rust: implement a #[vtable] macro

Use a single `#[vtable]` macro to replace the current boilerplating
of `ToUse`, `USE_NONE`, `declare_file_operations` required for
declaring and implementing traits that maps to Linux's pure vtables and
contains optional methods.

Signed-off-by: Gary Guo <gary@garyguo.net>

Author: nbdd0121

rust/kernel/file_operations.rs

@@ -16,6 +16,7 @@ use crate::{
     from_kernel_result,
     io_buffer::{IoBufferReader, IoBufferWriter},
     iov_iter::IovIter,
+    prelude::*,
     sync::CondVar,
     types::PointerWrapper,
     user_ptr::{UserSlicePtr, UserSlicePtrReader, UserSlicePtrWriter},
@@ -254,24 +255,24 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
     const VTABLE: bindings::file_operations = bindings::file_operations {
         open: Some(open_callback::<A, T>),
         release: Some(release_callback::<T>),
-        read: if T::TO_USE.read {
+        read: if T::HAS_READ {
             Some(read_callback::<T>)
         } else {
             None
         },
-        write: if T::TO_USE.write {
+        write: if T::HAS_WRITE {
             Some(write_callback::<T>)
         } else {
             None
         },
-        llseek: if T::TO_USE.seek {
+        llseek: if T::HAS_SEEK {
             Some(llseek_callback::<T>)
         } else {
             None
         },
 
         check_flags: None,
-        compat_ioctl: if T::TO_USE.compat_ioctl {
+        compat_ioctl: if T::HAS_COMPAT_IOCTL {
             Some(compat_ioctl_callback::<T>)
         } else {
             None
@@ -282,7 +283,7 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
         fasync: None,
         flock: None,
         flush: None,
-        fsync: if T::TO_USE.fsync {
+        fsync: if T::HAS_FSYNC {
             Some(fsync_callback::<T>)
         } else {
             None
@@ -292,19 +293,19 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
         iterate_shared: None,
         iopoll: None,
         lock: None,
-        mmap: if T::TO_USE.mmap {
+        mmap: if T::HAS_MMAP {
             Some(mmap_callback::<T>)
         } else {
             None
         },
         mmap_supported_flags: 0,
         owner: ptr::null_mut(),
-        poll: if T::TO_USE.poll {
+        poll: if T::HAS_POLL {
             Some(poll_callback::<T>)
         } else {
             None
         },
-        read_iter: if T::TO_USE.read_iter {
+        read_iter: if T::HAS_READ {
             Some(read_iter_callback::<T>)
         } else {
             None
@@ -315,12 +316,12 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
         show_fdinfo: None,
         splice_read: None,
         splice_write: None,
-        unlocked_ioctl: if T::TO_USE.ioctl {
+        unlocked_ioctl: if T::HAS_IOCTL {
             Some(unlocked_ioctl_callback::<T>)
         } else {
             None
         },
-        write_iter: if T::TO_USE.write_iter {
+        write_iter: if T::HAS_WRITE {
             Some(write_iter_callback::<T>)
         } else {
             None
@@ -337,69 +338,6 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
     }
 }
 
