linux-kernel - Re: [RFC][PATCH] exec: Freeze the other threads during a multi-threaded exec

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Message-ID: <87bljzkf36.fsf@x220.int.ebiederm.org>
Date:   Tue, 28 Jul 2020 07:18:37 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-fsdevel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        linux-pm@...r.kernel.org
Subject: Re: [RFC][PATCH] exec: Freeze the other threads during a multi-threaded exec

Aleksa Sarai <cyphar@...har.com> writes:

> On 2020-07-27, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> To the best of my knowledge processes with more than one thread
>> calling exec are not common, and as all of the threads will be killed
>> by exec there does not appear to be any useful work a thread can
>> reliably do during exec.
>
> Every Go program which calls exec (this includes runc, Docker, LXD,
> Kubernetes, et al) fills the niche of "multi-threaded program that calls
> exec" -- all Go programs are multi-threaded and there's no way of
> disabling this. This will most likely cause pretty bad performance
> regression for basically all container workloads.

So it is a good point that container runtimes use Go, and that
fundamentally anything that uses Go will be multi-threaded.  Having just
looked closely at this I don't think in practice this is an issue even
at this early state of my code.

If those other threads are sleeping the code I have implemented should
be a no-op.

If those threads aren't sleeping you have undefined behavior, as at
some point the kernel will come through and kill those threads.

Further unless I am completely mistaken the container runtimes use
forkAndExecInChild from go/src/syscall/exec_linux.go which performs a
vfork before performing the exec.

Eric