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Subject: Re: [PATCH v6 1/1] ns: add binfmt_misc to the user namespace
From:   Laurent Vivier <laurent@vivier.eu>
To:     Eric Biederman <ebiederm@xmission.com>
Cc:     Andrei Vagin <avagin@gmail.com>, linux-kernel@vger.kernel.org,
        Jann Horn <jannh@google.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        containers@lists.linux-foundation.org,
        Dmitry Safonov <dima@arista.com>
References: <20181010161430.11633-1-laurent@vivier.eu>
 <20181010161430.11633-2-laurent@vivier.eu> <20181016162245.GA7697@gmail.com>
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Message-ID: <4a1ec0cf-dce7-4193-6946-d46d63398c2d@vivier.eu>
Date:   Tue, 30 Oct 2018 09:51:30 +0100
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Le 24/10/2018 à 19:15, Laurent Vivier a écrit :
> On 16/10/2018 17:22, Andrei Vagin wrote:
>> On Wed, Oct 10, 2018 at 06:14:30PM +0200, Laurent Vivier wrote:
>>> This patch allows to have a different binfmt_misc configuration
>>> for each new user namespace. By default, the binfmt_misc configuration
>>> is the one of the previous level, but if the binfmt_misc filesystem is
>>> mounted in the new namespace a new empty binfmt instance is created and
>>> used in this namespace.
>>>
>>> For instance, using "unshare" we can start a chroot of another
>>> architecture and configure the binfmt_misc interpreter without being root
>>> to run the binaries in this chroot.
>>>
>>> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
>>
>> Acked-by: Andrei Vagin <avagin@gmail.com>
>>
>> Thanks,
>> Andrei
>>
> 
> I don't konw who is the maintainer for this part, but is there any
> chance to have this merged in 4.20?
> 

I'd really want to have this merged.

I have some real use cases for this:

1- to allow a non root user to run a container (with "unshare" for
instance) with its own binfmt_misc configuration. For instance, like we
provide a disk image and ask an ordinary user to run it with his
favorite VM hypervisor, we can provide a tar.gz containing our own
interpreter and just ask him to unshare+chroot to the exploded file tree,

2- to allow to run automatic tests of an interpreter on a machine
without having to change the global configuration of the system. I have
in mind to add some tests in Avocado to automatically test
qemu-linux-user in containers, so the interpreter path can depend on the
build path and possibly run them concurrently,

3- to select an interpreter by container. For instance, on the
qemu-devel mailing list, we have a waiting patch to add the bFLT
interpreter binfmt_misc configuration, but the bFLT doesn't provide the
CPU type in magic/mask. So it would be interesting to be able to select
also a bFLT interpreter by container, as we know the CPU architecture we
have in each chroot/container,

4- another example to select an interpreter by container is qemu-mips
and qemu-misn32 share the same magic/mask because only the kernel API
changes, so we can't configure both on the system (but I agree it's a
QEMU bug: they should be merged and the kernel API be selected at runtime).

Thanks,
Laurent