-/// Represents which fields of [`struct file_operations`] should be populated with pointers.
-pub struct ToUse {
-    /// The `read` field of [`struct file_operations`].
-    pub read: bool,
-
-    /// The `read_iter` field of [`struct file_operations`].
-    pub read_iter: bool,
-
-    /// The `write` field of [`struct file_operations`].
-    pub write: bool,
-
-    /// The `write_iter` field of [`struct file_operations`].
-    pub write_iter: bool,
-
-    /// The `llseek` field of [`struct file_operations`].
-    pub seek: bool,
-
-    /// The `unlocked_ioctl` field of [`struct file_operations`].
-    pub ioctl: bool,
-
-    /// The `compat_ioctl` field of [`struct file_operations`].
-    pub compat_ioctl: bool,
-
-    /// The `fsync` field of [`struct file_operations`].
-    pub fsync: bool,
-
-    /// The `mmap` field of [`struct file_operations`].
-    pub mmap: bool,
-
-    /// The `poll` field of [`struct file_operations`].
-    pub poll: bool,
-}
-
-/// A constant version where all values are to set to `false`, that is, all supported fields will
-/// be set to null pointers.
-pub const USE_NONE: ToUse = ToUse {
-    read: false,
-    read_iter: false,
-    write: false,
-    write_iter: false,
-    seek: false,
-    ioctl: false,
-    compat_ioctl: false,
-    fsync: false,
-    mmap: false,
-    poll: false,
-};
-
-/// Defines the [`FileOperations::TO_USE`] field based on a list of fields to be populated.
-#[macro_export]
-macro_rules! declare_file_operations {
-    () => {
-        const TO_USE: $crate::file_operations::ToUse = $crate::file_operations::USE_NONE;
-    };
-    ($($i:ident),+) => {
-        const TO_USE: kernel::file_operations::ToUse =
-            $crate::file_operations::ToUse {
-                $($i: true),+ ,
-                ..$crate::file_operations::USE_NONE
-            };
-    };
-}
-
 /// Allows the handling of ioctls defined with the `_IO`, `_IOR`, `_IOW`, and `_IOWR` macros.
 ///
 /// For each macro, there is a handler function that takes the appropriate types as arguments.
@@ -527,10 +465,8 @@ impl<T: FileOperations<Wrapper = Box<T>> + Default> FileOpener<()> for T {
 /// File descriptors may be used from multiple threads/processes concurrently, so your type must be
 /// [`Sync`]. It must also be [`Send`] because [`FileOperations::release`] will be called from the
 /// thread that decrements that associated file's refcount to zero.
+#[vtable]
 pub trait FileOperations: Send + Sync + Sized {
-    /// The methods to use to populate [`struct file_operations`].
-    const TO_USE: ToUse;
-
     /// The pointer type that will be used to hold ourselves.
     type Wrapper: PointerWrapper = Box<Self>;
 

rust/kernel/prelude.rs

@@ -15,7 +15,7 @@ pub use alloc::{borrow::ToOwned, string::String};
 
 pub use super::build_assert;
 
-pub use macros::{module, module_misc_device};
+pub use macros::{module, module_misc_device, vtable};
 
 pub use super::{pr_alert, pr_crit, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
 

rust/macros/lib.rs

@@ -8,6 +8,7 @@
 #![deny(clippy::style)]
 
 mod module;
+mod vtable;
 
 use proc_macro::TokenStream;
 
@@ -130,3 +131,54 @@ pub fn module(ts: TokenStream) -> TokenStream {
 pub fn module_misc_device(ts: TokenStream) -> TokenStream {
     module::module_misc_device(ts)
 }
+
+/// Declares or implements a vtable trait.
+///
+/// Linux's use of pure vtables is very close to Rust traits, but they differ
+/// in how unimplemented functions are represented. In Rust, traits can provide
+/// default implementation for all non-required methods (and the default
+/// implementation could just return `Error::EINVAL`); Linux typically use C
+/// `NULL` pointers to represent these functions.
+///
+/// This attribute is intended to close the gap. Traits can be declared and
+/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
+/// will be generated for each method in the trait, indicating if the implementor
+/// has overriden a method.
+///
+/// This attribute is not needed if all methods are required.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// use kernel::prelude::*;
+///
+/// // Declares a `#[vtable]` trait
+/// #[vtable]
+/// pub trait FileOperations: Send + Sync + Sized {
+///     fn read<T: IoBufferWriter>(&self, _file: &File, _data: &mut T, _offset: u64) -> Result<usize> {
+///         Err(Error::EINVAL)
+///     }
+///
+///     fn write<T: IoBufferWriter>(&self, _file: &File, _data: &mut T, _offset: u64) -> Result<usize> {
+///         Err(Error::EINVAL)
+///     }
+/// }
+///
+/// struct RustFile;
+///
+/// // Implements the `#[vtable]` trait
+/// #[vtable]
+/// impl FileOperations for RustFile {
+///     fn read<T: IoBufferWriter>(&self, _file: &File, _data: &mut T, _offset: u64) -> Result<usize> {
+/// #        Err(Error::EINVAL)
+///         /* ... */
+///     }
+/// }
+///
+/// assert_eq!(<RustFile as FileOperations>::HAS_READ, true);
+/// assert_eq!(<RustFile as FileOperations>::HAS_WRITE, false);
+/// ```
+#[proc_macro_attribute]
+pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream {
+    vtable::vtable(attr, ts)
+}

