[PATCH v7 5/6] 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              | 45 ++++++++++++++++++++
 rust/kernel/debugfs/display_file.rs | 85 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 129 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 929e55ee5629f6888edf29997b9ed77d274e11c8..d74b599e8534536b10502e6db8c6f3197f7ab4a5 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -10,7 +10,9 @@
 #[cfg(CONFIG_DEBUG_FS)]
 use crate::sync::Arc;
 use crate::types::ForeignOwnable;
+use core::fmt;
 use core::fmt::Display;
+use core::ops::Deref;
 
 #[cfg(CONFIG_DEBUG_FS)]
 mod display_file;
@@ -157,6 +159,49 @@ pub fn display_file<D: ForeignOwnable + Send + Sync>(&self, name: &CStr, data: D
         self.create_file(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<
+        D: ForeignOwnable + Send + Sync,
+        T,
+        F: Fn(&T, &mut fmt::Formatter<'_>) -> fmt::Result + Send + Sync,
+    >(
+        &self,
+        name: &CStr,
+        data: D,
+        f: &'static F,
+    ) -> File
+    where
+        for<'a> D::Borrowed<'a>: Deref<Target = T>,
+    {
+        #[cfg(CONFIG_DEBUG_FS)]
+        let data_adapted = display_file::FormatAdapter::new(data, f);
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        let data_adapted = {
+            // Mark used
+            let (_, _) = (data, f);
+            &0
+        };
+        self.display_file(name, data_adapted)
+    }
+
     /// Create a new directory in DebugFS at the root.
     ///
     /// # Examples
diff --git a/rust/kernel/debugfs/display_file.rs b/rust/kernel/debugfs/display_file.rs
index 0c2dd756fa866425d1b7771beceaa2fb43bf11e5..b38675a90e1b2e359fb54afd91062b26d1c32ba2 100644
--- a/rust/kernel/debugfs/display_file.rs
+++ b/rust/kernel/debugfs/display_file.rs
@@ -5,7 +5,9 @@
 use crate::seq_file::SeqFile;
 use crate::seq_print;
 use crate::types::ForeignOwnable;
-use core::fmt::Display;
+use core::fmt::{Display, Formatter, Result};
+use core::marker::PhantomData;
+use core::ops::Deref;
 
 /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`.
 ///
@@ -70,3 +72,84 @@ impl<D: ForeignOwnable + Sync> DisplayFile for D
         ..unsafe { core::mem::zeroed() }
     };
 }
+
+/// Adapter to implement `Display` via a callback with the same representation as `T`.
+///
+/// # Invariants
+///
+/// If an instance for `FormatAdapter<_, F>` is constructed, `F` is inhabited.
+#[repr(transparent)]
+pub(crate) struct FormatAdapter<D, F> {
+    inner: D,
+    _formatter: PhantomData<F>,
+}
+
+impl<D, F> FormatAdapter<D, F> {
+    pub(crate) fn new(inner: D, _f: &'static F) -> Self {
+        // INVARIANT: We were passed a reference to F, so it is inhabited.
+        FormatAdapter {
+            inner,
+            _formatter: PhantomData,
+        }
+    }
+}
+
+pub(crate) struct BorrowedAdapter<'a, D: ForeignOwnable, F> {
+    borrowed: D::Borrowed<'a>,
+    _formatter: PhantomData<F>,
+}
+
+// SAFETY: We delegate to D's implementation of `ForeignOwnable`, so `into_foreign` produced aligned
+// pointers.
+unsafe impl<D: ForeignOwnable, F> ForeignOwnable for FormatAdapter<D, F> {
+    type PointedTo = D::PointedTo;
+    type Borrowed<'a> = BorrowedAdapter<'a, D, F>;
+    type BorrowedMut<'a> = Self::Borrowed<'a>;
+    fn into_foreign(self) -> *mut Self::PointedTo {
+        self.inner.into_foreign()
+    }
+    unsafe fn from_foreign(foreign: *mut Self::PointedTo) -> Self {
+        Self {
+            // SAFETY: `into_foreign` is delegated, so a delegated `from_foreign` is safe.
+            inner: unsafe { D::from_foreign(foreign) },
+            _formatter: PhantomData,
+        }
+    }
+    unsafe fn borrow<'a>(foreign: *mut Self::PointedTo) -> Self::Borrowed<'a> {
+        BorrowedAdapter {
+            // SAFETY: `into_foreign` is delegated, so a delegated `borrow` is safe.
+            borrowed: unsafe { D::borrow(foreign) },
+            _formatter: PhantomData,
+        }
+    }
+    unsafe fn borrow_mut<'a>(foreign: *mut Self::PointedTo) -> Self::BorrowedMut<'a> {
+        // SAFETY: `borrow_mut` has stricter requirements than `borrow`
+        unsafe { Self::borrow(foreign) }
+    }
+}
+
+impl<'a, D: ForeignOwnable<Borrowed<'a>: Deref<Target = T>>, T, F> Display
+    for BorrowedAdapter<'a, D, F>
+where
+    F: Fn(&T, &mut Formatter<'_>) -> Result + 'static,
+{
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
+        // SAFETY: FormatAdapter<_, F> can only be constructed if F is inhabited
+        let f: &F = unsafe { materialize_zst_fmt() };
+        f(&self.borrowed, fmt)
+    }
+}
+
+/// For types with a unique value, produce a static reference to it.
+///
+/// # 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() }
+}

-- 
2.50.0.714.g196bf9f422-goog