[RFC PATCH v2 01/30] rust: fs: add registration/unregistration of file systems

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

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

Allow basic registration and unregistration of Rust file system types.
Unregistration happens automatically when a registration variable is
dropped (e.g., when it goes out of scope).

File systems registered this way are visible in `/proc/filesystems` but
cannot be mounted yet because `init_fs_context` fails.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/kernel/error.rs |  2 --
 rust/kernel/fs.rs    | 75 ++++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs   |  1 +
 3 files changed, 76 insertions(+), 2 deletions(-)
 create mode 100644 rust/kernel/fs.rs

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index b248a4c22fb4..f4fa2847e210 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -308,8 +308,6 @@ pub(crate) fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
 ///     })
 /// }
 /// ```
-// TODO: Remove `dead_code` marker once an in-kernel client is available.
-#[allow(dead_code)]
 pub(crate) fn from_result<T, F>(f: F) -> T
 where
     T: From<i16>,
diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
new file mode 100644
index 000000000000..cc1ed7ed2f54
--- /dev/null
+++ b/rust/kernel/fs.rs
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Kernel file systems.
+//!
+//! This module allows Rust code to register new kernel file systems.
+//!
+//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
+
+use crate::error::{code::*, from_result, to_result, Error};
+use crate::types::Opaque;
+use crate::{bindings, init::PinInit, str::CStr, try_pin_init, ThisModule};
+use core::{ffi, pin::Pin};
+use macros::{pin_data, pinned_drop};
+
+/// A file system type.
+pub trait FileSystem {
+    /// The name of the file system type.
+    const NAME: &'static CStr;
+}
+
+/// A registration of a file system.
+#[pin_data(PinnedDrop)]
+pub struct Registration {
+    #[pin]
+    fs: Opaque<bindings::file_system_type>,
+}
+
+// SAFETY: `Registration` doesn't provide any `&self` methods, so it is safe to pass references
+// to it around.
+unsafe impl Sync for Registration {}
+
+// SAFETY: Both registration and unregistration are implemented in C and safe to be performed
+// from any thread, so `Registration` is `Send`.
+unsafe impl Send for Registration {}
+
+impl Registration {
+    /// Creates the initialiser of a new file system registration.
+    pub fn new<T: FileSystem + ?Sized>(module: &'static ThisModule) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            fs <- Opaque::try_ffi_init(|fs_ptr: *mut bindings::file_system_type| {
+                // SAFETY: `try_ffi_init` guarantees that `fs_ptr` is valid for write.
+                unsafe { fs_ptr.write(bindings::file_system_type::default()) };
+
+                // SAFETY: `try_ffi_init` guarantees that `fs_ptr` is valid for write, and it has
+                // just been initialised above, so it's also valid for read.
+                let fs = unsafe { &mut *fs_ptr };
+                fs.owner = module.0;
+                fs.name = T::NAME.as_char_ptr();
+                fs.init_fs_context = Some(Self::init_fs_context_callback);
+                fs.kill_sb = Some(Self::kill_sb_callback);
+                fs.fs_flags = 0;
+
+                // SAFETY: Pointers stored in `fs` are static so will live for as long as the
+                // registration is active (it is undone in `drop`).
+                to_result(unsafe { bindings::register_filesystem(fs_ptr) })
+            }),
+        })
+    }
+
+    unsafe extern "C" fn init_fs_context_callback(_fc: *mut bindings::fs_context) -> ffi::c_int {
+        from_result(|| Err(ENOTSUPP))
+    }
+
+    unsafe extern "C" fn kill_sb_callback(_sb_ptr: *mut bindings::super_block) {}
+}
+
+#[pinned_drop]
+impl PinnedDrop for Registration {
+    fn drop(self: Pin<&mut Self>) {
+        // SAFETY: If an instance of `Self` has been successfully created, a call to
+        // `register_filesystem` has necessarily succeeded. So it's ok to call
+        // `unregister_filesystem` on the previously registered fs.
+        unsafe { bindings::unregister_filesystem(self.fs.get()) };
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 4b629aa94735..e664f80b8141 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -31,6 +31,7 @@
 mod build_assert;
 pub mod error;
 pub mod file;
+pub mod fs;
 pub mod init;
 pub mod ioctl;
 #[cfg(CONFIG_KUNIT)]
-- 
2.34.1