linux-kernel - Re: [PATCH v1 2/4] pid: add pidctl()

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Date:   Tue, 26 Mar 2019 09:33:52 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Jann Horn <jannh@...gle.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v1 2/4] pid: add pidctl()

On Tue, Mar 26, 2019 at 9:23 AM Christian Brauner <christian@...uner.io> wrote:
>
> On Tue, Mar 26, 2019 at 09:17:07AM -0700, Daniel Colascione wrote:
> > Thanks for the patch.
> >
> > On Tue, Mar 26, 2019 at 8:55 AM Christian Brauner <christian@...uner.io> wrote:
> > >
> > > The pidctl() syscalls builds on, extends, and improves translate_pid() [4].
> > > I quote Konstantins original patchset first that has already been acked and
> > > picked up by Eric before and whose functionality is preserved in this
> > > syscall:
> >
> > We still haven't had a much-needed conversation about splitting this
> > system call into smaller logical operations. It's important that we
> > address this point before this patch is merged and becomes permanent
> > kernel ABI.
>
> I don't particularly mind splitting this into an additional syscall like
> e.g.  pidfd_open() but then we have - and yes, I know you'll say
> syscalls are cheap - translate_pid(), and pidfd_open(). What I like
> about this rn is that it connects both apis in a single syscall
> and allows pidfd retrieval across pid namespaces. So I guess we'll see
> what other people think.

Thanks. I also appreciate a clean unification of related
functionality, but I'm concerned that this API in particular --- due
in part to its *ctl() name --- will become a catch-all facility for
doing *anything* with processes. (Granted, heavy use of a new, good,
and clean API would be a good problem to have.) This
single-system-call state of affairs would make it more awkward than
necessary to do system-call level logging (say, strace -e), enable or
disable tracing of specific operations with ftrace, apply some kinds
of SELinux policy, and so on, and the only advantage of the single
system call design that I can see right now is the logical
cleanliness.

I'd propose splitting the call, or if we can't do that, renaming it to
something else --- pidfd_query --- so that it's less likely to become
a catch-all operation holder.