From patchwork Mon Sep 23 18:19:06 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sami Tolvanen X-Patchwork-Id: 13809923 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9F2001A4F36 for ; Mon, 23 Sep 2024 18:19:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727115585; cv=none; b=ju5Z0YsHezh9aHdFU9G+xcwTOLIruIdkWFsgIUnw3/a+Gk7FNTrJgrvrw7w5Q/6MFvteNj14ryaGSnS/M9cvhQ7WtWTJSbvD/zFDAuhxPJW/MT1PnU5yRzAM85Ez2Ml+eaI4Jdb0IIgpyS2kSPYqJni+8wxfTAU1PlO7Q2qACFI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727115585; c=relaxed/simple; bh=MUvyVDOVnoZUdt93DJbmEjcHlGHywIDAJExoIyBJwTc=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=EyLGkwUgeVnAad+WXFPK4b/zFh1fQw+oIDd2zYoqntnSoHVdVKlEfQLyiD+yF/nNJqESHzhm4mBKIEpRqDkoEo1YLwwPHpS1DbXoD9m5HG7DupcV3SYNyoEKDO+Go/HkigxrbNfBt/IBX+WX5GVXVJiUx1lAJSpIXs68jGm1t3g= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--samitolvanen.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=nPTJd6au; arc=none smtp.client-ip=209.85.128.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--samitolvanen.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="nPTJd6au" Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-6e1fbe2a9a2so15592887b3.1 for ; Mon, 23 Sep 2024 11:19:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1727115582; x=1727720382; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=LIeNJExclvncrOtPvfgf/zFSBPEGc6h5JAKcbMZPgWU=; b=nPTJd6auPw5uWBT+1NZ2Qr+KeCYyBTCENVaVKqEi4cw4jtR9GbbGsgNgVnfsvWKxi3 uYHOcO/8ry9UgJwHIeNNvzbfASJL4iutgp/+MH/rSxfy7VOqtaGHC5A+52Vc40gvlSZx CHjDDR1uHFqhC7oikBKkwYKy7EzbkKEuGNFd9AGn1Fv/VH652cwowyp6N37Ewk3XhNDK alNvbD7e74f3QNsogumpxrmDF+Gci+djYeVAsTEbcoBHE+iwbhVgatkoB8JkzqMrhURE 2gCYmHgEypyZ90Cv6izAlZdAckRtuzhyQxRgGHBBuXBWYciwtPlNCsngbkG8VV8nZMoL 3iaQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1727115582; x=1727720382; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=LIeNJExclvncrOtPvfgf/zFSBPEGc6h5JAKcbMZPgWU=; b=dZNzGZoXslCWKEsYJaJkWVE3LPFvugGXueKhnQUBKNKY9pd03puVzaHIrZlhd2UxcC L9PutHlY5Og51rAg9fHizOQ1y5k0t9KXmC7tiIbOG9n63yMNrOh26GwGb5TD5tfL5nZV Bo8FHGyjsVnQMu2npk5Ga7hOJgX4JdzmtCPOzr3avTUiwOO33TW5wXivbkzbUDOWPZqO Fypb6ZCoNAsfmnyLzYyABSdUZtO/lC803VQReGjnVSZq9DEFTfP8qn/Msn4u6+L1ET4i ooxfDKt6DgX8rOn28Fkovh574bFcPNUjYcZ+oUH/4rskBO/7lFjL7ltvhiutUqqvv/cI /F8w== X-Forwarded-Encrypted: i=1; AJvYcCV/qSGG4EUZcZLlxM3wVVKKUlIFpK/tjKcQwQIeJSsqiPMwtQWQY2szgUIHnKtu86QUB3bQHA36CmG36GWz@vger.kernel.org X-Gm-Message-State: AOJu0YxXzy+CYAbqvzoTTb//QaXJfr7y6j4WLBXLFaZxy1+DXOgDvl20 GLZT1yU9qjDszJ+7543XqXo99n6o7+O22eUfIh5cb9xTCc5RDVHui4sustebbJlosiDGfDANy97 yXlKdWlk4uBLBMBwT28Oo86bPQQ== X-Google-Smtp-Source: AGHT+IGn5NLkaYtJf4zx1Oj6VLn9yvEjY3yp8FW/8aCXhpJ0JMKN//NTWxYZpRpKz1KNy1nm96hS6Hsq1+IESPg824U= X-Received: from samitolvanen.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:4f92]) (user=samitolvanen job=sendgmr) by 2002:a81:a704:0:b0:6db:c6eb:bae9 with SMTP id 00721157ae682-6dff270d59bmr1849607b3.2.1727115581692; Mon, 23 Sep 2024 11:19:41 -0700 (PDT) Date: Mon, 23 Sep 2024 18:19:06 +0000 In-Reply-To: <20240923181846.549877-22-samitolvanen@google.com> Precedence: bulk X-Mailing-List: linux-modules@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240923181846.549877-22-samitolvanen@google.com> X-Developer-Key: i=samitolvanen@google.com; a=openpgp; fpr=35CCFB63B283D6D3AEB783944CB5F6848BBC56EE X-Developer-Signature: v=1; a=openpgp-sha256; l=11935; i=samitolvanen@google.com; h=from:subject; bh=MUvyVDOVnoZUdt93DJbmEjcHlGHywIDAJExoIyBJwTc=; b=owGbwMvMwCEWxa662nLh8irG02pJDGkfNwq0Kpa5t5WHuQfeK867smrOH7WfMx615Xc9cWI/0 nJk7U62jlIWBjEOBlkxRZaWr6u37v7ulPrqc5EEzBxWJpAhDFycAjCRfysZGXqF3h5okQ0XnzJd XO2QbLnqPp7e67f0XmxWXJO+MWTdxycM/3QUre208y5l+cxZzrFqk8Xq4MllL5aEetl6GBjGZfR v4wYA X-Mailer: git-send-email 2.46.0.792.g87dc391469-goog Message-ID: <20240923181846.549877-42-samitolvanen@google.com> Subject: [PATCH v3 20/20] Documentation/kbuild: Add DWARF module versioning From: Sami Tolvanen To: Masahiro Yamada , Luis Chamberlain , Miguel Ojeda , Greg Kroah-Hartman Cc: Matthew Maurer , Alex Gaynor , Wedson Almeida Filho , Gary Guo , Petr Pavlu , Neal Gompa , Hector Martin , Janne Grunau , Miroslav Benes , Asahi Linux , linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-modules@vger.kernel.org, rust-for-linux@vger.kernel.org, Sami Tolvanen Add documentation for gendwarfksyms changes, and the kABI stability features that can be useful for distributions even though they're not used in mainline kernels. Signed-off-by: Sami Tolvanen --- Documentation/kbuild/gendwarfksyms.rst | 274 +++++++++++++++++++++++++ Documentation/kbuild/index.rst | 1 + 2 files changed, 275 insertions(+) create