rust: fs: introduce INode<T>

It's an inode from filesystem type `T`.

This is preparation for creating new inodes (for example, to create the
root inode of a new superblock), which will come in a subsequent commit.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

Author: wedsonaf

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.

rust/kernel/fs.rs

@@ -12,9 +12,9 @@ pub const MAX_LFS_FILESIZE: i64 = bindings::MAX_LFS_FILESIZE;
 /// Read-only file systems.
 pub mod ro {
     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};
 
     /// A read-only file system type.
@@ -93,6 +93,50 @@ pub mod ro {
         }
     }
 
+    /// 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: Type + ?Sized>(Opaque<bindings::inode>, PhantomData<T>);
+
+    impl<T: Type + ?Sized> INode<T> {
+        /// Returns the number of the inode.
+        pub fn ino(&self) -> u64 {
+            // 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: Type + ?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`.