do_fork: Propagate error code returned by mcexec

Refs: #731
Change-Id: I7eb52c1c76103d65d108b18b7beaf8041b51cd03

test/issues/731/g310a.c

@@ -0,0 +1,55 @@
+/*
+ * g310a: If superuser try to fork() after seteuid(bin), ...
+ */
+
+#include <pwd.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+int main(void)
+{
+	int error;
+	struct passwd *pwd;
+	pid_t pid;
+	int ws;
+
+	if (geteuid()) {
+		printf("not a superuser\n");
+		return 2;
+	}
+
+	pwd = getpwnam("bin");
+	if (!pwd) {
+		perror("getpwnam");
+		return 1;
+	}
+
+	error = seteuid(pwd->pw_uid);
+	if (error) {
+		perror("seteuid");
+		return 1;
+	}
+
+	pid = fork();
+	if (pid == -1) {
+		perror("fork");
+	}
+
+	if (!pid) {
+		return 0;
+	}
+
+	pid = waitpid(pid, &ws, 0);
+	if (pid == -1) {
+		perror("waitpid");
+		return 1;
+	}
+	if (ws) {
+		printf("ws: %#x\n", ws);
+		return 1;
+	}
+
+	printf("done.\n");
+	return 0;
+}