linux-kernel - Re: [PATCH v11 2/7] rust: debugfs: Add support for read-only files

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Message-Id: <DCND4SAEHX7T.3I2S7MDMUNSNL@kernel.org>
Date: Mon, 08 Sep 2025 12:56:19 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc: "Matthew Maurer" <mmaurer@...gle.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>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Rafael J. Wysocki"
 <rafael@...nel.org>, "Sami Tolvanen" <samitolvanen@...gle.com>, "Timur
 Tabi" <ttabi@...dia.com>, "Benno Lossin" <lossin@...nel.org>, "Dirk Beheme"
 <dirk.behme@...bosch.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v11 2/7] rust: debugfs: Add support for read-only files

On Mon Sep 8, 2025 at 12:54 PM CEST, Danilo Krummrich wrote:
> On Mon Sep 8, 2025 at 12:17 PM CEST, Greg Kroah-Hartman wrote:
>> I tried using this in a "tiny" test module I had written, and I get the
>> following build error:
>>
>>    --> samples/rust/rust_debugfs2.rs:64:53
>>     |
>> 64  |         _file = root.read_only_file(c_str!("name"), &hw_soc_info.name);
>>     |                      --------------                 ^^^^^^^^^^^^^^^^^ expected `&u32`, found `&&CStr`
>>     |                      |
>>     |                      arguments to this method are incorrect
>>     |
>>     = note: expected reference `&u32`
>>                found reference `&&'static kernel::prelude::CStr`
>>
>> I'm trying to "just" print a CStr, which is defined as:
>>
>> struct HwSocInfo {
>>     id: u32,
>>     ver: u32,
>>     raw_id: u32,
>>     foundry: u32,
>>     name: &'static CStr,
>> }
>>
>> Is this just a "user is holding it wrong" error on my side, or can this api not
>> handle CStr values?
>
> What you're doing should fundamentally work.
>
> The above error suggests that your declaration of `_file` is File<&u32> rather
> than File<&'static CStr>.
>
> Also note the double reference you create with `&hw_soc_info.name`, this should
> just be `hw_soc_info.name`.
>
> You can also test this case by applying the following diff the the sample in v5:

*sample in patch 5

>
> diff --git a/samples/rust/rust_debugfs.rs b/samples/rust/rust_debugfs.rs
> index b26eea3ee723..475502f30b1a 100644
> --- a/samples/rust/rust_debugfs.rs
> +++ b/samples/rust/rust_debugfs.rs
> @@ -59,6 +59,8 @@ struct RustDebugFs {
>      #[pin]
>      _compatible: File<CString>,
>      #[pin]
> +    _test: File<&'static CStr>,
> +    #[pin]
>      counter: File<AtomicUsize>,
>      #[pin]
>      inner: File<Mutex<Inner>>,
> @@ -140,6 +142,7 @@ fn new(pdev: &platform::Device<Core>) -> impl PinInit<Self, Error> + '_ {
>                          .property_read::<CString>(c_str!("compatible"))
>                          .required_by(dev)?,
>                  ),
> +                _test <- debugfs.read_only_file(c_str!("test"), c_str!("some_value")),
>                  counter <- Self::build_counter(&debugfs),
>                  inner <- Self::build_inner(&debugfs),
>                  _debugfs: debugfs,