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

[Alice Ryhl]

On Mon, Sep 8, 2025 at 3:30 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 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.)

All of these things is why I recommended to Matthew that he should add
back the scoped API, since it doesn't have all these drawbacks.

Alice