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Message-ID: <20250430-debugfs-rust-v2-2-2e8d3985812b@google.com>
Date: Wed, 30 Apr 2025 23:31:57 +0000
From: Matthew Maurer <mmaurer@...gle.com>
To: 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>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
Sami Tolvanen <samitolvanen@...gle.com>, Timur Tabi <ttabi@...dia.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
Matthew Maurer <mmaurer@...gle.com>
Subject: [PATCH v2 2/4] rust: debugfs: Bind file creation for long-lived Display
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()
+ }
+ }
}
impl Drop for Dir {
@@ -133,3 +175,63 @@ fn drop(&mut self) {
}
}
}
+
+#[cfg(CONFIG_DEBUG_FS)]
+mod helpers {
+ use crate::seq_file::SeqFile;
+ use crate::seq_print;
+ use core::fmt::Display;
+
+ /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`
+ ///
+ /// # Safety
+ /// * inode's private pointer must point to a value of type T which will outlive the inode and
+ /// will not be mutated during this call
+ /// * file must point to a live, not-yet-initialized file object
+ pub(crate) unsafe extern "C" fn display_open<T: Display>(
+ inode: *mut bindings::inode,
+ file: *mut bindings::file,
+ ) -> i32 {
+ // SAFETY:
+ // * file is acceptable by caller precondition
+ // * print_act will be called on a seq_file with private data set to the third argument,
+ // so we meet its safety requirements
+ // * The data pointer passed in the third argument is a valid T pointer that outlives this
+ // call by caller preconditions
+ unsafe { bindings::single_open(file, Some(display_act::<T>), (*inode).i_private) }
+ }
+
+ /// Prints private data stashed in a seq_file to that seq file
+ /// # Safety
+ /// `seq` must point to a live `seq_file` whose private data is a live pointer to a `T` which is
+ /// not being mutated.
+ pub(crate) unsafe extern "C" fn display_act<T: Display>(
+ seq: *mut bindings::seq_file,
+ _: *mut core::ffi::c_void,
+ ) -> i32 {
+ // SAFETY: By caller precondition, this pointer is live, points to a value of type T, and is
+ // not being mutated.
+ let data = unsafe { &*((*seq).private as *mut T) };
+ // SAFETY: By caller precondition, seq_file points to a live seq_file, so we can lift it
+ let seq_file = unsafe { SeqFile::from_raw(seq) };
+ seq_print!(seq_file, "{}", data);
+ 0
+ }
+
+ // Work around lack of generic const items
+ pub(crate) trait DisplayFile: Display + Sized {
+ const VTABLE: bindings::file_operations = bindings::file_operations {
+ read: Some(bindings::seq_read),
+ llseek: Some(bindings::seq_lseek),
+ release: Some(bindings::single_release),
+ open: Some(display_open::<Self> as _),
+ // SAFETY: file_operations supports zeroes in all fields
+ ..unsafe { core::mem::zeroed() }
+ };
+ }
+
+ impl<T: Display + Sized> DisplayFile for T {}
+}
+
+#[cfg(CONFIG_DEBUG_FS)]
+use helpers::*;
--
2.49.0.906.g1f30a19c02-goog
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