp + ...] = pointer to next stack record?
 * [rbp + ...] = stack parameters
 * [rbp + 16] = size of stack segment
 * [rbp + 8] = return address for upper frame 
 * [rbp + 0] = RBP (from push rbp \n mov rbp, rsp)
 * 
	In our model,
	the frame pointer is adjusted just prior to a procedure
	call to point to the new frame, and is adjusted after the
	called routine returns to point back to the old frame. In
	order for this to work, the frame pointer must still (or
	again) point to the called routine’s frame on return. The
	compiler generating code for the calling procedure must
	keep track of the displacement between the start of the
	calling procedure’s frame and the start of the called procedure’s
	frame in order to adjust the frame pointer both
	before and after the call. In both cases, the adjustment
	is performed by a single instruction to add (subtract)
	the displacement to (from) the frame pointer.
 */

void segfault(int signal, siginfo_t* si, void* arg)
{
	/* This is POSIX non-compliant... but whatever */
	printf("caught segfault %p\n", si->si_addr);
	exit(0);
}

int main(int argc, char** argv)
{

	/* Install segmentation fault handler */
	struct sigaction sa;
	memset(&sa, 0, sizeof(struct sigaction));
	sigemptyset(&sa.sa_mask);
	sa.sa_sigaction = segfault;
	sa.sa_flags = SA_SIGINFO;
	if(sigaction(SIGSEGV, &sa, NULL)) {
		fprintf(stderr, "Fatal error - could not install signal handler\n");
		exit(0);
	}

	/* Request mapping for a stack, RW privileges */
	void* addr = mmap(NULL, 0x10000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
	if (addr == MAP_FAILED) {
		fprintf(stderr, "Fatal error - memory mapping failed\n");
		exit(0);
	}

	printf("mmap gave up %p\n", addr);
	int* ptr =addr;
	*(ptr-1) = 12;

	munmap(addr, 0x10000);
	return 0;
}