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Wysocki" , Sami Tolvanen , Timur Tabi , Benno Lossin Cc: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, Matthew Maurer Content-Type: text/plain; charset="utf-8" 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. Signed-off-by: Matthew Maurer --- rust/kernel/debugfs.rs | 62 +++++++++++++++++++++++++++++++++++++ rust/kernel/debugfs/display_file.rs | 60 +++++++++++++++++++++++++++++++++++ rust/kernel/debugfs/entry.rs | 8 +++++ 3 files changed, 130 insertions(+) diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs index e390278b8b7384f1d9e2b3d7a4f5f365f6447d0f..6a89557d8cf49327d2984d15741ffb6640defd70 100644 --- a/rust/kernel/debugfs.rs +++ b/rust/kernel/debugfs.rs @@ -10,7 +10,10 @@ use crate::str::CStr; #[cfg(CONFIG_DEBUG_FS)] use crate::sync::Arc; +use core::fmt::Display; +#[cfg(CONFIG_DEBUG_FS)] +mod display_file; #[cfg(CONFIG_DEBUG_FS)] mod entry; @@ -57,6 +60,43 @@ fn create(_name: &CStr, _parent: Option<&Dir>) -> Self { Self() } + #[cfg(CONFIG_DEBUG_FS)] + fn create_file(&self, name: &CStr, data: &'static T) -> File { + let Some(parent) = &self.0 else { + return File { + _entry: entry::Entry::empty(), + }; + }; + // 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. + let ptr = unsafe { + bindings::debugfs_create_file_full( + name.as_char_ptr(), + 0o444, + parent.as_ptr(), + data as *const _ as *mut _, + core::ptr::null(), + &::VTABLE, + ) + }; + + // SAFETY: `debugfs_create_file_full` either returns an error code or a legal + // dentry pointer, so `Entry::new` is safe to call here. + let entry = unsafe { entry::Entry::new(ptr, Some(parent.clone())) }; + + File { _entry: entry } + } + + #[cfg(not(CONFIG_DEBUG_FS))] + fn create_file(&self, _name: &CStr, _data: &'static T) -> File { + File {} + } + /// Create a DebugFS subdirectory. /// /// Subdirectory handles cannot outlive the directory handle they were created from. @@ -73,6 +113,22 @@ pub fn subdir(&self, name: &CStr) -> Self { Dir::create(name, Some(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); + /// // "my_debugfs_dir/foo" now contains the number 200. + /// ``` + pub fn display_file(&self, name: &CStr, data: &'static T) -> File { + self.create_file(name, data) + } + /// Create a new directory in DebugFS at the root. /// /// # Examples @@ -86,3 +142,9 @@ pub fn new(name: &CStr) -> Self { Dir::create(name, None) } } + +/// Handle to a DebugFS file. +pub struct File { + #[cfg(CONFIG_DEBUG_FS)] + _entry: entry::Entry, +} diff --git a/rust/kernel/debugfs/display_file.rs b/rust/kernel/debugfs/display_file.rs new file mode 100644 index 0000000000000000000000000000000000000000..65e37e34d7b587482492dc296637a3bc517b9fe3 --- /dev/null +++ b/rust/kernel/debugfs/display_file.rs @@ -0,0 +1,60 @@ +// SPDX-License-Identifier: GPL-2.0 +// Copyright (C) 2025 Google LLC. + +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( + 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::), (*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( + 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:: as _), + // SAFETY: `file_operations` supports zeroes in all fields. + ..unsafe { core::mem::zeroed() } + }; +} + +impl DisplayFile for T {} diff --git a/rust/kernel/debugfs/entry.rs b/rust/kernel/debugfs/entry.rs index ae0e2c4e1d58e878ebb081a71e4ac0f4a7d99b91..2baaf31c326c3071b92b5bc37552907fa1102246 100644 --- a/rust/kernel/debugfs/entry.rs +++ b/rust/kernel/debugfs/entry.rs @@ -38,6 +38,14 @@ pub(crate) unsafe fn new(entry: *mut bindings::dentry, parent: Option } } + /// Constructs a placeholder DebugFS [`Entry`]. + pub(crate) fn empty() -> Self { + Self { + entry: core::ptr::null_mut(), + _parent: None, + } + } + /// Returns the pointer representation of the DebugFS directory. /// /// # Guarantees -- 2.50.0.rc2.696.g1fc2a0284f-goog