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Message-ID: <aBRoNKjB063QhGZo@pollux>
Date: Fri, 2 May 2025 08:37:40 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Matthew Maurer <mmaurer@...gle.com>
Cc: 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>,
Greg Kroah-Hartman <gregkh@...uxfoundation.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 v3 1/4] rust: debugfs: Bind DebugFS directory creation
On Thu, May 01, 2025 at 10:47:41PM +0000, Matthew Maurer wrote:
> +/// Owning handle to a DebugFS directory.
> +///
> +/// This directory will be cleaned up when it goes out of scope.
> +///
> +/// # Invariants
> +///
> +/// The wrapped pointer will always be `NULL`, an error, or an owned DebugFS `dentry`.
> +#[repr(transparent)]
> +pub struct Dir(#[cfg(CONFIG_DEBUG_FS)] *mut bindings::dentry);
Should probably use Opaque instead of a raw pointer.
> +// SAFETY: Dir is just a `dentry` under the hood, which the API promises can be transferred
[`Dir`]
> +// between threads.
> +unsafe impl Send for Dir {}
> +
> +// SAFETY: All the native functions we re-export use interior locking, and the contents of the
> +// struct are opaque to Rust.
> +unsafe impl Sync for Dir {}
> +
> +impl Dir {
> + /// Create a new directory in DebugFS at the root.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// # use kernel::c_str;
> + /// # use kernel::debugfs::Dir;
> + /// {
> + /// let parent = Dir::new(c_str!("parent"));
> + /// // The path "parent" exists in DebugFS here.
> + /// }
> + /// // It does not exist here.
This ready like an explanation for scopes; I think we should drop those comments
and the scope.
> + /// ```
> + pub fn new(name: &CStr) -> Self {
> + Self::create(name, None)
> + }
> +
> + /// Create a DebugFS subdirectory.
> + ///
> + /// This returns a [`SubDir`], which will not be automatically cleaned up when it leaves scope.
> + /// To convert this to a handle governing the lifetime of the directory, use [`Dir::from`].
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// # use kernel::c_str;
> + /// # use kernel::debugfs::Dir;
> + /// {
> + /// let parent = Dir::new(c_str!("parent"));
> + /// // The path "parent" exists in DebugFS here.
> + /// {
> + /// let child = parent.subdir(c_str!("child"));
> + /// // The path "parent/child" exists in DebugFS here.
> + /// }
> + /// // The path "parent/child" still exists.
> + /// {
> + /// let child2 = Dir::from(parent.subdir(c_str!("child2")));
> + /// // The path "parent/child2" exists in DebugFS here.
> + /// }
> + /// // The path "parent/child2" is gone.
> + /// }
> + /// // None of the paths exist here.
I think the fact that you need all those comments here proves that it's not
really intuitive. Please see me comment on SubDir below.
> + /// ```
> + pub fn subdir(&self, name: &CStr) -> SubDir {
> + SubDir::new(Self::create(name, Some(self)))
> + }
> +
> + /// Create a new directory in DebugFS. If `parent` is [`None`], it will be created at the root.
> + #[cfg(CONFIG_DEBUG_FS)]
> + fn create(name: &CStr, parent: Option<&Self>) -> Self {
> + let parent_ptr = match parent {
> + Some(parent) => parent.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.
> + // * `debugfs_create_dir` either returns an error code or a legal `dentry` pointer,
> + // so we can call `Self::from_ptr`.
> + unsafe { Self::from_ptr(bindings::debugfs_create_dir(name.as_char_ptr(), parent_ptr)) }
Please split up in two calls, such that we don't have two unsafe function calls
in a single unsafe block.
> + }
> +
> + #[cfg(not(CONFIG_DEBUG_FS))]
> + fn create(_name: &CStr, _parent: Option<&Self>) -> Self {
> + Self()
> + }
> +
> + /// Constructs a new DebugFS [`Dir`] from the underlying pointer.
> + ///
> + /// # Safety
> + ///
> + /// The pointer must either be an error code, `NULL`, or represent a transfer of ownership of a
> + /// live DebugFS directory.
> + #[cfg(CONFIG_DEBUG_FS)]
> + unsafe fn from_ptr(ptr: *mut bindings::dentry) -> Self {
> + Self(ptr)
> + }
> +
> + /// Returns the pointer representation of the DebugFS directory.
> + ///
> + /// Due to the type invariant, the value returned from this function will always be an error
> + /// code, NUL, or a live DebugFS directory.
Maybe put this in a '# Guarantees' section.
> + // If this function is ever needed with `not(CONFIG_DEBUG_FS)`, hardcode it to return `ENODEV`.
I think you mean ERR_PTR(ENODEV).
> + #[cfg(CONFIG_DEBUG_FS)]
> + fn as_ptr(&self) -> *mut bindings::dentry {
> + self.0
> + }
> +}
> +
> +impl Drop for Dir {
> + fn drop(&mut self) {
> + // SAFETY: `debugfs_remove` can take `NULL`, error values, and legal DebugFS dentries.
> + // `as_ptr` guarantees that the pointer is of this form.
> + #[cfg(CONFIG_DEBUG_FS)]
> + unsafe {
> + bindings::debugfs_remove(self.as_ptr())
> + }
> + }
> +}
> +
> +/// Handle to a DebugFS directory that will stay alive after leaving scope.
> +#[repr(transparent)]
> +pub struct SubDir(ManuallyDrop<Dir>);
I think it's not very intuitive if the default is that a SubDir still exists
after it has been dropped. I think your first approach being explicit about this
with keep() consuming the SubDir was much better; please keep this approach.
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