Re: [PATCH] pidfd: add NSpid entries to fdinfo

From: Jann Horn <jannh@google.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Christian Kellner <christian@kellner.me>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Roman Gushchin <guro@fb.com>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	Linux API <linux-api@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Michal Hocko <mhocko@suse.com>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH] pidfd: add NSpid entries to fdinfo
Date: Mon, 14 Oct 2019 17:10:01 +0200	[thread overview]
Message-ID: <CAG48ez1Yf05j2irdZxT2TA=2n3xo25KN48nRQdp_F8j14K36rA@mail.gmail.com> (raw)
In-Reply-To: <20191014103157.h2wph2ujjidsrhyw@wittgenstein>

On Mon, Oct 14, 2019 at 12:32 PM Christian Brauner
<christian.brauner@ubuntu.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.