linux-kernel - Re: [RFC PATCH 0/4] namespacefs: Proof-of-Concept

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Message-ID: <63f54c213253b80fcf3f8653766d5c6f5761034a.camel@HansenPartnership.com>
Date:   Mon, 22 Nov 2021 10:47:00 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Yordan Karadzhov <y.karadz@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        viro@...iv.linux.org.uk, mingo@...hat.com, hagen@...u.net,
        rppt@...nel.org, akpm@...ux-foundation.org, vvs@...tuozzo.com,
        shakeelb@...gle.com, christian.brauner@...ntu.com,
        mkoutny@...e.com, Linux Containers <containers@...ts.linux.dev>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [RFC PATCH 0/4] namespacefs: Proof-of-Concept

On Mon, 2021-11-22 at 17:00 +0200, Yordan Karadzhov wrote:
> 
> On 22.11.21 г. 15:44 ч., James Bottomley wrote:
> > Well, no, the information may not all exist.  However, the point is
> > we can add it without adding additional namespace objects.
> > 
> > > Let's look the following case (oversimplified just to get the
> > > idea):
> > > 1. The process X is a parent of the process Y and both are in
> > > namespace 'A'.
> > > 3. "unshare" is used to place process Y (and all its child
> > > processes) in a new namespace B (A is a parent namespace of B).
> > > 4. "setns" is s used to move process X in namespace C.
> > > 
> > > How would you find the parent namespace of B?
> > Actually this one's quite easy: the parent of X in your setup still
> > has it.
> 
> Hmm, Isn't that true only if somehow we know that (3) happened before
> (4).

This depends.  There are only two parented namespaces: pid and user. 
You said you were only interested in pid for now.  setns on the process
only affects pid_for_children because you have to fork to enter the pid
namespace, so in your scenario X has a new ns/pid_for_children but its
own ns/pid never changed.  It's the ns/pid not the ns/pid_for_children
which is the parent.  This makes me suspect that the specific thing
you're trying to do: trace the pid parentage, can actually be done with
the information we have now.

If you do this with the user_ns, then you have a problem because it's
not fork on entry.  But, as I listed in the examples, there are a load
of other problems with tracing the user_ns tree.

James