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Message-ID: <abc477fb3bd8fbf7b4d7e53d079dd1d8902e54af.camel@kellner.me>
Subject: Re: [PATCH] pidfd: add NSpid entries to fdinfo
From:   Christian Kellner <christian@kellner.me>
To:     Christian Brauner <christian.brauner@ubuntu.com>, jannh@google.com
Cc:     aarcange@redhat.com, akpm@linux-foundation.org, cyphar@cyphar.com,
        elena.reshetova@intel.com, guro@fb.com, ldv@altlinux.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        mhocko@suse.com, mingo@kernel.org, peterz@infradead.org,
        tglx@linutronix.de, viro@zeniv.linux.org.uk
Date:   Mon, 14 Oct 2019 11:43:01 +0200
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On Sat, 2019-10-12 at 12:21 +0200, Christian Brauner wrote:
> I think this might be more what we want.
Yep, indeed.

> 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").

> Christian, this is just a quick stab I took. Feel free to pick this
> up as a template.
Thanks! I slightly re-worked it, with the reasoning above in mind, to
get rid of one of the branches:

+#ifdef CONFIG_PID_NS
+	seq_put_decimal_ull(m, "\nNSpid:\t", nr);
+	if (nr) {
+		int i;
+
+		/* If nr is non-zero it means that 'pid' is valid and that
+		 * ns, i.e. the pid namespace associated with the procfs
+		 * instance, is in the pid namespace hierarchy of pid.
+		 * Start at one level below and print all descending pids.
+		 */
+		for (i = ns->level + 1; i <= pid->level; i++) {
+			ns = pid->numbers[i].ns;
+			seq_put_decimal_ull(m, "\t", pid_nr_ns(pid, ns));
+		}
+	}
+#endif

But I now just realized that with the very same reasoning, if nr is
non-zero, we don't need to redo all the checks and can just do:

for (i = ns->level + 1; i <= pid->level; i++)
	seq_put_decimal_ull(m, "\t", pid->numbers[i].nr);

If this sounds good to you I resend the patches with the change above.

Thanks,
Christian