Merge pull request torvalds#812 from wedsonaf/not-mountable

rust: add ability to register a file system

Author: wedsonaf

rust/kernel/fs.rs

@@ -4,8 +4,119 @@
 //!
 //! C headers: [`include/linux/fs.h`](../../../../include/linux/fs.h)
 
-use crate::{bindings, AlwaysRefCounted};
-use core::{cell::UnsafeCell, ptr};
+use crate::{bindings, error::code::*, str::CStr, to_result, AlwaysRefCounted, Result, ThisModule};
+use core::{cell::UnsafeCell, marker::PhantomPinned, pin::Pin, ptr};
+
+/// A file system type.
+pub trait Type {
+    /// The name of the file system type.
+    const NAME: &'static CStr;
+
+    /// The flags of this file system type.
+    ///
+    /// It is a combination of the flags in the [`flags`] module.
+    const FLAGS: i32;
+}
+
+/// File system flags.
+pub mod flags {
+    use crate::bindings;
+
+    /// The file system requires a device.
+    pub const REQUIRES_DEV: i32 = bindings::FS_REQUIRES_DEV as _;
+
+    /// The options provided when mounting are in binary form.
+    pub const BINARY_MOUNTDATA: i32 = bindings::FS_BINARY_MOUNTDATA as _;
+
+    /// The file system has a subtype. It is extracted from the name and passed in as a parameter.
+    pub const HAS_SUBTYPE: i32 = bindings::FS_HAS_SUBTYPE as _;
+
+    /// The file system can be mounted by userns root.
+    pub const USERNS_MOUNT: i32 = bindings::FS_USERNS_MOUNT as _;
+
+    /// Disables fanotify permission events.
+    pub const DISALLOW_NOTIFY_PERM: i32 = bindings::FS_DISALLOW_NOTIFY_PERM as _;
+
+    /// The file system has been updated to handle vfs idmappings.
+    pub const ALLOW_IDMAP: i32 = bindings::FS_ALLOW_IDMAP as _;
+
+    /// The file systen will handle `d_move` during `rename` internally.
+    pub const RENAME_DOES_D_MOVE: i32 = bindings::FS_RENAME_DOES_D_MOVE as _;
+}
+
+/// A file system registration.
+#[derive(Default)]
+pub struct Registration {
+    is_registered: bool,
+    fs: UnsafeCell<bindings::file_system_type>,
+    _pin: PhantomPinned,
+}
+
+// SAFETY: `Registration` doesn't really 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 a new file system registration.
+    ///
+    /// It is not visible or accessible yet. A successful call to [`Registration::register`] needs
+    /// to be made before users can mount it.
+    pub fn new() -> Self {
+        Self {
+            is_registered: false,
+            fs: UnsafeCell::new(bindings::file_system_type::default()),
+            _pin: PhantomPinned,
+        }
+    }
+
+    /// Registers a file system so that it can be mounted by users.
+    ///
+    /// The file system is described by the [`Type`] argument.
+    ///
+    /// It is automatically unregistered when the registration is dropped.
+    pub fn register<T: Type>(self: Pin<&mut Self>, module: &'static ThisModule) -> Result {
+        // SAFETY: We never move out of `this`.
+        let this = unsafe { self.get_unchecked_mut() };
+
+        if this.is_registered {
+            return Err(EINVAL);
+        }
+
+        let mut fs = this.fs.get_mut();
+        fs.owner = module.0;
+        fs.name = T::NAME.as_char_ptr();
+        fs.fs_flags = T::FLAGS;
+        fs.init_fs_context = Some(Self::init_fs_context_callback::<T>);
+        fs.kill_sb = Some(Self::kill_sb_callback::<T>);
+        // SAFETY: Pointers stored in `fs` are either static so will live for as long as the
+        // registration is active (it is undone in `drop`).
+        to_result(unsafe { bindings::register_filesystem(this.fs.get()) })?;
+        this.is_registered = true;
+        Ok(())
+    }
+
+    unsafe extern "C" fn init_fs_context_callback<T: Type>(
+        _fc_ptr: *mut bindings::fs_context,
+    ) -> core::ffi::c_int {
+        EINVAL.to_kernel_errno()
+    }
+
+    unsafe extern "C" fn kill_sb_callback<T: Type>(_sb_ptr: *mut bindings::super_block) {}
+}
+
+impl Drop for Registration {
+    fn drop(&mut self) {
+        if self.is_registered {
+            // SAFETY: When `is_registered` is `true`, a previous call to `register_filesystem` has
+            // succeeded, so it is safe to unregister here.
+            unsafe { bindings::unregister_filesystem(self.fs.get()) };
+        }
+    }
+}
 
 /// Wraps the kernel's `struct inode`.
 ///

samples/rust/Kconfig

@@ -118,6 +118,14 @@ config SAMPLE_RUST_PLATFORM
 	  To compile this as a module, choose M here:
 	  the module will be called rust_platform.
 
+config SAMPLE_RUST_FS
+	tristate "File system"
+	help
+	  This option builds the Rust file system sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_fs.
+
 	  If unsure, say N.
 
 config SAMPLE_RUST_NETFILTER

samples/rust/Makefile

@@ -13,5 +13,6 @@ obj-$(CONFIG_SAMPLE_RUST_RANDOM)		+= rust_random.o
 obj-$(CONFIG_SAMPLE_RUST_PLATFORM)		+= rust_platform.o
 obj-$(CONFIG_SAMPLE_RUST_NETFILTER)		+= rust_netfilter.o
 obj-$(CONFIG_SAMPLE_RUST_ECHO_SERVER)		+= rust_echo_server.o
+obj-$(CONFIG_SAMPLE_RUST_FS)			+= rust_fs.o
 
 subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs

samples/rust/rust_fs.rs

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust file system sample.
+
+use kernel::prelude::*;
+use kernel::{c_str, fs};
+
+module! {
+    type: FsModule,
+    name: b"rust_fs",
+    author: b"Rust for Linux Contributors",
+    license: b"GPL",
+}
+
+struct RustFs;
+impl fs::Type for RustFs {
+    const NAME: &'static CStr = c_str!("rustfs");
+    const FLAGS: i32 = fs::flags::USERNS_MOUNT;
+}
+
+struct FsModule {
+    _fs: Pin<Box<fs::Registration>>,
+}
+
+impl kernel::Module for FsModule {
+    fn init(_name: &'static CStr, module: &'static ThisModule) -> Result<Self> {
+        let mut reg = Pin::from(Box::try_new(fs::Registration::new())?);
+        reg.as_mut().register::<RustFs>(module)?;
+        Ok(Self { _fs: reg })
+    }
+}