File descriptor passing with sendmsg(2) and recvmsg(2) over Unix domain socket

fd_passing.c

#include <sys/param.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define	SIG	SIGUSR1

static void
diec(int error, const char *msg)
{

	fprintf(stderr, "%s: %s\n", msg, strerror(error));
	exit(1);
}

static void
die(const char *msg)
{

	diec(errno, msg);
}

#if 0
static void
print_alive_fd(const char *tag)
{
	struct stat sb;
	int i;

	for (i = 0; i < 32; i++) {
		if (fstat(i, &sb) == -1)
			continue;
		printf("%s: fd %d is alive.\n", tag, i);
	}
}
#endif

static int
do_sendmsg(int sock, const char *sockpath, int fd)
{
	struct msghdr msg;
	struct cmsghdr *cmsghdr;
	struct iovec iov[1];
	ssize_t nbytes;
	int i, *p;
	char buf[CMSG_SPACE(sizeof(int))], c;

	c = '*';
	iov[0].iov_base = &c;
	iov[0].iov_len = sizeof(c);
	memset(buf, 0x0b, sizeof(buf));
	cmsghdr = (struct cmsghdr *)buf;
	cmsghdr->cmsg_len = CMSG_LEN(sizeof(int));
	cmsghdr->cmsg_level = SOL_SOCKET;
	cmsghdr->cmsg_type = SCM_RIGHTS;
	msg.msg_name = NULL;
	msg.msg_namelen = 0;
	msg.msg_iov = iov;
	msg.msg_iovlen = sizeof(iov) / sizeof(iov[0]);
	msg.msg_control = cmsghdr;
	msg.msg_controllen = CMSG_LEN(sizeof(int));
	msg.msg_flags = 0;
	p = (int *)CMSG_DATA(buf);
	*p = fd;
	printf("sendmsg: %d\n", fd);

	nbytes = sendmsg(sock, &msg, 0);
	if (nbytes == -1)
		return (1);

	return (0);
}

static int
server_main(pid_t ppid, const char *sockpath)
{
	struct sockaddr_storage storage;
	struct sockaddr_un *addr;
	int error, fd, s, sock;
	const char *filepath = "fd_passing.txt";

	sock = socket(PF_LOCAL, SOCK_STREAM, 0);
	if (sock == -1)
		die("socket(2)");
	addr = (struct sockaddr_un *)&storage;
	addr->sun_family = AF_LOCAL;
	strlcpy(addr->sun_path, sockpath, sizeof(addr->sun_path));
	addr->sun_len = SUN_LEN(addr);
	if (bind(sock, (struct sockaddr *)addr, addr->sun_len) == -1)
		die("bind(2)");
	if (listen(sock, 0) == -1)
		goto fail;
	if (kill(ppid, SIG) == -1)
		goto fail;
	if ((s = accept(sock, NULL, 0)) == -1)
		goto fail;
	if (unlink(filepath) == -1)
		goto fail;
	if ((fd = open(filepath, O_WRONLY | O_CREAT, 0644)) == -1)
		goto fail;

	if (do_sendmsg(s, sockpath, fd) != 0)
		goto fail;

	if (close(fd) == -1)
		goto fail;
	if (close(s) == -1)
		goto fail;
	if (close(sock) == -1)
		goto fail;
	if (unlink(sockpath) == -1)
		goto fail;

	return (0);

fail:
	error = errno;
	unlink(sockpath);
	diec(error, "");

	/* NOTREACHED */
	return (1);
}

static int
do_recvmsg(int sock)
{
	struct msghdr msg;
	struct cmsghdr *cmsghdr;
	struct iovec iov[1];
	FILE *fp;
	ssize_t nbytes;
	int i, *p;
	char buf[CMSG_SPACE(sizeof(int))], c;

	iov[0].iov_base = &c;
	iov[0].iov_len = sizeof(c);
	memset(buf, 0x0d, sizeof(buf));
	cmsghdr = (struct cmsghdr *)buf;
	cmsghdr->cmsg_len = CMSG_LEN(sizeof(int));
	cmsghdr->cmsg_level = SOL_SOCKET;
	cmsghdr->cmsg_type = SCM_RIGHTS;
	msg.msg_name = NULL;
	msg.msg_namelen = 0;
	msg.msg_iov = iov;
	msg.msg_iovlen = sizeof(iov) / sizeof(iov[0]);
	msg.msg_control = cmsghdr;
	msg.msg_controllen = CMSG_LEN(sizeof(int));
	msg.msg_flags = 0;

	nbytes = recvmsg(sock, &msg, 0);
	if (nbytes == -1)
		return (1);

	p = (int *)CMSG_DATA(buf);
	printf("recvmsg: %d\n", *p);
	fp = fdopen(*p, "w");
	fprintf(fp, "OK\n");
	fclose(fp);

	return (0);
}


static int
client_main(pid_t pid, const char *sockpath)
{
	struct sockaddr_storage sockaddr;
	struct sockaddr_un *addr;
	sigset_t set;
	int sig, sock, status;
	const char *signame;

	if (sigemptyset(&set) == -1)
		die("sigemptyset(3)");
	if (sigaddset(&set, SIG) == -1)
		die("sigaddset(3)");
	if (sigwait(&set, &sig) != 0)
		die("sigwait(2)");
	if (sig != SIG)
		return (2);
	sock = socket(PF_LOCAL, SOCK_STREAM, 0);
	if (sock == -1)
		die("socket(2)");
	addr = (struct sockaddr_un *)&sockaddr;
	addr->sun_family = AF_LOCAL;
	strlcpy(addr->sun_path, sockpath, sizeof(addr->sun_path));
	addr->sun_len = SUN_LEN(addr);
	if (connect(sock, (struct sockaddr *)addr, addr->sun_len) == -1)
		die("connect(2)");

	if (do_recvmsg(sock) == -1)
		return (3);

	if (close(sock) == -1)
		die("close(2)");
	if (wait4(pid, &status, 0, NULL) == -1)
		die("wait4(2)");
	if (!WIFEXITED(status))
		return (4);
	if (WEXITSTATUS(status) != 0)
		return (32 + WEXITSTATUS(status));

	return (0);
}

int
main(int argc, const char *argv[])
{
	sigset_t set;
	pid_t pid, ppid;
	char sockpath[MAXPATHLEN];

	if (sigfillset(&set) == -1)
		die("sigfillset(3)");
	if (sigprocmask(SIG_BLOCK, &set, NULL) == -1)
		die("sigprocmask(2)");

	ppid = getpid();
	snprintf(sockpath, sizeof(sockpath), "%d.sock", ppid);

	pid = fork();
	switch (pid) {
	case -1:
		die("fork(2)");
	case 0:
		return (server_main(ppid, sockpath));
	default:
		break;
	}

	return (client_main(pid, sockpath));
}