[PATCH v9 4/5] 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              | 49 +++++++++++++++++++++++++++++++++++++
 rust/kernel/debugfs/display_file.rs | 39 ++++++++++++++++++++++++++++-
 2 files changed, 87 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index a1a84dd309216f455ae8fe3d3c0fd00f957f82a9..083c49007cd7ae5b3d7954bf859c24b7eb62d557 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -9,6 +9,7 @@
 use crate::str::CStr;
 #[cfg(CONFIG_DEBUG_FS)]
 use crate::sync::Arc;
+use core::fmt;
 use core::fmt::Display;
 use core::marker::PhantomPinned;
 use core::ops::Deref;
@@ -194,6 +195,54 @@ pub fn display_file<'b, T: Display + Send + Sync, E, TI: PinInit<T, E>>(
         unsafe { self.create_file(name, data, vtable) }
     }
 
+    /// 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<
+        'b,
+        T: Send + Sync,
+        E,
+        TI: PinInit<T, E>,
+        F: Fn(&T, &mut fmt::Formatter<'_>) -> fmt::Result + Send + Sync,
+    >(
+        &self,
+        name: &'b CStr,
+        data: TI,
+        _f: &'static F,
+    ) -> impl PinInit<File<T>, E> + use<'_, 'b, T, TI, E, F> {
+        #[cfg(CONFIG_DEBUG_FS)]
+        let vtable = &<display_file::FormatAdapter<T, F> as display_file::DisplayFile>::VTABLE;
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        let vtable = ();
+
+        // SAFETY: `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 `FormatAdapter<T, F>` that will outlive it.
+        // `create_file`'s safety requirements provide the lifetime aspect of this, but we are
+        // using a private `T` pointer. This is legal because:
+        // 1. `FormatAdapter<T, F>` is a `#[repr(transparent)]` wrapper around `T`, so the
+        //    implicit transmute is legal.
+        // 2. The invariant in `FormatAdapter` that `F` is inhabited is upheld because we have
+        //    `_f`, so constructing a `FormatAdapter<T, F> is legal.
+        unsafe { self.create_file(name, data, vtable) }
+    }
+
     /// 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 2a58ca2685258b050089e4cfd62188885f7f5f04..6275283b9dabd8dae84a9335c8832e7943707d56 100644
--- a/rust/kernel/debugfs/display_file.rs
+++ b/rust/kernel/debugfs/display_file.rs
@@ -4,7 +4,8 @@
 use crate::prelude::*;
 use crate::seq_file::SeqFile;
 use crate::seq_print;
-use core::fmt::Display;
+use core::fmt::{Display, Formatter, Result};
+use core::marker::PhantomData;
 
 /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`.
 ///
@@ -61,3 +62,39 @@ impl<T: Display + Sync> DisplayFile for T {
         ..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> Display for FormatAdapter<D, F>
+where
+    F: Fn(&D, &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.inner, 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() }
+}

-- 
2.50.0.727.gbf7dc18ff4-goog