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

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Message-Id: <D9VS7Q1LVMU0.14MJI8ACKVXLU@kernel.org>
Date: Wed, 14 May 2025 11:38:50 +0200
From: "Benno Lossin" <lossin@...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>, "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 10:49 AM CEST, Greg Kroah-Hartman wrote:
> 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 :)

Ah that makes sense, thanks for explaining :)

---
Cheers,
Benno