linux-kernel - Re: [PATCH] copy_process(): Move fd_install() out of sighand->siglock critical section

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Date:   Tue, 8 Feb 2022 13:51:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christian Brauner <brauner@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexey Gladkov <legion@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] copy_process(): Move fd_install() out of sighand->siglock
 critical section

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?

I am afraid that I am not knowledgeable enough to talk about the 
CLONE_PIDFD expectation. May other people chime in on this?

Cheers,
Longman