linux-kernel - Re: [PATCH] rust: add basic ELF sections parser

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Message-Id: <DAACKTM8B9A1.3SI9LRGNMSBH3@nvidia.com>
Date: Sat, 31 May 2025 21:33:38 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Greg KH" <gregkh@...uxfoundation.org>, "Timur Tabi" <timur@...nel.org>
Cc: "Danilo Krummrich" <dakr@...nel.org>, "John Hubbard"
 <jhubbard@...dia.com>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH] rust: add basic ELF sections parser

Hi Greg,

On Sat May 31, 2025 at 2:45 PM JST, Greg KH wrote:
> On Fri, May 30, 2025 at 01:10:50PM -0500, Timur Tabi wrote:
>> On Fri, May 30, 2025 at 10:42 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>> >
>> > On Fri, May 30, 2025 at 11:34:02PM +0900, Alexandre Courbot wrote:
>> > > So to try to answer your question, I am not disagreeing that userspace
>> > > is capable of doing what we currently do in the kernel. My follow-up
>> > > questions to that are: how do we command userspace to do that work for
>> > > us when we request the firmware, how do we provide the result to the
>> > > kernel, and is this something that distros can adopt easily? I'm happy
>> > > to consider doing things this way, but would need a few pointers to look
>> > > into.
>> >
>> > Again, look at how your firmware for your devices in your laptop are
>> > loaded today.
>
> Note, I am talking about non-gpu firmare images here (wifi, usb
> controllers, etc.) that are using the firmware download subsystem for
> ages as examples of what to look at as to how to trigger a firmware
> image to be loaded by userspace into the device.

I would really appreciate it if you could point me precisely to one
example (a link, a function, a file) of what you are describing because
I'm starting to wonder whether we are talking about the same thing.

Previously I mentioned udev and CONFIG_FW_LOADER_USER_HELPER, but you
haven't confirmed whether that was what you had in mind or not. Assuming
that udev is involved, I tried to snoop events while a
`request_firwmare` call is performed using `udevadm monitor`, but that
revealed no event related to firmware loading. Then looking deeper into
the kernel documentation confirmed that the kernel does indeed a direct
filesystem lookup in request_firmware [1]. IOW, the kernel looks for the
requested file, and if it cannot find it it's game over. This matches my
observations with udevadm, as I tried requesting a non-existing file and
no uevent was generated. I don't see what user-space can do here.

I also tried to look up this "firmware download subsystem" you
mentioned, but couldn't find anything under that name - I suspect you
are talking about the sysfs loading mechanism, but AFAIU this depends on 
CONFIG_FW_LOADER_USER_HELPER which doesn't seem to be widely enabled
(not on my distro at least).

What I am trying to say is that I am more than willing to look into
having this work done by user-space, but despite this conversation and
hours of research and testing I still don't have even a thread to
unravel. If you know the answer for how to do this, please share it.

[1] https://docs.kernel.org/driver-api/firmware/direct-fs-lookup.html