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Date: Mon, 20 May 2024 12:38:19 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Wedson Almeida Filho <wedsonaf@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
Dave Chinner <david@...morbit.com>,
Kent Overstreet <kent.overstreet@...il.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-fsdevel@...r.kernel.org, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org,
Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [RFC PATCH v2 04/30] rust: fs: introduce `FileSystem::fill_super`
On Tue, May 14, 2024 at 10:16:45AM -0300, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@...rosoft.com>
>
> Allow Rust file systems to initialise superblocks, which allows them
> to be mounted (though they are still empty).
>
> Some scaffolding code is added to create an empty directory as the root.
> It is replaced by proper inode creation in a subsequent patch in this
> series.
>
> Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> ---
> rust/bindings/bindings_helper.h | 5 ++
> rust/kernel/fs.rs | 147 ++++++++++++++++++++++++++++++--
> rust/kernel/fs/sb.rs | 50 +++++++++++
> samples/rust/rust_rofs.rs | 6 ++
> 4 files changed, 202 insertions(+), 6 deletions(-)
> create mode 100644 rust/kernel/fs/sb.rs
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 1bef4dff3019..dabb5a787e0d 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -12,6 +12,7 @@
> #include <linux/ethtool.h>
> #include <linux/file.h>
> #include <linux/fs.h>
> +#include <linux/fs_context.h>
> #include <linux/jiffies.h>
> #include <linux/mdio.h>
> #include <linux/phy.h>
> @@ -32,3 +33,7 @@ const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
>
> const slab_flags_t RUST_CONST_HELPER_SLAB_RECLAIM_ACCOUNT = SLAB_RECLAIM_ACCOUNT;
> const slab_flags_t RUST_CONST_HELPER_SLAB_ACCOUNT = SLAB_ACCOUNT;
> +
> +const unsigned long RUST_CONST_HELPER_SB_RDONLY = SB_RDONLY;
> +
> +const loff_t RUST_CONST_HELPER_MAX_LFS_FILESIZE = MAX_LFS_FILESIZE;
> diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
> index fb7a9b200b85..263b4b6186ae 100644
> --- a/rust/kernel/fs.rs
> +++ b/rust/kernel/fs.rs
> @@ -6,16 +6,30 @@
> //!
> //! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
>
> -use crate::error::{code::*, from_result, to_result, Error};
> +use crate::error::{code::*, from_result, to_result, Error, Result};
> use crate::types::Opaque;
> use crate::{bindings, init::PinInit, str::CStr, try_pin_init, ThisModule};
> use core::{ffi, marker::PhantomData, pin::Pin};
> use macros::{pin_data, pinned_drop};
> +use sb::SuperBlock;
> +
> +pub mod sb;
> +
> +/// The offset of a file in a file system.
This is really the position of some data within a file, in bytes.
> +///
> +/// This is C's `loff_t`.
> +pub type Offset = i64;
Ergh, I really wish this was loff (or LOff if we're really doing
camelcase for rust code) for somewhat better greppability.
> +
> +/// Maximum size of an inode.
> +pub const MAX_LFS_FILESIZE: Offset = bindings::MAX_LFS_FILESIZE;
>
> /// A file system type.
> pub trait FileSystem {
> /// The name of the file system type.
> const NAME: &'static CStr;
> +
> + /// Initialises the new superblock.
> + fn fill_super(sb: &mut SuperBlock<Self>) -> Result;
> }
>
> /// A registration of a file system.
> @@ -46,7 +60,7 @@ pub fn new<T: FileSystem + ?Sized>(module: &'static ThisModule) -> impl PinInit<
> 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.init_fs_context = Some(Self::init_fs_context_callback::<T>);
> fs.kill_sb = Some(Self::kill_sb_callback);
> fs.fs_flags = 0;
>
> @@ -57,11 +71,22 @@ pub fn new<T: FileSystem + ?Sized>(module: &'static ThisModule) -> impl PinInit<
> })
> }
>
> - unsafe extern "C" fn init_fs_context_callback(_fc: *mut bindings::fs_context) -> ffi::c_int {
> - from_result(|| Err(ENOTSUPP))
> + unsafe extern "C" fn init_fs_context_callback<T: FileSystem + ?Sized>(
> + fc_ptr: *mut bindings::fs_context,
> + ) -> ffi::c_int {
> + from_result(|| {
> + // SAFETY: The C callback API guarantees that `fc_ptr` is valid.
