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Date: Fri, 27 Feb 2026 11:28:44 +0100
From: Christian Brauner <brauner@kernel.org>
To: Song Liu <song@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko <andrii@kernel.org>, 
	Martin KaFai Lau <martin.lau@linux.dev>, Tejun Heo <tj@kernel.org>, KP Singh <kpsingh@kernel.org>, 
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, 
	Lennart Poettering <lennart@poettering.net>
Subject: Re: [PATCH 1/4] ns: add bpf hooks
Message-ID: <20260227-nullnummer-eisdiele-08db4c8fe99e@brauner>
References: <20260220-work-bpf-namespace-v1-0-866207db7b83@kernel.org>
 <20260220-work-bpf-namespace-v1-1-866207db7b83@kernel.org>
 <CAPhsuW63sEvK50ELaxo4LxjCS-2RdfxDzuMYhW59PDUHfF0-iQ@mail.gmail.com>
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In-Reply-To: <CAPhsuW63sEvK50ELaxo4LxjCS-2RdfxDzuMYhW59PDUHfF0-iQ@mail.gmail.com>

On Tue, Feb 24, 2026 at 03:04:43PM -0800, Song Liu wrote:
> On Thu, Feb 19, 2026 at 4:38 PM Christian Brauner <brauner@kernel.org> wrote:
> [...]
> > @@ -1,6 +1,7 @@
> >  // SPDX-License-Identifier: GPL-2.0-only
> >  /* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
> >
> > +#include <linux/bpf_lsm.h>
> >  #include <linux/ns_common.h>
> >  #include <linux/nstree.h>
> >  #include <linux/proc_ns.h>
> > @@ -77,6 +78,7 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
> >                 ret = proc_alloc_inum(&ns->inum);
> >         if (ret)
> >                 return ret;
> > +
> >         /*
> >          * Tree ref starts at 0. It's incremented when namespace enters
> >          * active use (installed in nsproxy) and decremented when all
> > @@ -86,11 +88,16 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
> >                 atomic_set(&ns->__ns_ref_active, 1);
> >         else
> >                 atomic_set(&ns->__ns_ref_active, 0);
> > -       return 0;
> > +
> > +       ret = bpf_lsm_namespace_alloc(ns);
> > +       if (ret && !inum)
> > +               proc_free_inum(ns->inum);
> > +       return ret;
> >  }
> 
> If we change the hook as
> 
>    bpf_lsm_namespace_alloc(ns, inum);
> 
> We can move it to the beginning of __ns_common_init().
> This change allows blocking __ns_common_init() before
> it makes any changes to the ns. Is this a better approach?

I don't think it matters tbh. We have no control when exactly
__ns_common_init() is called. That's up to the containing namespace. We
can't rely on the namespace to have been correctly set up at this time.
My main goal was to have struct ns_common to be fully initialized
already so that direct access to it's field already makes sense.

The containing namespace my already have to rollback a bunch of stuff
anyway.