rust/macros/vtable.rs

@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
+use std::fmt::Write;
+
+pub fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
+    let mut it = ts.into_iter();
+
+    let mut tokens = Vec::new();
+
+    // Scan for the `trait` or `impl` keyword
+    let is_trait = loop {
+        let token = it.next().expect("unexpected end");
+        let keyword = match &token {
+            TokenTree::Ident(ident) => match ident.to_string().as_str() {
+                "trait" => Some(true),
+                "impl" => Some(false),
+                _ => None,
+            },
+            _ => None,
+        };
+        tokens.push(token);
+        if let Some(v) = keyword {
+            break v;
+        }
+    };
+
+    // Scan for the main body
+    // FIXME: `{}` is allowed within type as well, e.g. `impl Foo for Bar<{0}>`.
+    // but these are very rare and we don't want to have generics parsing code.
+    let body = loop {
+        match it.next() {
+            Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => {
+                break group;
+            }
+            Some(token) => tokens.push(token),
+            None => panic!("unexpected end"),
+        }
+    };
+
+    assert!(it.next().is_none(), "expected end");
+
+    let mut body_it = body.stream().into_iter();
+    let mut functions = Vec::new();
+    while let Some(token) = body_it.next() {
+        match token {
+            TokenTree::Ident(ident) if ident.to_string() == "fn" => {
+                let fn_name = match body_it.next() {
+                    Some(TokenTree::Ident(ident)) => ident.to_string(),
+                    _ => panic!("expected identifier after `fn`"),
+                };
+                functions.push(fn_name);
+            }
+            _ => (),
+        }
+    }
+
+    let mut const_items;
+    if is_trait {
+        const_items =  "/// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable) attribute when implementing this trait.
+        const USE_VTABLE_ATTR: ();".to_owned();
+
+        for f in functions {
+            write!(
+                const_items,
+                "/// Indicates if the `{}` method is overriden by the implementor.
+                const HAS_{}: bool = false;",
+                f,
+                f.to_uppercase()
+            )
+            .unwrap();
+        }
+    } else {
+        const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();
+
+        for f in functions {
+            write!(const_items, "const HAS_{}: bool = true;", f.to_uppercase()).unwrap();
+        }
+    }
+
+    let new_body = vec![const_items.parse().unwrap(), body.stream()]
+        .into_iter()
+        .collect();
+    tokens.push(TokenTree::Group(Group::new(Delimiter::Brace, new_body)));
+    tokens.into_iter().collect()
+}

samples/rust/rust_chrdev.rs

@@ -21,9 +21,8 @@ module! {
 #[derive(Default)]
 struct RustFile;
 
-impl FileOperations for RustFile {
-    kernel::declare_file_operations!();
-}
+#[vtable]
+impl FileOperations for RustFile {}
 
 struct RustChrdev {
     _dev: Pin<Box<chrdev::Registration<2>>>,

samples/rust/rust_miscdev.rs

@@ -67,11 +67,10 @@ impl FileOpener<Pin<Arc<SharedState>>> for Token {
     }
 }
 
+#[vtable]
 impl FileOperations for Token {
     type Wrapper = Box<Self>;
 
-    kernel::declare_file_operations!(read, write);
-
     fn read<T: IoBufferWriter>(&self, _: &File, data: &mut T, offset: u64) -> Result<usize> {
         // Succeed if the caller doesn't provide a buffer or if not at the start.
         if data.is_empty() || offset != 0 {

samples/rust/rust_random.rs

@@ -18,9 +18,8 @@ use kernel::{
 #[derive(Default)]
 struct RandomFile;
 
+#[vtable]
 impl FileOperations for RandomFile {
-    kernel::declare_file_operations!(read, write, read_iter, write_iter);
-
     fn read<T: IoBufferWriter>(&self, file: &File, buf: &mut T, _offset: u64) -> Result<usize> {
         let total_len = buf.len();
         let mut chunkbuf = [0; 256];

samples/rust/rust_semaphore.rs

@@ -22,7 +22,7 @@ use core::{
     sync::atomic::{AtomicU64, Ordering},
 };
 use kernel::{
-    c_str, condvar_init, declare_file_operations,
+    c_str, condvar_init,
     file::File,
     file_operations::{FileOpener, FileOperations, IoctlCommand, IoctlHandler},
     io_buffer::{IoBufferReader, IoBufferWriter},
@@ -79,11 +79,10 @@ impl FileOpener<Arc<Semaphore>> for FileState {
     }
 }
 
+#[vtable]
 impl FileOperations for FileState {
     type Wrapper = Box<Self>;
 
-    declare_file_operations!(read, write, ioctl);
-
     fn read<T: IoBufferWriter>(&self, _: &File, data: &mut T, offset: u64) -> Result<usize> {
         if data.is_empty() || offset > 0 {
             return Ok(0);