linux-kernel - Re: [PATCH v5 1/4] rust: debugfs: Bind DebugFS directory creation

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Message-ID: <2025051451-charm-lankiness-8d59@gregkh>
Date: Wed, 14 May 2025 10:49:06 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Benno Lossin <lossin@...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>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Timur Tabi <ttabi@...dia.com>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v5 1/4] rust: debugfs: Bind DebugFS directory creation

On Wed, May 14, 2025 at 09:33:05AM +0200, Benno Lossin wrote:
> On Tue May 6, 2025 at 1:51 AM CEST, Matthew Maurer wrote:
> > +impl<'a> Dir<'a> {
> > +    /// Create a new directory in DebugFS. If `parent` is [`None`], it will be created at the root.
> > +    #[cfg(CONFIG_DEBUG_FS)]
> > +    fn create<'b>(name: &CStr, parent: Option<&'a Dir<'b>>) -> Self {
> > +        let parent_ptr = match parent {
> > +            Some(parent) => parent.0.as_ptr(),
> > +            None => core::ptr::null_mut(),
> > +        };
> > +        // SAFETY:
> > +        // * `name` argument points to a NUL-terminated string that lives across the call, by
> > +        //   invariants of `&CStr`.
> > +        // * If `parent` is `None`, `parent` accepts null pointers to mean create at root.
> > +        // * If `parent` is `Some`, `parent` accepts live dentry debugfs pointers.
> > +        //   so we can call `Self::from_ptr`.
> > +        let dir = unsafe { bindings::debugfs_create_dir(name.as_char_ptr(), parent_ptr) };
> > +
> > +        // SAFETY: `debugfs_create_dir` either returns an error code or a legal `dentry` pointer,
> > +        Self(unsafe { Entry::from_ptr(dir) })
> > +    }
> > +
> > +    #[cfg(not(CONFIG_DEBUG_FS))]
> > +    fn create<'b>(_name: &CStr, _parent: Option<&'a Dir<'b>>) -> Self {
> > +        Self(Entry::new())
> > +    }
> > +
> > +    /// Create a DebugFS subdirectory.
> 
> I'm not familiar with debugfs, if I run `Dir::create(c"foo", None)`
> twice, will both of the returned values refer to the same or different
> directories? 

You can not create a directory, or file, in the same location with the
same name.  The call will fail, so don't do that :)

> What if I give a parent?

Same thing, it will fail.

> If the answer in both cases is that they will refer to the same
> directory, then I'd change the docs to mention that.

Nope, that does not happen.

> So instead of
> "Creates" we could say "Finds or creates" or something better.

Find does not happen.

> If they refer to different files, then I am confused how that would look
> like in user-land :)

Agreed, which is why that does not happen :)

thanks,

greg k-h