linux-kernel - Re: [RFC PATCH] Implement /proc/pid/kill

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Date:   Mon, 29 Oct 2018 20:21:51 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill

On Mon, Oct 29, 2018 at 3:11 PM Daniel Colascione <dancol@...gle.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'.
>
> 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.
>
> 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".

How long does the 'inspection' procedure take? If its a short
duration, then is PID reuse really an issue, I mean the PIDs are not
reused until wrap around and the only reason this can be a problem is
if you have the wrap around while the 'inspecting some aspect'
procedure takes really long.

Also the proc fs is typically not the right place for this. Some
entries in proc are writeable, but those are for changing values of
kernel data structures. The title of man proc(5) is "proc - process
information pseudo-filesystem". So its "information" right?

IMO without a really good reason for this, it could really be a hard
sell but the RFC was worth it anyway to discuss it ;-)

thanks,

- Joel