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Date:   Wed, 7 Aug 2019 19:55:12 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Adrian Reber <areber@redhat.com>
Cc:     Eric Biederman <ebiederm@xmission.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        linux-kernel@vger.kernel.org, Andrei Vagin <avagin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID
Message-ID: <20190807175511.xchtx764hkkervu3@wittgenstein>
References: <20190806191551.22192-1-areber@redhat.com>
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On Tue, Aug 06, 2019 at 09:15:50PM +0200, Adrian Reber wrote:
> The main motivation to add set_tid to clone3() is CRIU.
> 
> To restore a process with the same PID/TID CRIU currently uses
> /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> ns_last_pid and then (quickly) does a clone(). This works most of the
> time, but it is racy. It is also slow as it requires multiple syscalls.
> 
> Extending clone3() to support set_tid makes it possible restore a
> process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> race free (as long as the desired PID/TID is available).
> 
> This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> on clone3() with set_tid as they are currently in place for ns_last_pid.
> 
> Signed-off-by: Adrian Reber <areber@redhat.com>

I'm currently on vacation until 12 August so I'm going to be a little
slow in responding. Sorry about that!

Christian