> + let fc = unsafe { &mut *fc_ptr };
> + fc.ops = &Tables::<T>::CONTEXT;
> + Ok(0)
> + })
> }
>
> - unsafe extern "C" fn kill_sb_callback(_sb_ptr: *mut bindings::super_block) {}
> + unsafe extern "C" fn kill_sb_callback(sb_ptr: *mut bindings::super_block) {
> + // SAFETY: In `get_tree_callback` we always call `get_tree_nodev`, so `kill_anon_super` is
> + // the appropriate function to call for cleanup.
> + unsafe { bindings::kill_anon_super(sb_ptr) };
> + }
> }
>
> #[pinned_drop]
> @@ -74,6 +99,113 @@ fn drop(self: Pin<&mut Self>) {
> }
> }
>
> +struct Tables<T: FileSystem + ?Sized>(T);
> +impl<T: FileSystem + ?Sized> Tables<T> {
> + const CONTEXT: bindings::fs_context_operations = bindings::fs_context_operations {
> + free: None,
> + parse_param: None,
> + get_tree: Some(Self::get_tree_callback),
> + reconfigure: None,
> + parse_monolithic: None,
> + dup: None,
> + };
> +
> + unsafe extern "C" fn get_tree_callback(fc: *mut bindings::fs_context) -> ffi::c_int {
> + // SAFETY: `fc` is valid per the callback contract. `fill_super_callback` also has
> + // the right type and is a valid callback.
> + unsafe { bindings::get_tree_nodev(fc, Some(Self::fill_super_callback)) }
> + }
> +
> + unsafe extern "C" fn fill_super_callback(
> + sb_ptr: *mut bindings::super_block,
> + _fc: *mut bindings::fs_context,
> + ) -> ffi::c_int {
> + from_result(|| {
> + // SAFETY: The callback contract guarantees that `sb_ptr` is a unique pointer to a
> + // newly-created superblock.
> + let new_sb = unsafe { SuperBlock::from_raw_mut(sb_ptr) };
> +
> + // SAFETY: The callback contract guarantees that `sb_ptr`, from which `new_sb` is
> + // derived, is valid for write.
> + let sb = unsafe { &mut *new_sb.0.get() };
> + sb.s_op = &Tables::<T>::SUPER_BLOCK;
> + sb.s_flags |= bindings::SB_RDONLY;
> +
> + T::fill_super(new_sb)?;
> +
> + // The following is scaffolding code that will be removed in a subsequent patch. It is
> + // needed to build a root dentry, otherwise core code will BUG().
> + // SAFETY: `sb` is the superblock being initialised, it is valid for read and write.
> + let inode = unsafe { bindings::new_inode(sb) };
> + if inode.is_null() {
> + return Err(ENOMEM);
> + }
> +
> + // SAFETY: `inode` is valid for write.
> + unsafe { bindings::set_nlink(inode, 2) };
> +
> + {
> + // SAFETY: This is a newly-created inode. No other references to it exist, so it is
> + // safe to mutably dereference it.
> + let inode = unsafe { &mut *inode };
> + inode.i_ino = 1;
> + inode.i_mode = (bindings::S_IFDIR | 0o755) as _;
> +
> + // SAFETY: `simple_dir_operations` never changes, it's safe to reference it.
> + inode.__bindgen_anon_3.i_fop = unsafe { &bindings::simple_dir_operations };
^^^^^^^^^^^^^^^^
This is a gross way to handle anonymous struct fields. What happens
when struct inode changes and we have to do a giant treewide sed?
(and yes, I understand that's likely going to be a rustc change...)
--D
> +
> + // SAFETY: `simple_dir_inode_operations` never changes, it's safe to reference it.
> + inode.i_op = unsafe { &bindings::simple_dir_inode_operations };
> + }
> +
> + // SAFETY: `d_make_root` requires that `inode` be valid and referenced, which is the
> + // case for this call.
> + //
> + // It takes over the inode, even on failure, so we don't need to clean it up.
> + let dentry = unsafe { bindings::d_make_root(inode) };
> + if dentry.is_null() {
> + return Err(ENOMEM);
> + }
> +
> + sb.s_root = dentry;
> +
> + Ok(0)
> + })
> + }
> +
> + const SUPER_BLOCK: bindings::super_operations = bindings::super_operations {
> + alloc_inode: None,
> + destroy_inode: None,
> + free_inode: None,
> + dirty_inode: None,
> + write_inode: None,
> + drop_inode: None,
> + evict_inode: None,
> + put_super: None,
> + sync_fs: None,
> + freeze_super: None,
> + freeze_fs: None,
> + thaw_super: None,
> + unfreeze_fs: None,
> + statfs: None,
> + remount_fs: None,
> + umount_begin: None,
> + show_options: None,
> + show_devname: None,
> + show_path: None,
> + show_stats: None,
> + #[cfg(CONFIG_QUOTA)]
> + quota_read: None,
> + #[cfg(CONFIG_QUOTA)]
> + quota_write: None,
> + #[cfg(CONFIG_QUOTA)]
> + get_dquots: None,
> + nr_cached_objects: None,
> + free_cached_objects: None,
> + shutdown: None,
> + };
> +}
> +
> /// Kernel module that exposes a single file system implemented by `T`.
