linux-kernel - Re: [RFC v2 v2 0/1] ns: introduce binfmt

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Message-ID: <a9997ce3-d63d-44b4-6a2a-09fd207a1550@vivier.eu>
Date:   Tue, 2 Oct 2018 18:47:14 +0200
From:   Laurent Vivier <laurent@...ier.eu>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        linux-kernel@...r.kernel.org
Cc:     Andrei Vagin <avagin@...nvz.org>, Dmitry Safonov <dima@...sta.com>,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

Le 02/10/2018 à 18:13, James Bottomley a écrit :
> On Tue, 2018-10-02 at 12:20 +0200, Laurent Vivier wrote:
>> v2: no new namespace, binfmt_misc data are now part of
>>     the mount namespace
>>     I put this in mount namespace instead of user namespace
>>     because the mount namespace is already needed and
>>     I don't want to force to have the user namespace for that.
>>     As this is a filesystem, it seems logic to have it here.
>>
>> This allows to define a new interpreter for each new container.
>>
>> But the main goal is to be able to chroot to a directory
>> using a binfmt_misc interpreter without being root.
> 
> Reading all this, I don't quite understand why this works for me and
> not for you (I think I get from your explanation that it doesn't work
> for you, but I might have missed something):
> 
> jejb@...vis:~> uname -m
> x86_64
> jejb@...vis:~> unshare -r -m
> root@...vis:~# chroot /home/jejb/containers/aarch64
> jarvis:/ # uname -m
> aarch64
> 
> Of course to get that to work I have an 'F' entry in
> /etc/binfmt.d/qemu-aarch64.conf
> 

I'd like to configure the interpreter without being root.

As a simple user can run a VM and a full system inside, I'd like to be
able to start a container/chroot without having to configure something
at the host level.

For instance, I'd like to provide to "someone" (with no admin rights) a
tar file with inside an OS environment for a given target and the
interpreter, and allow him to run the binaries inside just by running a
simple command (like qemu-system-XXX -hda my.img)

It's also interesting for a test purpose: I can test concurrently
different interpreters for the same target without modifying the target
root filesystem (with the 'F' flag but on a per directory basis) or the
host configuration.

Another case is we can't configure qemu-mips/qemu-mipsel (old kernel
API) and qemu-mipsn32/qemu-mipsne32el (new kernel API) interpreters on
the same system because they share the same ELF signature (to be honest
qemu should have only one binary for the old and the new interface and
dynamically change it according to the ELF binary that is loaded, as it
is done for ARM).

But if no one thinks it's useful, I don't want to push this more than
that...

Thanks,
Laurent