From f347fa1e02df4f1a0de24fd451df509287f66f33 Mon Sep 17 00:00:00 2001 From: Benno Lossin Date: Thu, 19 Oct 2023 17:15:53 +0000 Subject: [PATCH] rust: macros: improve `#[vtable]` documentation Traits marked with `#[vtable]` need to provide default implementations for optional functions. The C side represents these with `NULL` in the vtable, so the default functions are never actually called. We do not want to replicate the default behavior from C in Rust, because that is not maintainable. Therefore we should use `build_error` in those default implementations. The error message for that is provided at `kernel::error::VTABLE_DEFAULT_ERROR`. Signed-off-by: Benno Lossin Reviewed-by: Alice Ryhl Reviewed-by: Ariel Miculas Link: https://lore.kernel.org/r/20231019171540.259173-1-benno.lossin@proton.me --- rust/kernel/error.rs | 4 ++++ rust/macros/lib.rs | 32 ++++++++++++++++++++++++-------- 2 files changed, 28 insertions(+), 8 deletions(-) diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs index 05fcab6abfe63e..1373cde025ef79 100644 --- a/rust/kernel/error.rs +++ b/rust/kernel/error.rs @@ -335,3 +335,7 @@ where Err(e) => T::from(e.to_errno() as i16), } } + +/// Error message for calling a default function of a [`#[vtable]`](macros::vtable) trait. +pub const VTABLE_DEFAULT_ERROR: &str = + "This function must not be called, see the #[vtable] documentation."; diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs index c42105c2ff9635..daf1ef8baa6210 100644 --- a/rust/macros/lib.rs +++ b/rust/macros/lib.rs @@ -87,27 +87,41 @@ pub fn module(ts: TokenStream) -> TokenStream { /// 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 overridden a method. +/// This attribute closes that gap. A trait can be annotated with the `#[vtable]` attribute. +/// Implementers of the trait will then also have to annotate the trait with `#[vtable]`. This +/// attribute generates a `HAS_*` associated constant bool for each method in the trait that is set +/// to true if the implementer has overridden the associated method. +/// +/// For a function to be optional, it must have a default implementation. But this default +/// implementation will never be executed, since these functions are exclusively called from +/// callbacks from the C side. This is because the vtable will have a `NULL` entry and the C side +/// will execute the default behavior. Since it is not maintainable to replicate the default +/// behavior in Rust, the default implementation should be: +/// +/// ```compile_fail +/// # use kernel::error::VTABLE_DEFAULT_ERROR; +/// kernel::build_error(VTABLE_DEFAULT_ERROR) +/// ``` +/// +/// note that you might need to import [`kernel::error::VTABLE_DEFAULT_ERROR`]. /// -/// This attribute is not needed if all methods are required. +/// This macro should not be used when all function are required. /// /// # Examples /// /// ```ignore +/// # use kernel::error::VTABLE_DEFAULT_ERROR; /// use kernel::prelude::*; /// /// // Declares a `#[vtable]` trait /// #[vtable] -/// pub trait Operations: Send + Sync + Sized { +/// pub trait Operations { /// fn foo(&self) -> Result<()> { -/// Err(EINVAL) +/// kernel::build_error(VTABLE_DEFAULT_ERROR) /// } /// /// fn bar(&self) -> Result<()> { -/// Err(EINVAL) +/// kernel::build_error(VTABLE_DEFAULT_ERROR) /// } /// } /// @@ -125,6 +139,8 @@ pub fn module(ts: TokenStream) -> TokenStream { /// assert_eq!(::HAS_FOO, true); /// assert_eq!(::HAS_BAR, false); /// ``` +/// +/// [`kernel::error::VTABLE_DEFAULT_ERROR`]: ../kernel/error/constant.VTABLE_DEFAULT_ERROR.html #[proc_macro_attribute] pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream { vtable::vtable(attr, ts)