Re: [RFC PATCH] Implement /proc/pid/kill

[Joel Fernandes]

On Tue, Oct 30, 2018 at 1:50 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>
> On Tue, Oct 30, 2018 at 3:21 AM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
> > On Mon, Oct 29, 2018 at 3:11 PM Daniel Colascione <dancol@xxxxxxxxxx> 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.
>
> It's a race. Would you make similar statements about a similar fix for
> a race condition involving a mutex and a double-free just because the
> race didn't crash most of the time? The issue I'm trying to fix here
> is the same problem, one level higher up in the abstraction hierarchy.

I was just curious that if this was a real issue you are hitting in a
production system, it wasn't clear from the commit message. When I
read your commit I thought "Does the inspection process take that long
that we wrap around an entire PID range?". So perhaps you should amend
your commit message to address that it is not really a problem you ARE
seeing, but rather something you anticipate and that this patch would
be a nice-to-have to avoid that. Typically there should be good
reasons/real-usecases to add a new interface to the kernel. Linus has
repeatedly rejected new interfaces on the grounds of non existent
use-cases or non real-world use cases. Again if I am missing something
here, then please improve the commit message so others don't have
similar questions :) Its completely upto you though..

> > 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 ;-)
>
> The traditional unix process API is down there at level -10 of Rusty
> Russel's old bad API scale: "It's impossible to get right". The races
> in the current API are unavoidable. That most programs don't hit these
> races most of the time doesn't mean that the race isn't present.
>
> We've moved to a model where we identify other system resources, like
> DRM fences, locks, sockets, and everything else via file descriptors.
> This change is a step toward using procfs file descriptors to work
> with processes, which makes the system more regular and easier to
> reason about. A clean API that's possible to use correctly is a
> worthwhile project.

Ok, agreed. thanks,

- Joel