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[2620:137:e000::3:2]) by mx.google.com with ESMTPS id k138-20020a252490000000b00d81755db936si223174ybk.240.2023.10.26.13.20.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Oct 2023 13:20:18 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:2 as permitted sender) client-ip=2620:137:e000::3:2; Authentication-Results: mx.google.com; dkim=pass header.i=@proton.me header.s=protonmail header.b=dGbPzAOc; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:2 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=proton.me Received: from out1.vger.email (depot.vger.email [IPv6:2620:137:e000::3:0]) by agentk.vger.email (Postfix) with ESMTP id EA4E281C00DA; Thu, 26 Oct 2023 13:20:14 -0700 (PDT) X-Virus-Status: Clean X-Virus-Scanned: clamav-milter 0.103.10 at agentk.vger.email Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344818AbjJZUUA (ORCPT + 26 others); Thu, 26 Oct 2023 16:20:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55514 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231971AbjJZUT7 (ORCPT ); Thu, 26 Oct 2023 16:19:59 -0400 Received: from mail-40131.protonmail.ch (mail-40131.protonmail.ch [185.70.40.131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6DEC31AC for ; Thu, 26 Oct 2023 13:19:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=proton.me; s=protonmail; t=1698351593; x=1698610793; bh=MY6UhJS4wgOAoosPta+TuxtbXCsYhD59Xf2pfqIYj2s=; h=Date:To:From:Cc:Subject:Message-ID:Feedback-ID:From:To:Cc:Date: Subject:Reply-To:Feedback-ID:Message-ID:BIMI-Selector; b=dGbPzAOcCpW+TbJACsMXT18aTk1rLFR5+rb05l3gHLPdhgtMrVbqZVRZ50zxXpY2o B7mIbgbT0foKPJ/yVeAjyepHdQbl8FwlcawjHZ3G9k940FIvLuXSVKIGt+Ya41Rnfl hclu30YXG9ikEA3d98cRg+N9ZFlE5ULWNGl/2Azr+CspZVqq8ncu3jmlU4DsNHvc4B sDvr5sJzuAsXe2dfSTsitxxpvm5+FVKPGITNVgASvMAjGcl5v/UdRYDj16Yh2FESpP Jh3bz3+/HbJvj674iAU5dmrPCQEUJlRyWlHFFGEl+TWqTpxsOiSXaX52w9YrVY40lo uJwBTi5C/5K7g== Date: Thu, 26 Oct 2023 20:19:33 +0000 To: Miguel Ojeda , Wedson Almeida Filho , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Martin Rodriguez Reboredo , Asahi Lina , Sven Van Asbroeck , Viktor Garske , Finn Behrens From: Benno Lossin Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v3] rust: macros: improve `#[vtable]` documentation Message-ID: <20231026201855.1497680-1-benno.lossin@proton.me> Feedback-ID: 71780778:user:proton MIME-Version: 1.0 X-Spam-Status: No, score=-0.8 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on agentk.vger.email Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (agentk.vger.email [0.0.0.0]); Thu, 26 Oct 2023 13:20:15 -0700 (PDT) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1780850746513897197 X-GMAIL-MSGID: 1780850746513897197 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: Martin Rodriguez Reboredo Reviewed-by: Finn Behrens Reviewed-by: Alice Ryhl Reviewed-by: Andreas Hindborg --- v2 -> v3: - don't hide the import of the constant in the example - fixed "function" typo - improve paragraph about optional functions - do not remove the `Send + Sync + Sized` bounds on the example v1 -> v2: - removed imperative mode in the paragraph describing optional functions. rust/kernel/error.rs | 4 ++++ rust/macros/lib.rs | 37 ++++++++++++++++++++++++++++++------- 2 files changed, 34 insertions(+), 7 deletions(-) base-commit: 3857af38e57a80b15b994e19d1f4301cac796481 diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs index 05fcab6abfe6..1373cde025ef 100644 --- a/rust/kernel/error.rs +++ b/rust/kernel/error.rs @@ -335,3 +335,7 @@ pub(crate) fn from_result(f: F) -> T 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 c42105c2ff96..917a51183c23 100644 --- a/rust/macros/lib.rs +++ b/rust/macros/lib.rs @@ -87,27 +87,48 @@ 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 trait method to be optional, it must have a default implementation. +/// This is also the case for traits annotated with `#[vtable]`, but in this +/// case the default implementation will never be executed. The reason for this +/// is that the functions will be called through function pointers installed in +/// C side vtables. When an optional method is not implemented on a `#[vtable]` +/// 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 `kernel::build_error`, which prevents +/// calls to this function at compile time: +/// +/// ```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 functions are required. /// /// # Examples /// /// ```ignore +/// use kernel::error::VTABLE_DEFAULT_ERROR; /// use kernel::prelude::*; /// /// // Declares a `#[vtable]` trait /// #[vtable] /// pub trait Operations: Send + Sync + Sized { /// 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 +146,8 @@ 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)