linux-kernel - Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON

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Message-ID: <f55261dbc41544363b4348820f67fb01c6d7aaec.camel@bitron.ch>
Date:   Thu, 29 Nov 2018 15:41:38 +0000
From:   Jürg Billeter <j@...ron.ch>
To:     Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT

Hi Oleg,

Thanks for the review.

On Thu, 2018-11-29 at 13:34 +0100, Oleg Nesterov wrote:
> To me it would be more clean to call walk_process_tree(kill_descendant_visitor)
> unconditionally in find_new_reaper() right before "if (has_child_subreaper)", but
> then we will need to shift read_lock(tasklist) from walk_process_tree().

Yes, that's the reason why I added the call before the tasklist lock.
Let me know if you want me to move the read lock from
walk_process_tree() to PR_SET_CHILD_SUBREAPER (the only caller)
instead.

> So I think the patch is mostly fine, the only problem I can see is that
> PR_SET_KILL_DESCENDANTS_ON_EXIT can race with PR_SET_CHILD_SUBREAPER, they both
> need to update the bits in the same word.

Good point. I'll make it a regular bool instead of a bitfield for v2,
unless you have another approach in mind to fix this.

Jürg