Example of sharing a page between processes after fork+exec.

fun.c

/*
 * Copyright (c) 2022 Dave Voutila <dave@sisu.io>
 *
 * 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.
 */

/*
 * An example of using shm_mkstemp, execve, and pledge altogether.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <fcntl.h>
#include <time.h>

#include <err.h>
#include <errno.h>

#include <sys/mman.h>
#include <sys/wait.h>

static const int TICKS = 8;

static const size_t SZ = 4096;
static char *PAGE = NULL;
static int FD = -1;

int main(int, char**);

int
read_int(void)
{
	return *(int*)(PAGE);
}

void
write_int(int i)
{
	int *p = (int*)PAGE;
	*p = i;
}

int
child(void)
{
	struct timespec timeout;
	int cnt = TICKS, ms = 1000;

	printf("child starting. my main is @ %p\n", main);

	timeout.tv_sec = ms / 1000;
	timeout.tv_nsec = ms * 1000000 % 1000000000;

	printf("pledging in child\n");

	if (pledge("stdio", NULL))
		return (errno);

	do {
		if (nanosleep(&timeout, NULL) == -1) {
			printf("! %s interrupted?!\n", __func__);
			break;
		}
		write_int(cnt);
		printf("...tick\n");
	} while ((--cnt) > 0);

	printf("child finishing\n");

	return (0);
}

int
parent(pid_t child)
{
	struct timespec timeout;
	int cnt = TICKS, ms = 900, status = 0;

	timeout.tv_sec = ms / 1000;
	timeout.tv_nsec = ms * 1000000 % 1000000000;

	printf("parent starting. my main is @ %p\n", main);

	printf("pledging in parent\n");
	if (pledge("stdio", NULL))
		err(1, "parent pledge");

	do {
		if (nanosleep(&timeout, NULL) == -1) {
			printf("! %s interrupted?!\n", __func__);
			break;
		}
		printf("... %d\n", read_int());
	} while ((--cnt) > 0);

	printf("parent waiting for child %d to finish\n", child);
	if (wait(&status) != child) {
		warn("waitpid");
		return (1);
	}

	return (0);
}

int
main(int argc, char **argv)
{
	char *nargv[3], arg[13], *path;
	int prot, flags, isParent = 0;
	pid_t pid = 0;

	path = calloc(32, sizeof(char));

	if (argc > 1) {
		// coming in via fork+exec
		const char *errstr = NULL;
		FD = strtonum(argv[1], 3, 10, &errstr);
		if (errstr != NULL)
			errx(1, "failed to parse fd argument: %s", errstr);
		printf("got FD = %d\n", FD);
	} else {
		isParent = 1;
		strlcpy(path, "/tmp/fun.XXXXXXX", 32);
		printf("calling shm_mkstemp(%s)\n", path);
		FD = shm_mkstemp(path);
		if (FD < 0)
			err(1, "shm_mkstemp");
		printf("made temp file, FD=%d\n", FD);

		/* We need to "size" the file */
		ftruncate(FD, SZ);

		fcntl(FD, F_SETFD, 0);

		shm_unlink(path); // cleanup filesystem
		free(path);
	}

	prot = PROT_READ | PROT_WRITE;
	flags = MAP_SHARED | MAP_CONCEAL;
	PAGE = (char*)mmap(NULL, SZ, prot, flags, FD, 0);
	if (PAGE == MAP_FAILED)
		err(1, "mmap");
	printf("%s MMAP'd @ %p\n", isParent ? "parent" : "child", PAGE);

	if (!isParent) {
		close(FD);
		child();
	} else {
		// zero our page
		memset(PAGE, 0, SZ);

		// fork you!
		printf("...forking\n");
		if ((pid = fork()) == -1) {
			if (munmap(PAGE, SZ))
				err(3, "munmap");
			err(2, "fork");
		}

		if (pid == 0) {
			// Child
			memset(&arg, 0, sizeof(arg));
			snprintf(arg, sizeof(arg), "%d", FD);

			nargv[0] = argv[0];
			nargv[1] = arg;
			nargv[2] = NULL;

			execvp(argv[0], nargv);
			err(3, "execvp");
			// catch you on the flip side
		}
		close(FD);
		parent(pid);
	}

	return (0);
}