linux-kernel - Re: [PATCH] pidfd: add NSpid entries to fdinfo

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Date:   Mon, 14 Oct 2019 17:10:01 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Roman Gushchin <guro@...com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] pidfd: add NSpid entries to fdinfo

On Mon, Oct 14, 2019 at 12:32 PM Christian Brauner
<christian.brauner@...ntu.com> wrote:
> On Mon, Oct 14, 2019 at 11:43:01AM +0200, Christian Kellner wrote:
> > On Sat, 2019-10-12 at 12:21 +0200, Christian Brauner wrote:
> > > I tried to think of cases where the first entry of Pid is not
> > > identical
> > > to the first entry of NSpid but I came up with none. Maybe you do,
> > > Jann?
> > Yeah, I don't think that can be the case. By looking at the source of
> > 'pid_nr_ns(pid, ns)' a non-zero return means that a) 'pid' valid, ie.
> > non-null and b) 'ns' is in the pid namespace hierarchy of 'pid' (at
> > pid->level, i.e. "pid->numbers[ns->level].ns == ns").

Agreed.

> You could probably do:
>
> #ifdef CONFIG_PID_NS
> seq_put_decimal_ull(m, "\nNSpid:\t", nr);
> for (i = ns->level + 1; i <= pid->level && nr; i++)
>         seq_put_decimal_ull(m, "\t", pid->numbers[i].nr);
> #endif

Personally, I dislike hiding the precondition for running the loop in
the loop statement like that. While it makes the code more concise, it
somewhat obfuscates the high-level logic at a first glance.