linux-kernel - Re: [PATCH v2 0/4] time namespace aware system boot time

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Message-ID: <20201009132129.3vzdgzyy7gloou6v@wittgenstein>
Date:   Fri, 9 Oct 2020 15:21:29 +0200
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, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2 0/4] time namespace aware system boot time

On Thu, Oct 08, 2020 at 07:39:40AM +0200, 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_bootime provided by the task_struct.
> 
> v2 Changes:
> Fixed compile errors with TIME_NS not set in config
> Reported-by: kernel test robot <lkp@...el.com>

Hey Michael,

Thanks for the patches. This looks like a good idea to me. Since
/proc/uptime is now virtualized according to the timens the caller is in
btime has to be virtualized too.

Christian