linux-kernel - Re: [PATCH v5 0/3] time namespace aware system boot time

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Message-ID: <20201029153301.aahwxau6ijyol357@wittgenstein>
Date:   Thu, 29 Oct 2020 16:33:01 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Subject: Re: [PATCH v5 0/3] time namespace aware system boot time

On Tue, Oct 27, 2020 at 09:42:55PM +0100, Michael Weiß wrote:
> Time namespaces make it possible to virtualize time inside of
> containers, e.g., it is feasible to reset the uptime of a container
> to zero by setting the time namespace offset for boottime to the
> negated current value of the CLOCK_BOOTTIME.
> 
> However, the boot time stamp provided by getboottime64() does not
> take care of time namespaces. The resulting boot time stamp 'btime'
> provided by /proc/stat does not show a plausible time stamp inside
> the time namespace of a container.
> 
> We address this by shifting the value returned by getboottime64()
> by subtracting the boottime offset of the time namespace.
> (A selftest to check the expected /proc/stat 'btime' inside the
> namespace is provided.)
> 
> Further, to avoid to show processes as time travelers inside of the
> time namespace the boottime offset then needs to be added to the
> start_boottime provided by the task_struct.
> 
> v5 Changes:
> Added tags, Thanks Andrei and Thomas for reviews and comments.
> Updated commit message of [2/3] as indicated by Thomas.
> 
> v4 Changes:
> Avoid type conversions back and forth between timespec64 and ktime_t
> in 'proc/stat.c' as suggested by Andrei.
> Introduced timens_sub_boottime() in 'time_namespace.h' to provide
> better coder readability/consistency.
> 
> v3 Changes:
> leave getboottime64() unchanged and shift the boot timestamp in
> 'fs/proc/stat.c' as result of the discussion with Andrei and Thomas.
> 
> v2 Changes:
> Fixed compile errors with TIME_NS not set in config

Thank you Michael,

Acked-by: Christian Brauner <christian.brauner@...ntu.com>

As discussed with Thomas, I'm picking this up now!

Thanks!
Christian