/*      $OpenBSD: trapstack.c,v 1.2 2020/11/07 23:36:24 bluhm Exp $     */
/*
 * Copyright (c) 2018 Todd Mortimer <mortimer@openbsd.org>
 * Copyright (c) 2019 Alexander Bluhm <bluhm@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/mman.h>

#include <err.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

#include "pivot.h"

void handler(int);
void dotrap(void);

static volatile char *trapmap;

int
main(int argc, char *argv[])
{
        stack_t ss;
        struct sigaction act;
        void (**newstack)(void);
        long pagesize;

        ss.ss_sp = malloc(SIGSTKSZ);
        if (ss.ss_sp == NULL)
                err(1, "malloc sigstack");
        ss.ss_size = SIGSTKSZ;
        ss.ss_flags = 0;
        if (sigaltstack(&ss, NULL) == -1)
                err(1, "sigaltstack");

        act.sa_handler = handler;
        sigemptyset(&act.sa_mask);
        act.sa_flags = SA_ONSTACK;

        /* set up an alt stack on the heap that just calls dotrap */
        pagesize = sysconf(_SC_PAGESIZE);
        if (pagesize == -1)
                err(1, "sysconf");
        newstack = malloc(pagesize > SIGSTKSZ ? pagesize : SIGSTKSZ);
        if (newstack == NULL)
                err(1, "malloc newstack");
        /* allow stack to change half a page up and down. */
        newstack[pagesize/sizeof(*newstack)/2] = dotrap;

        trapmap = mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS,
            -1, 0);
        if (trapmap == MAP_FAILED)
                err(1, "mmap");

        if (sigaction(SIGSEGV, &act, NULL) == -1)
                err(1, "sigaction");
        pivot(&newstack[pagesize/sizeof(*newstack)/2]);
        return 3;
}

void
handler(int signum)
{
        _exit(0);
}

void
dotrap(void)
{
        trapmap[0] = 'x';
        exit(2);
}