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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 21 Mar 2023 19:23:21 -0700
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Subject: Re: [PATCH 0/5] usermode_driver: Add management library and API
To:     Roberto Sassu <roberto.sassu@huaweicloud.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
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        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
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        Eric Dumazet <edumazet@google.com>,
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        Roberto Sassu <roberto.sassu@huawei.com>
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On Fri, Mar 17, 2023 at 7:53=E2=80=AFAM Roberto Sassu
<roberto.sassu@huaweicloud.com> wrote:
>
> From: Roberto Sassu <roberto.sassu@huawei.com>
>
> A User Mode Driver (UMD) is a specialization of a User Mode Helper (UMH),
> which runs a user space process from a binary blob, and creates a
> bidirectional pipe, so that the kernel can make a request to that process=
,
> and the latter provides its response. It is currently used by bpfilter,
> although it does not seem to do any useful work.

FYI the new home for bpfilter is here:
https://github.com/facebook/bpfilter

> The problem is, if other users would like to implement a UMD similar to
> bpfilter, they would have to duplicate the code. Instead, make an UMD
> management library and API from the existing bpfilter and sockopt code,
> and move it to common kernel code.
>
> Also, define the software architecture and the main components of the
> library: the UMD Manager, running in the kernel, acting as the frontend
> interface to any user or kernel-originated request; the UMD Loader, also
> running in the kernel, responsible to load the UMD Handler; the UMD
> Handler, running in user space, responsible to handle requests from the U=
MD
> Manager and to send to it the response.

That doesn't look like a generic interface for UMD.
It was a quick hack to get bpfilter off the ground, but certainly
not a generic one.

> I have two use cases, but for sake of brevity I will propose one.
>
> I would like to add support for PGP keys and signatures in the kernel, so
> that I can extend secure boot to applications, and allow/deny code
> execution based on the signed file digests included in RPM headers.
>
> While I proposed a patch set a while ago (based on a previous work of Dav=
id
> Howells), the main objection was that the PGP packet parser should not ru=
n
> in the kernel.
>
> That makes a perfect example for using a UMD. If the PGP parser is moved =
to
> user space (UMD Handler), and the kernel (UMD Manager) just instantiates
> the key and verifies the signature on already parsed data, this would
> address the concern.

I don't think PGP parser belongs to UMD either.
Please do it as a normal user space process and define a proper
protocol for communication between kernel and user space.