[PATCH v2] clone.2: Fix outdated statement about CLONE_NEWPID

[Alejandro Colomar <alx@kernel.org>]

From: Sargun Dhillon <sargun@sargun.me>

[TO += Serge]

Hi Sargun, Serge!

Sargun, I've slightly changed your patch, to keep the historical
information that in old versions of Linux this restriction was in place.
I also cleaned up the example program a little bit, and fixed the
formatting mistake I mentioned.  See scissor patch below.

However, reading the Linux commit that changed this, I had some doubts.
I've added Serge, in case he can remember the details.

Serge, I see that the commit message for (Linux) 1f7f4dde5c94 quotes
some email of yours, and mentions that the commit fixes a regression.
So my doubt is: was this restriction in place before v3.13 as a stable
thing, or was it only accidentally introduced temporarily and soon
fixed?  How should we document it?

Cheers,
Alex

---8<-----

It appears like the documentation is based on out of date information in
regards to using CLONE_NEWPID and CLONE_PARENT together.  Since Linux
v3.13, this restriction has been lifted.  See commit 1f7f4dde5c94
("fork:  Allow CLONE_PARENT after setns(CLONE_NEWPID)").

For example, in this test program, one can see that it works:

 #include <assert.h>
 #include <err.h>
 #include <linux/sched.h>
 #include <sched.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/syscall.h>
 #include <unistd.h>

static pid_t sys_clone3(struct clone_args *args);

int
main(void)
{
	int                ret;
	struct clone_args  args = {
		.flags = CLONE_PARENT | CLONE_NEWPID,
	};

	printf("main program: pid: %d, and ppid: %d\n", getpid(), getppid());

	ret = sys_clone3(&args);
	switch (ret) {
	case -1:
		err(EXIT_FAILURE, "clone3");
	case 0:
		printf("child: pid: %d, and ppid: %d\n", getpid(), getppid());
		exit(EXIT_SUCCESS);
	default:
		exit(EXIT_SUCCESS);
	}
}

static pid_t
sys_clone3(struct clone_args *args)
{
	fflush(stdout);
	fflush(stderr);
	return syscall(SYS_clone3, args, sizeof(*args));
}

This test program (successfully) outputs:

    # ./a.out
    main program: pid: 34663, and ppid: 34662
    child: pid: 1, and ppid: 0

Cowritten-by: Sargun Dhillon <sargun@sargun.me>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: John Watts <contact@jookia.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---

 man2/clone.2 | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/man2/clone.2 b/man2/clone.2
index b91b71831..225fef86d 100644
--- a/man2/clone.2
+++ b/man2/clone.2
@@ -736,9 +736,11 @@ .SS The flags mask
 can employ
 .BR CLONE_NEWPID .
 This flag can't be specified in conjunction with
-.B CLONE_THREAD
-or
-.BR CLONE_PARENT .
+.BR CLONE_THREAD .
+Before Linux 3.13,
+this flag couldn't be specified in conjunction with
+.B CLONE_PARENT
+either.
 .TP
 .B CLONE_NEWUSER
 (This flag first became meaningful for
-- 
2.40.1