linux-kernel - Re: [PATCHv7 01/33] ns: Introduce Time Namespace

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Date:   Wed, 16 Oct 2019 16:33:42 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv7 01/33] ns: Introduce Time Namespace

On Wed, Oct 16, 2019 at 12:39:11PM +0200, Thomas Gleixner wrote:
> On Wed, 16 Oct 2019, Vincenzo Frascino wrote:
> 
> < Trim 250+ lines ( 3+ pages) of pointlessly wasted electrons >
> 
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -1096,6 +1096,13 @@ config UTS_NS
> > >  	  In this namespace tasks see different info provided with the
> > >  	  uname() system call
> > >  
> > > +config TIME_NS
> > > +	bool "TIME namespace"
> > > +	default y
> > 
> > Having CONFIG_TIME_NS "default y" makes so that the option is selected even on
> > the architectures that have no support for time namespaces.
> > The direct consequence is that the fallbacks defined in this patch are never
> > selected and this ends up in kernel compilation errors due to missing symbols.
> > 
> > The error below shows what happens on arm64 (similar behavior on other
> > architectures):
> > 
> > aarch64-linux-gnu-ld: kernel/time/namespace.o: in function `timens_on_fork':
> > kernel/time/namespace.c:321: undefined reference to `vdso_join_timens'
> > 
> > My proposal is to keep TIME_NS "default n" (just remove "default y"), let the
> > architectures that enable time namespaces select it and make CONFIG_TIME_NS
> > select GENERIC_VDSO_TIME_NS if arch has HAVE_GENERIC_VDSO.
> 
> Nah.
> 
> config TIME_NS
> 	bool "TIME namespace"
> 	depends on GENERIC_VDSO_TIME_NS

I was thinking to fix this by the same way with a small difference.

If GENERIC_GETTIMEOFDAY isn't set, it should be safe to allow enabling
TIME_NS. In this case, clock_gettime works via system call and we don't
have arch-specific code in this case. Does this sound reasonable?

        depends on (!GENERIC_GETTIMEOFDAY || GENERIC_VDSO_TIME_NS)

Thanks,
Andrei