[RFC PATCH 05/19] rust: fs: introduce `INode<T>`

[Wedson Almeida Filho <wedsonaf@gmail.com>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

Allow Rust file systems to handle typed and ref-counted inodes.

This is in preparation for creating new inodes (for example, to create
the root inode of a new superblock), which comes in the next patch in
the series.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/helpers.c    |  7 +++++++
 rust/kernel/fs.rs | 53 +++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/rust/helpers.c b/rust/helpers.c
index 4c86fe4a7e05..fe45f8ddb31f 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -25,6 +25,7 @@
 #include <linux/build_bug.h>
 #include <linux/err.h>
 #include <linux/errname.h>
+#include <linux/fs.h>
 #include <linux/mutex.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
@@ -144,6 +145,12 @@ struct kunit *rust_helper_kunit_get_current_test(void)
 }
 EXPORT_SYMBOL_GPL(rust_helper_kunit_get_current_test);
 
+off_t rust_helper_i_size_read(const struct inode *inode)
+{
+	return i_size_read(inode);
+}
+EXPORT_SYMBOL_GPL(rust_helper_i_size_read);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
index 31cf643aaded..30fa1f312f33 100644
--- a/rust/kernel/fs.rs
+++ b/rust/kernel/fs.rs
@@ -7,9 +7,9 @@
 //! C headers: [`include/linux/fs.h`](../../include/linux/fs.h)
 
 use crate::error::{code::*, from_result, to_result, Error, Result};
-use crate::types::Opaque;
+use crate::types::{AlwaysRefCounted, Opaque};
 use crate::{bindings, init::PinInit, str::CStr, try_pin_init, ThisModule};
-use core::{marker::PhantomData, marker::PhantomPinned, pin::Pin};
+use core::{marker::PhantomData, marker::PhantomPinned, pin::Pin, ptr};
 use macros::{pin_data, pinned_drop};
 
 /// Maximum size of an inode.
@@ -94,6 +94,55 @@ fn drop(self: Pin<&mut Self>) {
     }
 }
 
+/// The number of an inode.
+pub type Ino = u64;
+
+/// A node in the file system index (inode).
+///
+/// Wraps the kernel's `struct inode`.
+///
+/// # Invariants
+///
+/// Instances of this type are always ref-counted, that is, a call to `ihold` ensures that the
+/// allocation remains valid at least until the matching call to `iput`.
+#[repr(transparent)]
+pub struct INode<T: FileSystem + ?Sized>(Opaque<bindings::inode>, PhantomData<T>);
+
+impl<T: FileSystem + ?Sized> INode<T> {
+    /// Returns the number of the inode.
+    pub fn ino(&self) -> Ino {
+        // SAFETY: `i_ino` is immutable, and `self` is guaranteed to be valid by the existence of a
+        // shared reference (&self) to it.
+        unsafe { (*self.0.get()).i_ino }
+    }
+
+    /// Returns the super-block that owns the inode.
+    pub fn super_block(&self) -> &SuperBlock<T> {
+        // SAFETY: `i_sb` is immutable, and `self` is guaranteed to be valid by the existence of a
+        // shared reference (&self) to it.
+        unsafe { &*(*self.0.get()).i_sb.cast() }
+    }
+
+    /// Returns the size of the inode contents.
+    pub fn size(&self) -> i64 {
+        // SAFETY: `self` is guaranteed to be valid by the existence of a shared reference.
+        unsafe { bindings::i_size_read(self.0.get()) }
+    }
+}
+
+// SAFETY: The type invariants guarantee that `INode` is always ref-counted.
+unsafe impl<T: FileSystem + ?Sized> AlwaysRefCounted for INode<T> {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::ihold(self.0.get()) };
+    }
+
+    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
+        unsafe { bindings::iput(obj.cast().as_ptr()) }
+    }
+}
+
 /// A file system super block.
 ///
 /// Wraps the kernel's `struct super_block`.
-- 
2.34.1