PR_DETACH: detach from parent

Hi.

I was trying to use daemon() library call to daemonize the process, and found out that it is not suitable for the process that have threads: all the threads are simply being killed.
The reason is that the daemon() function does fork(), then exit() in parent and setsid() in child, which is a bit ugly, and doesn't work with threads (fork() doesn't fork threads).
I've found no solution to that, and just hacked up one instead.
The attached patch adds the PR_DETACH prctl command, which detaches the process from its parent. With that, the daemon() function can be implemented without the fork/exit/setsid hacks, and here is an example of it:

---
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/prctl.h>

#ifndef PR_DETACH
#define PR_DETACH 35
#endif

int daemon2(int nochdir, int noclose)

{
    int err, fd;

    err = prctl(PR_DETACH);
    if (err == -1) {
        if (errno == EINVAL)
            errno = ENOTSUP;
        return -1;
    }

    fd = open("/dev/tty", O_RDWR);
    if (fd != -1) {
        ioctl(fd, TIOCNOTTY, NULL);
        close(fd);
    }

    if (!nochdir && chdir("/") == -1)
        return -1;

    if (!noclose) {
        fd = open("/dev/null", O_RDWR, 0);
        if (fd == -1)
            return -1;
        if (dup2(fd, STDIN_FILENO) == -1)
            return -1;
        if (dup2(fd, STDOUT_FILENO) == -1)
            return -1;
        if (dup2(fd, STDERR_FILENO) == -1)
            return -1;
        if (fd > 2)
            close(fd);
    }
    return 0;
}

---

With this implementation, all threads survive the daemonization. Thoughts?

index a3baeb2..fbd2451 100644
@@ -102,4 +102,6 @@
 
 #define PR_MCE_KILL_GET 34
 
+#define PR_DETACH 35
+
 #endif /* _LINUX_PRCTL_H */

index 4e3cff1..2cd495a 100644
@@ -1450,10 +1450,10 @@ int do_notify_parent(struct task_struct *tsk, int sig)
 
 	BUG_ON(sig == -1);
 
- 	/* do_notify_parent_cldstop should have been called instead.  */
- 	BUG_ON(task_is_stopped_or_traced(tsk));
+	/* do_notify_parent_cldstop should have been called instead.  */
+	BUG_ON(task_is_stopped_or_traced(tsk));
 
-	BUG_ON(!task_ptrace(tsk) &&
+	BUG_ON(!task_ptrace(tsk) && (tsk->flags & PF_EXITING) &&
 	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
 
 	info.si_signo = sig;

index 7f5a0cd..fa10732 100644
@@ -1727,6 +1727,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			else
 				error = PR_MCE_KILL_DEFAULT;
 			break;
+		case PR_DETACH:
+			error = -EPERM;
+			if (me->real_parent == init_pid_ns.child_reaper)
+				break;
+			if (me->group_leader != me)
+				break;
+			exit_ptrace(me);
+			do_notify_parent(me, me->exit_signal);
+			me->exit_signal = SIGCHLD;
+			me->parent = me->real_parent =
+				init_pid_ns.child_reaper;
+			list_move_tail(&me->sibling,
+				&me->real_parent->children);
+			error = 0;
+			break;
 		default:
 			error = -EINVAL;
 			break;