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However, these constants will have a fixed value so shouldn't be used. Thus, hide them. Signed-off-by: Gary Guo --- rust/macros/vtable.rs | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff --git a/rust/macros/vtable.rs b/rust/macros/vtable.rs index c6510b0c4ea1..e28976a5919f 100644 --- a/rust/macros/vtable.rs +++ b/rust/macros/vtable.rs @@ -9,7 +9,10 @@ Ident, TokenStream, // }; -use quote::ToTokens; +use quote::{ + quote, + ToTokens, // +}; use syn::{ parse_quote, Error, @@ -40,12 +43,18 @@ fn handle_trait(mut item: ItemTrait) -> Result { =20 // We don't know on the implementation-site whether a method i= s required or provided // so we have to generate a const for all methods. + // However, hide it for required methods as it will always be = true. + let doc =3D if fn_item.default.is_some() { + let comment =3D + format!("Indicates if the `{name}` method is overridde= n by the implementor."); + quote!(#[doc =3D #comment]) + } else { + quote!(#[doc(hidden)]) + }; let cfg_attrs =3D crate::helpers::gather_cfg_attrs(&fn_item.at= trs); - let comment =3D - format!("Indicates if the `{name}` method is overridden by= the implementor."); 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a `NULL` pointer is inserted to the vtable and the C subsystem does thing differently when it sees `NULL`. Streamline this use case by providing a `#[optional]` attribute that does what the `build_error!(VTABLE_DEFAULT_ERROR)` boilerplate does. Signed-off-by: Gary Guo --- rust/macros/lib.rs | 39 +++++++++++---------------------------- rust/macros/vtable.rs | 26 +++++++++++++++++++++++++- 2 files changed, 36 insertions(+), 29 deletions(-) diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs index 0514fc7c0a55..de55dee4d866 100644 --- a/rust/macros/lib.rs +++ b/rust/macros/lib.rs @@ -156,44 +156,29 @@ pub fn module(input: TokenStream) -> TokenStream { /// associated constant bool for each method in the trait that is set to t= rue if /// the implementer has overridden the associated method. /// -/// For a trait method to be optional, it must have a default implementati= on. -/// This is also the case for traits annotated with `#[vtable]`, but in th= is -/// case the default implementation will never be executed. The reason for= this -/// is that the functions will be called through function pointers install= ed in -/// C side vtables. When an optional method is not implemented on a `#[vta= ble]` -/// trait, a `NULL` entry is installed in the vtable. Thus the default -/// implementation is never called. Since these traits are not designed to= be -/// used on the Rust side, it should not be possible to call the default -/// implementation. This is done to ensure that we call the vtable methods -/// through the C vtable, and not through the Rust vtable. Therefore, the -/// default implementation should call `build_error!`, which prevents -/// calls to this function at compile time: -/// -/// ```compile_fail -/// # // Intentionally missing `use`s to simplify `rusttest`. -/// build_error!(VTABLE_DEFAULT_ERROR) -/// ``` -/// -/// Note that you might need to import [`kernel::error::VTABLE_DEFAULT_ERR= OR`]. +/// For a trait method to be optional for normal traits, it must have a de= fault implementation. +/// However, for many users of `#[vtable]`, the functions will be called t= hrough function pointers +/// installed in C side vtables. When an optional method is not implemente= d on a `#[vtable]` trait, +/// a `NULL` entry is installed in the vtable; thus the default implementa= tion is never called. If +/// this is the case, `#[optional]` can be applied on the optional method.= Methods annotated as such +/// do not need to be implemented nor need a default implementation. Calli= ng these methods on types +/// that do not implement them will fail to build using the `build_error!`= mechanism. /// /// This macro should not be used when all functions are required. /// /// # Examples /// /// ``` -/// use kernel::error::VTABLE_DEFAULT_ERROR; /// use kernel::prelude::*; /// /// // Declares a `#[vtable]` trait /// #[vtable] /// pub trait Operations: Send + Sync + Sized { -/// fn foo(&self) -> Result<()> { -/// build_error!(VTABLE_DEFAULT_ERROR) -/// } +/// #[optional] +/// fn foo(&self) -> Result<()>; /// -/// fn bar(&self) -> Result<()> { -/// build_error!(VTABLE_DEFAULT_ERROR) -/// } +/// #[optional] +/// fn bar(&self) -> Result<()>; /// } /// /// struct Foo; @@ -210,8 +195,6 @@ pub fn module(input: TokenStream) -> TokenStream { /// assert_eq!(::HAS_FOO, true); /// assert_eq!