linux-kernel - Re: [RFC][PATCH v2 3/5] clone: Disallown CLONE_THREAD with a shared sighand

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Message-ID: <20170405162458.GF14536@redhat.com>
Date:   Wed, 5 Apr 2017 18:24:59 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <asarai@...e.com>,
        Andy Lutomirski <luto@...capital.net>,
        Attila Fazekas <afazekas@...hat.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Ulrich Obergfell <uobergfe@...hat.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [RFC][PATCH v2 3/5] clone: Disallown CLONE_THREAD with a
        shared sighand_struct

On 04/02, Eric W. Biederman wrote:
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1515,6 +1515,13 @@ static __latent_entropy struct task_struct *copy_process(
>  	if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
>  		return ERR_PTR(-EINVAL);
>
> +	/* Disallow CLONE_THREAD with a shared SIGHAND structure.  No
> +	 * one cares

Well, can't resists... I won't argue, but we can't know if no one cares
or not. I agree that most probably this won't break something, but who
knows... I am always scared when we add the incompatible changes.

> and supporting it leads to unnecessarily complex
> +	 * code.
> +	 */
> +	if ((clone_flags & CLONE_THREAD) && (atomic_read(&current->sighand->count) > 1))
> +		return ERR_PTR(-EINVAL);

Perhaps the comment should explain why we do this and say that
sighand-unsharing in de_thread() depends on this.

Oleg.