linux-kernel - Re: [PATCH v5 0/6] arm64: add the time namespace support

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Date:   Fri, 24 Jul 2020 15:40:28 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Andrei Vagin <avagin@...il.com>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH v5 0/6] arm64: add the time namespace support

On Fri, Jul 24, 2020 at 03:30:39PM +0200, Christian Brauner wrote:
> On Thu, Jul 23, 2020 at 10:41:40AM -0700, Andrei Vagin wrote:
> > On Wed, Jul 22, 2020 at 07:15:06PM +0100, Catalin Marinas wrote:
> > > On Mon, Jul 13, 2020 at 06:57:43PM -0700, Andrei Vagin wrote:
> > > > On Sat, Jul 04, 2020 at 11:40:55PM -0700, Andrei Vagin wrote:
> > > > > On Wed, Jun 24, 2020 at 01:33:15AM -0700, Andrei Vagin wrote:
> > > > > > Allocate the time namespace page among VVAR pages and add the logic
> > > > > > to handle faults on VVAR properly.
> > > > > > 
> > > > > > If a task belongs to a time namespace then the VVAR page which contains
> > > > > > the system wide VDSO data is replaced with a namespace specific page
> > > > > > which has the same layout as the VVAR page. That page has vdso_data->seq
> > > > > > set to 1 to enforce the slow path and vdso_data->clock_mode set to
> > > > > > VCLOCK_TIMENS to enforce the time namespace handling path.
> > > > > > 
> > > > > > The extra check in the case that vdso_data->seq is odd, e.g. a concurrent
> > > > > > update of the VDSO data is in progress, is not really affecting regular
> > > > > > tasks which are not part of a time namespace as the task is spin waiting
> > > > > > for the update to finish and vdso_data->seq to become even again.
> > > > > > 
> > > > > > If a time namespace task hits that code path, it invokes the corresponding
> > > > > > time getter function which retrieves the real VVAR page, reads host time
> > > > > > and then adds the offset for the requested clock which is stored in the
> > > > > > special VVAR page.
> > > > > > 
> > > > > 
> > > > > > v2: Code cleanups suggested by Vincenzo.
> > > > > > v3: add a comment in __arch_get_timens_vdso_data.
> > > > > > v4: - fix an issue reported by the lkp robot.
> > > > > >     - vvar has the same size with/without CONFIG_TIME_NAMESPACE, but the
> > > > > >       timens page isn't allocated on !CONFIG_TIME_NAMESPACE. This
> > > > > >       simplifies criu/vdso migration between different kernel configs.
> > > > > > v5: - Code cleanups suggested by Mark Rutland.
> > > > > >     - In vdso_join_timens, mmap_write_lock is downgraded to
> > > > > >       mmap_read_lock. The VMA list isn't changed there, zap_page_range
> > > > > >       doesn't require mmap_write_lock.
> > > > > > 
> > > > > > Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
> > > > > > Reviewed-by: Dmitry Safonov <dima@...sta.com>
> > > > > 
> > > > > Hello Will and Catalin,
> > > > > 
> > > > > Have you had a chance to look at this patch set? I think it is ready to be
> > > > > merged. Let me know if you have any questions.
> > > > 
> > > > *friendly ping*
> > > > 
> > > > If I am doing something wrong, let me know.
> > > 
> > > Not really, just haven't got around to looking into it. Mark Rutland
> > > raised a concern (in private) about the safety of multithreaded apps
> > > but I think you already replied that timens_install() checks for this
> > > already [1].
> > > 
> > > Maybe a similar atomicity issue to the one raised by Mark but for
> > > single-threaded processes: the thread is executing vdso code, gets
> > > interrupted and a signal handler invokes setns(). Would resuming the
> > > execution in the vdso code on sigreturn cause any issues?
> > 
> > It will not cause any issues in the kernel. In the userspace,
> > clock_gettime() can return a clock value with an inconsistent offset, if
> > a process switches between two non-root namespaces. And it can triggers
> > SIGSEGV if it switches from a non-root to the root time namespace,
> > because a time namespace isn't mapped in the root time namespace.
> > 
> > I don't think that we need to handle this case in the kernel. Users
> > must understand what they are doing and have to write code so that avoid
> > these sort of situations. In general, I would say that in most cases it
> > is a bad idea to call setns from a signal handler.
> 
> I would argue that calling any function not in the list of
> man 7 signal-safety
> without checking the kernel implementation is "you get to keep the
> pieces territory". There's a whole range of syscalls that are not safe
> in signal handlers and we don't have any special precautions for them so
> I'm not sure we'd need one for setns(). But maybe I'm missing the bigger
> picture here.

Good point (I don't read man pages very often ;)). Thanks for
clarifying.

-- 
Catalin