[PATCH v2 3/4] rust: debugfs: Support format hooks

[Matthew Maurer <mmaurer@google.com>]

Rather than always using Display, allow hooking arbitrary functions to
arbitrary files. Display technically has the expressiveness to do this,
but requires a new type be declared for every different way to render
things, which can be very clumsy.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 rust/kernel/debugfs.rs | 110 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 109 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 87de94da3b27c2a399bb377afd47280f65208d41..2935c7ffbfaf460fff5b5f1ffc768f803c2da345 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;
 use core::fmt::Display;
 
 /// Handle to a DebugFS directory.
@@ -136,6 +137,47 @@ pub fn keep(self) {
     /// // "my_debugfs_dir/foo" now contains the number 200.
     /// ```
     pub fn display_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) -> Self {
+        // SAFETY: As `data` lives for the static lifetime, it outlives the file.
+        unsafe { self.display_file_raw(name, data) }
+    }
+
+    /// Create a file in a DebugFS directory with the provided name, and contents from invoking `f`
+    /// on the provided reference.
+    ///
+    /// `f` must be a function item or a non-capturing closure, or this will fail to compile.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use core::sync::atomic::{AtomicU32, Ordering};
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let dir = Dir::new(c_str!("foo"));
+    /// static MY_ATOMIC: AtomicU32 = AtomicU32::new(3);
+    /// let file = dir.fmt_file(c_str!("bar"), &MY_ATOMIC, &|val, f| {
+    ///   let out = val.load(Ordering::Relaxed);
+    ///   writeln!(f, "{out:#010x}")
+    /// });
+    /// MY_ATOMIC.store(10, Ordering::Relaxed);
+    /// ```
+    pub fn fmt_file<T, F: Fn(&T, &mut fmt::Formatter<'_>) -> fmt::Result>(
+        &self,
+        name: &CStr,
+        data: &'static T,
+        f: &'static F,
+    ) -> Self {
+        // SAFETY: As `data` lives for the static lifetime, it outlives the file
+        unsafe { self.fmt_file_raw(name, data, f) }
+    }
+
+    /// Creates a DebugFS file backed by the display implementation of the provided pointer.
+    ///
+    /// # Safety
+    /// The pointee of `data` must outlive the accessibility of the `Dir` returned by this function.
+    /// This means that before `data` may become invalid, either:
+    /// * The refcount must go to zero
+    /// * The file must be rendered inaccessible, e.g. via `debugfs_remove`
+    unsafe fn display_file_raw<T: Display + Sized>(&self, name: &CStr, data: *const 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
@@ -163,6 +205,32 @@ pub fn display_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) ->
             Self()
         }
     }
+
+    /// Create a file in a DebugFS directory with the provided name, and contents from invoking the
+    /// fomatter on the attached data.
+    ///
+    /// The attached function must be a ZST, and will cause a compilation error if it is not.
+    ///
+    /// # Safety
+    ///
+    /// `data` must outlive the resulting file's accessibility
+    unsafe fn fmt_file_raw<T, F: Fn(&T, &mut fmt::Formatter<'_>) -> fmt::Result>(
+        &self,
+        name: &CStr,
+        data: &T,
+        f: &'static F,
+    ) -> Self {
+        #[cfg(CONFIG_DEBUG_FS)]
+        let data_adapted = FormatAdapter::new(data, f);
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        let data_adapted = {
+            // Mark used
+            let (_, _) = (data, f);
+            &0
+        };
+        // SAFETY: data outlives the file's accessibility, so data_adapted does too
+        unsafe { self.display_file_raw(name, data_adapted) }
+    }
 }
 
 impl Drop for Dir {
@@ -180,7 +248,9 @@ fn drop(&mut self) {
 mod helpers {
     use crate::seq_file::SeqFile;
     use crate::seq_print;
-    use core::fmt::Display;
+    use core::fmt;
+    use core::fmt::{Display, Formatter};
+    use core::marker::PhantomData;
 
     /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`
     ///
@@ -231,6 +301,44 @@ pub(crate) trait DisplayFile: Display + Sized {
     }
 
     impl<T: Display + Sized> DisplayFile for T {}
+
+    // INVARIANT: F is inhabited
+    #[repr(transparent)]
+    pub(crate) struct FormatAdapter<T, F> {
+        inner: T,
+        _formatter: PhantomData<F>,
+    }
+
+    impl<T, F> FormatAdapter<T, F> {
+        pub(crate) fn new<'a>(inner: &'a T, _f: &'static F) -> &'a Self {
+            // SAFETY: FormatAdapater is a repr(transparent) wrapper around T, so
+            // casting a reference is legal
+            // INVARIANT: We were passed a reference to F, so it is inhabited.
+            unsafe { core::mem::transmute(inner) }
+        }
+    }
+
+    impl<T, F> Display for FormatAdapter<T, F>
+    where
+        F: Fn(&T, &mut Formatter<'_>) -> fmt::Result + 'static,
+    {
+        fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+            // SAFETY: FormatAdapter<_, F> can only be constructed if F is inhabited
+            let f: &F = unsafe { materialize_zst_fmt() };
+            f(&self.inner, fmt)
+        }
+    }
+
+    /// # Safety
+    /// The caller asserts that F is inhabited
+    unsafe fn materialize_zst_fmt<F>() -> &'static F {
+        const { assert!(core::mem::size_of::<F>() == 0) };
+        let zst_dangle: core::ptr::NonNull<F> = core::ptr::NonNull::dangling();
+        // SAFETY: While the pointer is dangling, it is a dangling pointer to a ZST, based on the
+        // assertion above. The type is also inhabited, by the caller's assertion. This means
+        // we can materialize it.
+        unsafe { zst_dangle.as_ref() }
+    }
 }
 
 #[cfg(CONFIG_DEBUG_FS)]

-- 
2.49.0.906.g1f30a19c02-goog