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Message-ID: <aBNO1rMcAwo-TNWQ@pollux>
Date: Thu, 1 May 2025 12:37:10 +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 v2 2/4] rust: debugfs: Bind file creation for long-lived
Display
On Wed, Apr 30, 2025 at 11:31:57PM +0000, Matthew Maurer wrote:
> Allows creation of files for references that live forever and lack
> metadata through the `Display` implementation.
>
> The reference must live forever because we do not have a maximum
> lifetime for the file we are creating.
>
> The `Display` implementation is used because `seq_printf` needs to route
> through `%pA`, which in turn routes through Arguments. A more generic
> API is provided later in the series, implemented in terms of this one.
>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> ---
> rust/kernel/debugfs.rs | 102 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 102 insertions(+)
>
> diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
> index b533ab21aaa775d4e3f33caf89e2d67ef85592f8..87de94da3b27c2a399bb377afd47280f65208d41 100644
> --- a/rust/kernel/debugfs.rs
> +++ b/rust/kernel/debugfs.rs
> @@ -7,6 +7,7 @@
> //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
>
> use crate::str::CStr;
> +use core::fmt::Display;
>
> /// Handle to a DebugFS directory.
> // INVARIANT: The wrapped pointer will always be NULL, an error, or an owned DebugFS `dentry`
> @@ -121,6 +122,47 @@ fn as_ptr(&self) -> *mut bindings::dentry {
> pub fn keep(self) {
> core::mem::forget(self)
> }
> +
> + /// Create a file in a DebugFS directory with the provided name, and contents from invoking
> + /// [`Display::fmt`] on the provided reference.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// # use kernel::c_str;
> + /// # use kernel::debugfs::Dir;
> + /// let dir = Dir::new(c_str!("my_debugfs_dir"));
> + /// dir.display_file(c_str!("foo"), &200).keep();
> + /// // "my_debugfs_dir/foo" now contains the number 200.
> + /// ```
> + pub fn display_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) -> Self {
> + // SAFETY:
> + // * `name` is a NUL-terminated C string, living across the call, by CStr invariant
> + // * `parent` is a live dentry since we have a reference to it
> + // * `vtable` is all stock `seq_file` implementations except for `open`.
> + // `open`'s only requirement beyond what is provided to all open functions is that the
> + // inode's data pointer must point to a `T` that will outlive it, which we know because
> + // we have a static reference.
> + // * debugfs_create_file_full either returns an error code or a legal dentry pointer, so
> + // `Self::from_ptr` is safe to call here.
> + #[cfg(CONFIG_DEBUG_FS)]
> + unsafe {
> + Self::from_ptr(bindings::debugfs_create_file_full(
> + name.as_char_ptr(),
> + 0o444,
> + self.as_ptr(),
> + data as *const _ as *mut _,
> + core::ptr::null(),
> + &<T as DisplayFile>::VTABLE,
> + ))
> + }
> + #[cfg(not(CONFIG_DEBUG_FS))]
> + {
> + // Mark parameters used
> + let (_, _) = (name, data);
> + Self()
> + }
> + }
Analogous to SubDir, this should be a new type, such that we can't leak the root
directory. Also, methods like subdir() don't really make sense for a file, no?
Besides that, don't we also need a separate type for a file to be able to attach
non-static data anyways? I.e. something like:
#[cfg(CONFIG_DEBUG_FS)]
struct File<T> {
dentry: *mut bindings::dentry,
data: T,
}
#[cfg(not(CONFIG_DEBUG_FS))]
struct File<T> {
_p: PhantomData<T>,
}
I'm not exactly sure how v1 did this; I haven't had time to look at v1 before v2
was posted. I seems like v1 relied on a separate structure storing the data,
which also held a reference to the corresponding dentry or something along those
lines?
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