[PATCH v2 2/4] rust: debugfs: Bind file creation for long-lived Display

[Matthew Maurer <mmaurer@google.com>]

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@google.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