linux-kernel - Re: [PATCH v2 2/5] clone: add CLONE

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Date:   Thu, 18 Apr 2019 15:12:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        jannh@...gle.com, dhowells@...hat.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, serge@...lyn.com, luto@...nel.org,
        arnd@...db.de, ebiederm@...ssion.com, keescook@...omium.org,
        tglx@...utronix.de, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, cyphar@...har.com,
        joel@...lfernandes.org, dancol@...gle.com
Subject: Re: [PATCH v2 2/5] clone: add CLONE_PIDFD

On 04/18, Christian Brauner wrote:
>
> @@ -1674,13 +1729,14 @@ static __latent_entropy struct task_struct *copy_process(
>  					unsigned long clone_flags,
>  					unsigned long stack_start,
>  					unsigned long stack_size,
> +					int __user *parent_tidptr,
>  					int __user *child_tidptr,
>  					struct pid *pid,
>  					int trace,
>  					unsigned long tls,
>  					int node)
>  {
> -	int retval;
> +	int pidfd = -1, retval;

it seems that initialization is unneeded, but this is cosmetic.

I see no technical problems, feel free to add my reviewed-by.

But let me ask a couple of questions...

Why O_CLOEXEC? I am just curious, I do not really care.

Should we allow CLONE_THREAD | CLONE_PIDFD ?

Are you sure we will never need to extend this interface? If not, then perhaps it
make sense to add something like

	if (CLONE_PIDFD) {
		unsigned long not_used_yet;
		if (get_user(not_used_yet, parent_tidptr) ||
		    not_used_yet != 0)
			return -EINVAL;
	}

this way we can easily add more arguments in future or even turn CLONE_PIDFD into
CLONE_MORE_ARGS_IN_PARENT_TIDPTR.

Not that I think this is really good idea, sys_clone2() makes more sense, but still.

Oleg.