mode 100644 Documentation/kbuild/gendwarfksyms.rst diff --git a/Documentation/kbuild/gendwarfksyms.rst b/Documentation/kbuild/gendwarfksyms.rst new file mode 100644 index 000000000000..4b89743d2a88 --- /dev/null +++ b/Documentation/kbuild/gendwarfksyms.rst @@ -0,0 +1,274 @@ +======================= +DWARF module versioning +======================= + +1. Introduction +=============== + +When CONFIG_MODVERSIONS is enabled, symbol versions for modules +are typically calculated from preprocessed source code using the +**genksyms** tool. However, this is incompatible with languages such +as Rust, where the source code has insufficient information about +the resulting ABI. With CONFIG_GENDWARFKSYMS (and CONFIG_DEBUG_INFO) +selected, **gendwarfksyms** is used instead to calculate symbol versions +from the DWARF debugging information, which contains the necessary +details about the final module ABI. + +1.1. Usage +========== + +gendwarfksyms accepts a list of object files on the command line, and a +list of symbol names (one per line) in standard input:: + + Usage: gendwarfksyms [options] elf-object-file ... < symbol-list + + Options: + -d, --debug Print debugging information + --dump-dies Dump DWARF DIE contents + --dump-die-map Print debugging information about die_map changes + --dump-types Dump type strings + --dump-versions Dump expanded type strings used for symbol versions + -s, --stable Support kABI stability features + -T, --symtypes file Write a symtypes file + -h, --help Print this message + + +2. Type information availability +================================ + +While symbols are typically exported in the same translation unit (TU) +where they're defined, it's also perfectly fine for a TU to export +external symbols. For example, this is done when calculating symbol +versions for exports in stand-alone assembly code. + +To ensure the compiler emits the necessary DWARF type information in the +TU where symbols are actually exported, gendwarfksyms adds a pointer +to exported symbols in the `EXPORT_SYMBOL()` macro using the following +macro:: + + #define __GENDWARFKSYMS_EXPORT(sym) \ + static typeof(sym) *__gendwarfksyms_ptr_##sym __used \ + __section(".discard.gendwarfksyms") = &sym; + + +When a symbol pointer is found in DWARF, gendwarfksyms can use its +type for calculating symbol versions even if the symbol is defined +elsewhere. The name of the symbol pointer is expected to start with +`__gendwarfksyms_ptr_`, followed by the name of the exported symbol. + +3. Symtypes output format +========================= + +Similarly to genksyms, gendwarfksyms supports writing a symtypes file +for each processed object that contain types for exported symbols and +each referenced type that was used in calculating symbol versions. These +files can be useful when trying to determine what exactly caused symbol +versions to change between builds. + +Matching the existing format, the first column of each line contains +either a type reference or a symbol name. Type references have a +one-letter prefix followed by "#" and the name of the type. Four +reference types are supported:: + + e# = enum + s# = struct + t# = typedef + u# = union + +Type names with spaces in them are wrapped in single quotes, e.g.:: + + s#'core::result::Result' + +The rest of the line contains a type string. Unlike with genksyms that +produces C-style type strings, gendwarfksyms uses the same simple parsed +DWARF format produced by **--dump-dies**, but with type references +instead of fully expanded strings. + +4. Maintaining a stable kABI +============================ + +Distribution maintainers often need the ability to make ABI compatible +changes to kernel data structures due to LTS updates or backports. Using +the traditional `#ifndef __GENKSYMS__` to hide these changes from symbol +versioning won't work when processing object files. To support this +use case, gendwarfksyms provides kABI stability features designed to +hide changes that won't affect the ABI when calculating versions. These +features are all gated behind the **--stable** command line flag and are +not used in the mainline kernel. + +Examples for using these features are provided in the +**scripts/gendwarfksyms/examples** directory, including helper macros +for source code annotation. Note that as these features are only used to +transform the inputs for symbol versioning, the user is responsible for +ensuring that their changes actually won't break the ABI. + +4.1. kABI rules +=============== + +kABI rules allow distributions to fine-tune certain parts +of gendwarfksyms output and thus control how symbol +versions