1 /* $OpenBSD: trapstack.c,v 1.2 2020/11/07 23:36:24 bluhm Exp $ */
2 /*
3 * Copyright (c) 2018 Todd Mortimer <mortimer@openbsd.org>
4 * Copyright (c) 2019 Alexander Bluhm <bluhm@openbsd.org>
5 *
6 * Permission to use, copy, modify, and distribute this software for any
7 * purpose with or without fee is hereby granted, provided that the above
8 * copyright notice and this permission notice appear in all copies.
9 *
10 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
11 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
12 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
13 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
14 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
15 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
16 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
17 */
18
19 #include <sys/mman.h>
20
21 #include <err.h>
22 #include <stdlib.h>
23 #include <signal.h>
24 #include <unistd.h>
25
26 #include "pivot.h"
27
28 void handler(int);
29 void dotrap(void);
30
31 static volatile char *trapmap;
32
33 int
main(int argc,char * argv[])34 main(int argc, char *argv[])
35 {
36 stack_t ss;
37 struct sigaction act;
38 void (**newstack)(void);
39 long pagesize;
40
41 ss.ss_sp = malloc(SIGSTKSZ);
42 if (ss.ss_sp == NULL)
43 err(1, "malloc sigstack");
44 ss.ss_size = SIGSTKSZ;
45 ss.ss_flags = 0;
46 if (sigaltstack(&ss, NULL) == -1)
47 err(1, "sigaltstack");
48
49 act.sa_handler = handler;
50 sigemptyset(&act.sa_mask);
51 act.sa_flags = SA_ONSTACK;
52
53 /* set up an alt stack on the heap that just calls dotrap */
54 pagesize = sysconf(_SC_PAGESIZE);
55 if (pagesize == -1)
56 err(1, "sysconf");
57 newstack = malloc(pagesize > SIGSTKSZ ? pagesize : SIGSTKSZ);
58 if (newstack == NULL)
59 err(1, "malloc newstack");
60 /* allow stack to change half a page up and down. */
61 newstack[pagesize/sizeof(*newstack)/2] = dotrap;
62
63 trapmap = mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS,
64 -1, 0);
65 if (trapmap == MAP_FAILED)
66 err(1, "mmap");
67
68 if (sigaction(SIGSEGV, &act, NULL) == -1)
69 err(1, "sigaction");
70 pivot(&newstack[pagesize/sizeof(*newstack)/2]);
71 return 3;
72 }
73
74 void
handler(int signum)75 handler(int signum)
76 {
77 _exit(0);
78 }
79
80 void
dotrap(void)81 dotrap(void)
82 {
83 trapmap[0] = 'x';
84 exit(2);
85 }
86