rust: file: add Rust abstraction for struct file

This abstraction makes it possible to manipulate the open files for a
process. The new `File` struct wraps the C `struct file`. When accessing
it using the smart pointer `ARef<File>`, the pointer will own a
reference count to the file. When accessing it as `&File`, then the
reference does not own a refcount, but the borrow checker will ensure
that the reference count does not hit zero while the `&File` is live.

Since this is intended to manipulate the open files of a process, we
introduce an `fget` constructor that corresponds to the C `fget`
method. In future patches, it will become possible to create a new fd in
a process and bind it to a `File`. Rust Binder will use these to send
fds from one process to another.

We also provide a method for accessing the file's flags. Rust Binder
will use this to access the flags of the Binder fd to check whether the
non-blocking flag is set, which affects what the Binder ioctl does.

This introduces a struct for the EBADF error type, rather than just
using the Error type directly. This has two advantages:
* `File::fget` returns a `Result<ARef<File>, BadFdError>`, which the
  compiler will represent as a single pointer, with null being an error.
  This is possible because the compiler understands that `BadFdError`
  has only one possible value, and it also understands that the
  `ARef<File>` smart pointer is guaranteed non-null.
* Additionally, we promise to users of the method that the method can
  only fail with EBADF, which means that they can rely on this promise
  without having to inspect its implementation.
That said, there are also two disadvantages:
* Defining additional error types involves boilerplate.
* The question mark operator will only utilize the `From` trait once,
  which prevents you from using the question mark operator on
  `BadFdError` in methods that return some third error type that the
  kernel `Error` is convertible into. (However, it works fine in methods
  that return `Error`.)

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240628-alice-file-v7-3-4d701f6335f3@google.com

Author: wedsonaf

fs/file.c

@@ -1127,6 +1127,13 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
  *
  * The fput_needed flag returned by fget_light should be passed to the
  * corresponding fput_light.
+ *
+ * (As an exception to rule 2, you can call filp_close between fget_light and
+ * fput_light provided that you capture a real refcount with get_file before
+ * the call to filp_close, and ensure that this real refcount is fput *after*
+ * the fput_light call.)
+ *
+ * See also the documentation in rust/kernel/file.rs.
  */
 static unsigned long __fget_light(unsigned int fd, fmode_t mask)
 {

rust/bindings/bindings_helper.h

@@ -9,6 +9,8 @@
 #include <kunit/test.h>
 #include <linux/errname.h>
 #include <linux/ethtool.h>
+#include <linux/file.h>
+#include <linux/fs.h>
 #include <linux/jiffies.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>

rust/helpers.c

@@ -27,6 +27,7 @@
 #include <linux/errname.h>
 #include <linux/gfp.h>
 #include <linux/highmem.h>
+#include <linux/fs.h>
 #include <linux/mutex.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
@@ -199,6 +200,12 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
 }
 EXPORT_SYMBOL_GPL(rust_helper_krealloc);
 
+struct file *rust_helper_get_file(struct file *f)
+{
+	return get_file(f);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_file);
+
 /*
  * `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.