(::HAS_BAR, false); /// ``` -/// -/// [`kernel::error::VTABLE_DEFAULT_ERROR`]: ../kernel/error/constant.VTAB= LE_DEFAULT_ERROR.html #[proc_macro_attribute] pub fn vtable(attr: TokenStream, input: TokenStream) -> TokenStream { parse_macro_input!(attr as syn::parse::Nothing); diff --git a/rust/macros/vtable.rs b/rust/macros/vtable.rs index e28976a5919f..54bd32e82a9c 100644 --- a/rust/macros/vtable.rs +++ b/rust/macros/vtable.rs @@ -33,7 +33,7 @@ fn handle_trait(mut item: ItemTrait) -> Result= { const USE_VTABLE_ATTR: (); }); =20 - for item in &item.items { + for item in &mut item.items { if let TraitItem::Fn(fn_item) =3D item { let name =3D &fn_item.sig.ident; let gen_const_name =3D Ident::new( @@ -41,6 +41,30 @@ fn handle_trait(mut item: ItemTrait) -> Result { name.span(), ); =20 + if fn_item + .attrs + .extract_if(.., |attr| attr.path().is_ident("optional")) + .count() + !=3D 0 + { + if let Some(default) =3D &fn_item.default { + Err(Error::new_spanned( + default, + "`#[optional]` methods must not have default imple= mentation", + ))?; + } + + // Optional methods in Rust need a default implementation.= Inject one that fails + // the build in compile-time if not overridden. + fn_item.default =3D Some(parse_quote!({ + ::kernel::build_assert::build_error!( + "This function must not be called, see the #[vtabl= e] documentation." + ); 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Tue, 11 Aug 2026 13:56:02 +0000 From: Gary Guo Date: Tue, 11 Aug 2026 14:55:57 +0100 Subject: [PATCH 3/3] rust: misc: use `#[optional]` attribute of `#[vtable]` macro Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260811-vtable_macro-v1-3-50c5aab3c211@garyguo.net> References: <20260811-vtable_macro-v1-0-50c5aab3c211@garyguo.net> In-Reply-To: <20260811-vtable_macro-v1-0-50c5aab3c211@garyguo.net> To: Miguel Ojeda , Boqun Feng , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , =?utf-8?q?Onur_=C3=96zkan?= , Arnd Bergmann , Greg Kroah-Hartman Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, Gary Guo X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1786456560; l=3150; i=gary@garyguo.net; s=20221204; h=from:subject:message-id; bh=DGqeYGJbOZC5QOnNInNezUiW6itP8noeWPvF08pVCd0=; b=C0QKnyiM+sMj7LXJKmLmL2niz4dMVDPx5ksoVzv+C4RFUtP16caUTc4yOqaP4zSuY0sf1tc7Z 08M2PgGAc7XDT4S98eGMBvte63CCw8Lw9PcLdC2qgCHw4SHHtQ8KKN1 X-Developer-Key: i=gary@garyguo.net; 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Signed-off-by: Gary Guo --- rust/kernel/miscdevice.rs | 35 +++++++++++++---------------------- 1 file changed, 13 insertions(+), 22 deletions(-) diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs index 3abef1b8543d..76cb45e2a5ee 100644 --- a/rust/kernel/miscdevice.rs +++ b/rust/kernel/miscdevice.rs @@ -11,10 +11,7 @@ use crate::{ bindings, device::Device, - error::{ - to_result, - VTABLE_DEFAULT_ERROR, // - }, + error::to_result, fs::{ File, Kiocb, // @@ -144,37 +141,33 @@ fn release(device: Self::Ptr, _file: &File) { /// the VMA initialization by calling methods of `vma`. If the functio= n does not return an /// error, the kernel will complete initialization of the VMA accordin= g to the properties of /// `vma`. + #[optional] fn mmap( _device: ::Borrowed<'_>, _file: &File, _vma: &VmaNew, - ) -> Result { - build_error!(VTABLE_DEFAULT_ERROR) - } + ) -> Result; =20 /// Read from this miscdevice. - fn read_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterDest<'_>)= -> Result { - build_error!(VTABLE_DEFAULT_ERROR) - } + #[optional] + fn read_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterDest<'_>)= -> Result; =20 /// Write to this miscdevice. - fn write_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterSource<'= _>) -> Result { - build_error!(VTABLE_DEFAULT_ERROR) - } + #[optional] + fn write_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterSource<'= _>) -> Result; =20 /// Handler for ioctls. /// /// The `cmd` argument is usually manipulated using the utilities in [= `kernel::ioctl`]. /// /// [`kernel::ioctl`]: mod@crate::ioctl + #[optional] fn ioctl( _device: ::Borrowed<'_>, _file: &File, _cmd: u32, _arg: usize, - ) -> Result { - build_error!(VTABLE_DEFAULT_ERROR) - } + ) -> Result; =20 /// Handler for ioctls. /// @@ -184,23 +177,21 @@ fn ioctl( /// that have different layout on 32-bit and 64-bit userspace. If no i= mplementation is /// provided, then `compat_ptr_ioctl` will be used instead. #[cfg(CONFIG_COMPAT)] + #[optional] fn compat_ioctl( _device: ::Borrowed<'_>, _file: &File, _cmd: u32, _arg: usize, - ) -> Result { - build_error!(VTABLE_DEFAULT_ERROR) - } + ) -> Result; =20 /// Show info for this fd. + #[optional] fn show_fdinfo( _device: ::Borrowed<'_>, _m: &SeqFile, _file: &File, - ) { - build_error!(VTABLE_DEFAULT_ERROR) - } + ); } =20 /// A vtable for the file operations of a Rust miscdevice. --=20 2.54.0