[PATCH v3] clone.2: Fix erroneous statement about CLONE_NEWPID|CLONE_PARENT

[Alejandro Colomar <alx@kernel.org>]

From: Sargun Dhillon <sargun@sargun.me>

CLONE_NEWPID|CLONE_PARENT was only prohibited during a short period.
That prohibition was introduced in Linux 3.12, in commit 40a0d32d1eaf
("fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks"), but
was a regression, and was fixed in Linux 3.13, in commit 1f7f4dde5c94
("fork:  Allow CLONE_PARENT after setns(CLONE_NEWPID)").

In this test program, one can see that it works:

 #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 | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/man2/clone.2 b/man2/clone.2
index b91b71831..7d2dc2339 100644
--- a/man2/clone.2
+++ b/man2/clone.2
@@ -736,9 +736,7 @@ .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 .
 .TP
 .B CLONE_NEWUSER
 (This flag first became meaningful for
-- 
2.40.1