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Date:   Tue, 8 Feb 2022 18:16:41 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Waiman Long <longman@redhat.com>
Cc:     Christian Brauner <brauner@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Alexey Gladkov <legion@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jann Horn <jannh@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] copy_process(): Move fd_install() out of
 sighand->siglock critical section
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References: <20220208163912.1084752-1-longman@redhat.com>
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On Tue, Feb 08, 2022 at 11:39:12AM -0500, Waiman Long wrote:

> One way to solve this problem is to move the fd_install() call out of
> the sighand->siglock critical section.
> 
> Before commit 6fd2fe494b17 ("copy_process(): don't use ksys_close()
> on cleanups"), the pidfd installation was done without holding both
> the task_list lock and the sighand->siglock. Obviously, holding these
> two locks are not really needed to protect the fd_install() call.
> So move the fd_install() call down to after the releases of both locks.

	Umm... That assumes we can delay it that far.  IOW, that nothing
relies upon having pidfd observable in /proc/*/fd as soon as the child
becomes visible there in the first place.

	What warranties are expected from CLONE_PIDFD wrt observation of
child's descriptor table?