subprocess(const char *tag, const char *command,
    int ac, char **av, FILE **child, u_int flags,
    struct passwd *pw, privdrop_fn *drop_privs, privrestore_fn *restore_privs)
{
	FILE *f = NULL;
	struct stat st;
	int fd, devnull, p[2], i;
	pid_t pid;
	char *cp, errmsg[512];
	u_int nenv = 0;
	char **env = NULL;

	/* If dropping privs, then must specify user and restore function */
	if (drop_privs != NULL && (pw == NULL || restore_privs == NULL)) {
		error("%s: inconsistent arguments", tag); /* XXX fatal? */
		return 0;
	}
	if (pw == NULL && (pw = getpwuid(getuid())) == NULL) {
		error("%s: no user for current uid", tag);
		return 0;
	}
	if (child != NULL)
		*child = NULL;

	debug3_f("%s command \"%s\" running as %s (flags 0x%x)",
	    tag, command, pw->pw_name, flags);

	/* Check consistency */
	if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0 &&
	    (flags & SSH_SUBPROCESS_STDOUT_CAPTURE) != 0) {
		error_f("inconsistent flags");
		return 0;
	}
	if (((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) == 0) != (child == NULL)) {
		error_f("inconsistent flags/output");
		return 0;
	}

	/*
	 * If executing an explicit binary, then verify the it exists
	 * and appears safe-ish to execute
	 */
	if (!path_absolute(av[0])) {
		error("%s path is not absolute", tag);
		return 0;
	}
	if (drop_privs != NULL)
		drop_privs(pw);
	if (stat(av[0], &st) == -1) {
		error("Could not stat %s \"%s\": %s", tag,
		    av[0], strerror(errno));
		goto restore_return;
	}
	if ((flags & SSH_SUBPROCESS_UNSAFE_PATH) == 0 &&
	    safe_path(av[0], &st, NULL, 0, errmsg, sizeof(errmsg)) != 0) {
		error("Unsafe %s \"%s\": %s", tag, av[0], errmsg);
		goto restore_return;
	}
	/* Prepare to keep the child's stdout if requested */
	if (pipe(p) == -1) {
		error("%s: pipe: %s", tag, strerror(errno));
 restore_return:
		if (restore_privs != NULL)
			restore_privs();
		return 0;
	}
	if (restore_privs != NULL)
		restore_privs();

	switch ((pid = fork())) {
	case -1: /* error */
		error("%s: fork: %s", tag, strerror(errno));
		close(p[0]);
		close(p[1]);
		return 0;
	case 0: /* child */
		/* Prepare a minimal environment for the child. */
		if ((flags & SSH_SUBPROCESS_PRESERVE_ENV) == 0) {
			nenv = 5;
			env = xcalloc(sizeof(*env), nenv);
			child_set_env(&env, &nenv, "PATH", _PATH_STDPATH);
			child_set_env(&env, &nenv, "USER", pw->pw_name);
			child_set_env(&env, &nenv, "LOGNAME", pw->pw_name);
			child_set_env(&env, &nenv, "HOME", pw->pw_dir);
			if ((cp = getenv("LANG")) != NULL)
				child_set_env(&env, &nenv, "LANG", cp);
		}

		for (i = 1; i < NSIG; i++)
			ssh_signal(i, SIG_DFL);

		if ((devnull = open(_PATH_DEVNULL, O_RDWR)) == -1) {
			error("%s: open %s: %s", tag, _PATH_DEVNULL,
			    strerror(errno));
			_exit(1);
		}
		if (dup2(devnull, STDIN_FILENO) == -1) {
			error("%s: dup2: %s", tag, strerror(errno));
			_exit(1);
		}

		/* Set up stdout as requested; leave stderr in place for now. */
		fd = -1;
		if ((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) != 0)
			fd = p[1];
		else if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0)
			fd = devnull;
		if (fd != -1 && dup2(fd, STDOUT_FILENO) == -1) {
			error("%s: dup2: %s", tag, strerror(errno));
			_exit(1);
		}
		closefrom(STDERR_FILENO + 1);

		if (geteuid() == 0 &&
		    initgroups(pw->pw_name, pw->pw_gid) == -1) {
			error("%s: initgroups(%s, %u): %s", tag,
			    pw->pw_name, (u_int)pw->pw_gid, strerror(errno));
			_exit(1);
		}
		if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1) {
			error("%s: setresgid %u: %s", tag, (u_int)pw->pw_gid,
			    strerror(errno));
			_exit(1);
		}
		if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1) {
			error("%s: setresuid %u: %s", tag, (u_int)pw->pw_uid,
			    strerror(errno));
			_exit(1);
		}
		/* stdin is pointed to /dev/null at this point */
		if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0 &&
		    dup2(STDIN_FILENO, STDERR_FILENO) == -1) {
			error("%s: dup2: %s", tag, strerror(errno));
			_exit(1);
		}
		if (env != NULL)
			execve(av[0], av, env);
		else
			execv(av[0], av);
		error("%s %s \"%s\": %s", tag, env == NULL ? "execv" : "execve",
		    command, strerror(errno));
		_exit(127);
	default: /* parent */
		break;
	}

	close(p[1]);
	if ((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) == 0)
		close(p[0]);
	else if ((f = fdopen(p[0], "r")) == NULL) {
		error("%s: fdopen: %s", tag, strerror(errno));
		close(p[0]);
		/* Don't leave zombie child */
		kill(pid, SIGTERM);
		while (waitpid(pid, NULL, 0) == -1 && errno == EINTR)
			;
		return 0;
	}
	/* Success */
	debug3_f("%s pid %ld", tag, (long)pid);
	if (child != NULL)
		*child = f;
	return pid;
}