linux-kernel - Re: [PATCH 16/32] x86/vdso: Generate vdso{,32}-timens.lds

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Date:   Thu, 7 Feb 2019 16:11:59 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Adrian Reber <adrian@...as.de>, Andrei Vagin <avagin@...nvz.org>,
        Andrei Vagin <avagin@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andy Tucker <agtucker@...gle.com>,
        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>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 16/32] x86/vdso: Generate vdso{,32}-timens.lds

Hi Rasmus,

On 2/7/19 8:31 AM, Rasmus Villemoes wrote:
> These (14-19, if I'm reading them right) seems to add quite a lot of
> complexity and fragility to the build, and other architectures would
> probably have to add something similar to their vdso builds.
> 
> I'm wondering why not make the rule be that a timens takes effect on
> next execve?

I believe, it would make setns() syscall much tricker than wanted:
At this moment the only exception is pidns which changes ns of the child
and not the process-callee.
If exec() would be required to join timens - it may be a challenging
problem for container systems: in order to enter it one needs to
exec("/proc/self/exe") and add some new arguments/options.
Furthermore, it seems to me that to enter container with this semantics,
one needs to enter timens before entering mountns.

IOW, I believe, this would move complexity from kernel build time to
userspace ABI. And I guess, it would require much more logic to
re-create possibly nested namespaces hierarchy.

Rather I've considered using some kind of dynamic patching on vdso_init():
o static_branch - it would add some nops to !timens vdso
o something new like static_retpoline which would put RET over call to
  clk_to_ns(), shouldn't be a rocket since.

But in my point of view, if something can be done in compile time
instead of patching code dynamically - than it reduces the complexity
(lesser depends on what compiler/toolchain does).

Thanks,
          Dmitry