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Message-ID: <2025090817-attendant-ungodly-78f6@gregkh>
Date: Mon, 8 Sep 2025 15:30:29 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.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 08, 2025 at 03:22:41PM +0200, Danilo Krummrich wrote:
> 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.
Ah, that's exactly what I was trying to do.
> 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);
> }
Oh that's not going to work well at all :(
Think about something "simple" like a pci config descriptor. You have a
structure, with fields, already sitting there. You want to expose those
fields in debugfs. So you want to only create debugfs files in one
location in a driver, you don't want ALL users of those fields to have
to go through a File<T> api, right? That would be crazy, all drivers
would end up always having File<T> everywhere.
> 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.
That's fine, but:
> Think of File<T> as a containers like you think of KBox<T>.
Ok, but again, you are now forcing all users to think of debugfs as the
main "interface" to those variables, which is not true (nor should it
be.)
> 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.
And what happens if debugfs is not enabled? What about if creating the
file fails? The variable still needs to be present and active and
working.
thanks,
greg k-h
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