linux-kernel - Re: [PATCH v6 1/2] fork: extend clone3() to support setting a PID

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Message-ID: <20190813143023.GC6971@redhat.com>
Date:   Tue, 13 Aug 2019 16:30:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Adrian Reber <areber@...hat.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v6 1/2] fork: extend clone3() to support setting a PID

On 08/12, 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@...hat.com>

Reviewed-by: Oleg Nesterov <oleg@...hat.com>