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

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Message-Id: <DCNG8UF8XFT2.12S9I7MBNV5PX@kernel.org>
Date: Mon, 08 Sep 2025 15:22:41 +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 2:48 PM CEST, Greg Kroah-Hartman wrote:
> On Mon, Sep 08, 2025 at 12:54:46PM +0200, Danilo Krummrich wrote:
>> 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")),
>
> Cool, but again, we do not want to ever be storing individual debugfs
> files.  Well, we can, but for 90% of the cases, we do not, we only want
> to remove the whole directory when that goes out of scope, which will
> clean up the files then.

This API does not work in the way that you have a struct storing the data you
want to expose *and* another one for the files with the data attached.

The File type contains the actual data. For instance, if you have a struct Foo,
where you want to expose the members through debugfs you would *not* do:

	struct Foo {
	   a: u32,
	   b: u32,
	}

	struct FooFiles {
	   a: File<&u32>,
	   b: File<&u32>
	}

and then create an instance of Foo *and* another instance of FooFiles to export
them via debugfs.

Instead you would change your struct Foo to just be:

	struct Foo {
	   a: File<u32>,
	   b: File<u32>,
	}

If you now create an instance of Foo (let's call it `foo`), then foo.a or foo.b
dereferences to the inner type, i.e. the u32. Or in other words `foo` still
behaves as if `a` and `b` would be u32 values. For instance:

   if foo.a == 42 {
      pr_info!("Foo::b = {}\n", foo.b);
   }

The fact that the backing files of `a` and `b` are removed from debugfs when Foo
is dropped is necessary since otherwise we create a UAF.

Think of File<T> as a containers like you think of KBox<T>.

KBox<T> behaves exactly like T, but silently manages the backing kmalloc()
allocation that T lives in.

With File<T> it's exactly the same, it behaves exactly like the T that lives
within File<T>, but silently manages the debugfs file the T is exposed by.