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Date:   Tue, 30 Oct 2018 16:00:12 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     linux-kernel@vger.kernel.org, timmurray@google.com,
        joelaf@google.com, surenb@google.com
Subject: Re: [RFC PATCH] Implement /proc/pid/kill
Message-ID: <20181030050012.u43lcvydy6nom3ul@yavin>
References: <20181029221037.87724-1-dancol@google.com>
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On 2018-10-29, Daniel Colascione <dancol@google.com> wrote:
> Add a simple proc-based kill interface. To use /proc/pid/kill, just
> write the signal number in base-10 ASCII to the kill file of the
> process to be killed: for example, 'echo 9 > /proc/$$/kill'.
>=20
> Semantically, /proc/pid/kill works like kill(2), except that the
> process ID comes from the proc filesystem context instead of from an
> explicit system call parameter. This way, it's possible to avoid races
> between inspecting some aspect of a process and that process's PID
> being reused for some other process.

(Aside from any UX concerns other folks might have.)

I think it would be a good idea to (at least temporarily) restrict this
so that only processes that are in the same PID namespace as the /proc
being resolved through may use this interface. Otherwise you might have
cases where partial container breakouts can start sending signals to
PIDs they wouldn't normally be able to address.

> With /proc/pid/kill, it's possible to write a proper race-free and
> safe pkill(1). An approximation follows. A real program might use
> openat(2), having opened a process's /proc/pid directory explicitly,
> with the directory file descriptor serving as a sort of "process
> handle".

I do like the idea of holding a dirfd to /proc/$pid to address
processes, and it something I considered doing in runc. (Unfortunately
there are lots of things that make it a bit difficult to use /proc/$pid
exclusively for introspection of a process -- especially in the context
of containers.)

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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