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

Serge E. Hallyn
2023-08-13T14:36:15Z

On Sun, Aug 13, 2023 at 03:55:25PM +0200, Alejandro Colomar wrote:
> 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>

Reviewed-by: 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