> #[pin_data]
> pub struct Module<T: FileSystem + ?Sized> {
> @@ -100,7 +232,7 @@ fn init(module: &'static ThisModule) -> impl PinInit<Self, Error> {
> ///
> /// ```
> /// # mod module_fs_sample {
> -/// use kernel::fs;
> +/// use kernel::fs::{sb::SuperBlock, self};
> /// use kernel::prelude::*;
> ///
> /// kernel::module_fs! {
> @@ -114,6 +246,9 @@ fn init(module: &'static ThisModule) -> impl PinInit<Self, Error> {
> /// struct MyFs;
> /// impl fs::FileSystem for MyFs {
> /// const NAME: &'static CStr = kernel::c_str!("myfs");
> +/// fn fill_super(_: &mut SuperBlock<Self>) -> Result {
> +/// todo!()
> +/// }
> /// }
> /// # }
> /// ```
> diff --git a/rust/kernel/fs/sb.rs b/rust/kernel/fs/sb.rs
> new file mode 100644
> index 000000000000..113d3c0d8148
> --- /dev/null
> +++ b/rust/kernel/fs/sb.rs
> @@ -0,0 +1,50 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! File system super blocks.
> +//!
> +//! This module allows Rust code to use superblocks.
> +//!
> +//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
> +
> +use super::FileSystem;
> +use crate::{bindings, types::Opaque};
> +use core::marker::PhantomData;
> +
> +/// A file system super block.
> +///
> +/// Wraps the kernel's `struct super_block`.
> +#[repr(transparent)]
> +pub struct SuperBlock<T: FileSystem + ?Sized>(
> + pub(crate) Opaque<bindings::super_block>,
> + PhantomData<T>,
> +);
> +
> +impl<T: FileSystem + ?Sized> SuperBlock<T> {
> + /// Creates a new superblock mutable reference from the given raw pointer.
> + ///
> + /// # Safety
> + ///
> + /// Callers must ensure that:
> + ///
> + /// * `ptr` is valid and remains so for the lifetime of the returned object.
> + /// * `ptr` has the correct file system type.
> + /// * `ptr` is the only active pointer to the superblock.
> + pub(crate) unsafe fn from_raw_mut<'a>(ptr: *mut bindings::super_block) -> &'a mut Self {
> + // SAFETY: The safety requirements guarantee that the cast below is ok.
> + unsafe { &mut *ptr.cast::<Self>() }
> + }
> +
> + /// Returns whether the superblock is mounted in read-only mode.
> + pub fn rdonly(&self) -> bool {
> + // SAFETY: `s_flags` only changes during init, so it is safe to read it.
> + unsafe { (*self.0.get()).s_flags & bindings::SB_RDONLY != 0 }
> + }
> +
> + /// Sets the magic number of the superblock.
> + pub fn set_magic(&mut self, magic: usize) -> &mut Self {
> + // SAFETY: This is a new superblock that is being initialised, so it's ok to write to its
> + // fields.
> + unsafe { (*self.0.get()).s_magic = magic as core::ffi::c_ulong };
> + self
> + }
> +}
> diff --git a/samples/rust/rust_rofs.rs b/samples/rust/rust_rofs.rs
> index d465b107a07d..022addf68891 100644
> --- a/samples/rust/rust_rofs.rs
> +++ b/samples/rust/rust_rofs.rs
> @@ -2,6 +2,7 @@
>
> //! Rust read-only file system sample.
>
> +use kernel::fs::sb;
> use kernel::prelude::*;
> use kernel::{c_str, fs};
>
> @@ -16,4 +17,9 @@
> struct RoFs;
> impl fs::FileSystem for RoFs {
> const NAME: &'static CStr = c_str!("rust_rofs");
> +
> + fn fill_super(sb: &mut sb::SuperBlock<Self>) -> Result {
> + sb.set_magic(0x52555354);
> + Ok(())
> + }
> }
> --
> 2.34.1
>
>
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