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Date:   Tue, 8 Feb 2022 19:07:41 +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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        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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On Tue, Feb 08, 2022 at 01:51:35PM -0500, Waiman Long wrote:
> On 2/8/22 13:16, Al Viro wrote:
> > 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?
> > 
> I think the fd_install() call can be moved after the release of
> sighand->siglock but before the release the tasklist_lock. Will that be good
> enough?

Looks like it should, but I'd rather hear from the CLONE_PIDFD authors first...
Christian, could you comment on that?