libbb: rework NOMMU helper API so that it makes more sense

and easier to use. Doesn't compile - need two more commits.

libbb/vfork_daemon_rexec.c

@@ -18,11 +18,68 @@
 #include <paths.h>
 #include "libbb.h"
 
-#ifdef BB_NOMMU
+/* This does a fork/exec in one call, using vfork().  Returns PID of new child,
+ * -1 for failure.  Runs argv[0], searching path if that has no / in it. */
+pid_t spawn(char **argv)
+{
+	/* Compiler should not optimize stores here */
+	volatile int failed;
+	pid_t pid;
+
+	// Be nice to nommu machines.
+	failed = 0;
+	pid = vfork();
+	if (pid < 0) /* error */
+		return pid;
+	if (!pid) { /* child */
+		/* Don't use BB_EXECVP tricks here! */
+		execvp(argv[0], argv);
+
+		/* We are (maybe) sharing a stack with blocked parent,
+		 * let parent know we failed and then exit to unblock parent
+		 * (but don't run atexit() stuff, which would screw up parent.)
+		 */
+		failed = errno;
+		_exit(0);
+	}
+	/* parent */
+	/* Unfortunately, this is not reliable: vfork()
+	 * can be equivalent to fork() according to standards */
+	if (failed) {
+		errno = failed;
+		return -1;
+	}
+	return pid;
+}
+
+/* Die with an error message if we can't spawn a child process. */
+pid_t xspawn(char **argv)
+{
+	pid_t pid = spawn(argv);
+	if (pid < 0) bb_perror_msg_and_die("%s", *argv);
+	return pid;
+}
+
+
+
+#if 0 //ndef BB_NOMMU
+// Die with an error message if we can't daemonize.
+void xdaemon(int nochdir, int noclose)
+{
+	if (daemon(nochdir, noclose))
+		bb_perror_msg_and_die("daemon");
+}
+#endif
+
+#if 0 // def BB_NOMMU
 void vfork_daemon_rexec(int nochdir, int noclose, char **argv)
 {
 	int fd;
 
+	/* Maybe we are already re-execed and come here again? */
+	if (re_execed)
+		return;
+
 	setsid();
 
 	if (!nochdir)
@@ -56,3 +113,78 @@ void vfork_daemon_rexec(int nochdir, int noclose, char **argv)
 	}
 }
 #endif /* BB_NOMMU */
+
+#ifdef BB_NOMMU
+static void daemon_or_rexec(char **argv)
+{
+	pid_t pid;
+	/* Maybe we are already re-execed and come here again? */
+	if (re_execed)
+		return;
+
+	pid = vfork();
+	if (pid < 0) /* wtf? */
+		bb_perror_msg_and_die("vfork");
+	if (pid) /* parent */
+		exit(0);
+	/* child - re-exec ourself */
+	/* high-order bit of first char in argv[0] is a hidden
+	 * "we have (alrealy) re-execed, don't do it again" flag */
+	argv[0][0] |= 0x80;
+	execv(CONFIG_BUSYBOX_EXEC_PATH, argv);
+	bb_perror_msg_and_die("exec %s", CONFIG_BUSYBOX_EXEC_PATH);
+}
+#else
+static void daemon_or_rexec(void)
+{
+	pid_t pid;
+	pid = fork();
+	if (pid < 0) /* wtf? */
+		bb_perror_msg_and_die("fork");
+	if (pid) /* parent */
+		exit(0);
+	/* child */
+}
+#define daemon_or_rexec(argv) daemon_or_rexec()
+#endif
+
+
+/* Due to a #define in libbb.h on MMU systems we actually have 1 argument -
+ * char **argv "vanishes" */
+void bb_daemonize_or_rexec(int flags, char **argv)
+{
+	int fd;
+
+	fd = xopen(bb_dev_null, O_RDWR);
+
+	if (flags & DAEMON_CHDIR_ROOT)
+		xchdir("/");
+
+	if (flags & DAEMON_DEVNULL_STDIO) {
+		close(0);
+		close(1);
+		close(2);
+	}
+
+	while ((unsigned)fd < 2)
+		fd = dup(fd); /* have 0,1,2 open at least to /dev/null */
+
+	if (!(flags & DAEMON_ONLY_SANITIZE)) {
+		daemon_or_rexec(argv);
+		/* if daemonizing, make sure we detach from stdio */
+		setsid();
+		dup2(fd, 0);
+		dup2(fd, 1);
+		dup2(fd, 2);
+	}
+	if (fd > 2)
+		close(fd--);
+	if (flags & DAEMON_CLOSE_EXTRA_FDS)
+		while (fd > 2)
+			close(fd--); /* close everything after fd#2 */
+}
+
+void bb_sanitize_stdio(void)
+{
+	bb_daemonize_or_rexec(DAEMON_ONLY_SANITIZE, NULL);
+}