are calculated. These rules are defined in the +`.discard.gendwarfksyms.kabi_rules` section of the object file and +consist of simple null-terminated strings with the following structure:: + + version\0type\0target\0value\0 + +This string sequence is repeated as many times as needed to express all +the rules. The fields are as follows: + +- `version`: Ensures backward compatibility for future changes to the + structure. Currently expected to be "1". +- `type`: Indicates the type of rule being applied. +- `target`: Specifies the target of the rule, typically the fully + qualified name of the DWARF Debugging Information Entry (DIE). +- `value`: Provides rule-specific data. + +The following helper macro, for example, can be used to specify rules +in the source code:: + + #define __KABI_RULE(hint, target, value) \ + static const char __PASTE(__gendwarfksyms_rule_, \ + __COUNTER__)[] __used __aligned(1) \ + __section(".discard.gendwarfksyms.kabi_rules") = \ + "1\0" #hint "\0" #target "\0" #value + + +Currently, only the rules discussed in this section are supported, but +the format is extensible enough to allow further rules to be added as +need arises. + +4.1.1. Managing structure visibility +==================================== + +A structure declaration can change into a full definition when +additional includes are pulled into the translation unit. This changes +the versions of any symbol that references the structure even if the ABI +remains unchanged. As it may not be possible to drop includes without +breaking the build, the `struct_declonly` rule can be used to specify a +data structure as declaration-only, even if the debugging information +contains the full definition. + +The rule fields are expected to be as follows: + +- `type`: "struct_declonly" +- `target`: The fully qualified name of the target data structure + (as shown in **--dump-dies** output). +- `value`: This field is ignored and is expected to have the value ";". + +Using the `__KABI_RULE` macro, this rule can be defined as:: + + #define KABI_STRUCT_DECLONLY(fqn) \ + __KABI_RULE(struct_declonly, fqn, ;) + +Example usage:: + + struct s { + /* definition */ + }; + + KABI_STRUCT_DECLONLY(s); + +4.1.2. Adding enumerators +========================= + +For enums, all enumerators and their values are included in calculating +symbol versions, which becomes a problem if we later need to add more +enumerators without changing symbol versions. The `enumerator_ignore` +rule allows us to hide named enumerators from the input. + +The rule fields are expected to be as follows: + +- `type`: "enumerator_ignore" +- `target`: The fully qualified name of the target enum + (as shown in **--dump-dies** output). +- `value`: The name of the enumerator to ignore. + +Using the `__KABI_RULE` macro, this rule can be defined as:: + + #define KABI_ENUMERATOR_IGNORE(fqn, field) \ + __KABI_RULE(enumerator_ignore, fqn, field) + +Example usage:: + + enum e { + A, B, C, D, + }; + + KABI_ENUMERATOR_IGNORE(e, B); + KABI_ENUMERATOR_IGNORE(e, C); + + +4.3. Adding structure members +============================= + +Perhaps the most common ABI compatible changeis adding a member to a +kernel data structure. When changes to a structure are anticipated, +distribution maintainers can pre-emptively reserve space in the +structure and take it into use later without breaking the ABI. If +changes are needed to data structures without reserved space, existing +alignment holes can potentially be used instead. While kABI rules could +be added for these type of changes, using unions is typically a more +natural method. This section describes gendwarfksyms support for using +reserved space in data structures and hiding members that don't change +the ABI when calculating symbol versions. + +4.3.1. Reserving space and replacing members +============================================ + +To reserve space in a struct, adding a member of any type with a name +that starts with `__kabi_` will result in the name being left out of +symbol versioning:: + + struct s { + long a; + long __kabi_reserved_0; /* reserved for future use */ + }; + +The space reserved by this member can be later taken into use by +wrapping it into a union, which includes the original type and the +replacement struct member:: + + struct s { + long a; + union { + long __kabi_reserved_0; /* original type */ + struct b b; /* replaced field */ + }; + }; + +As long as the reserved member's name in the union starts with +`__kabi_reserved_`, the original type will be used for symbol +versioning and rest of the union is ignored. The examples include +`KABI_(RESERVE|USE)*` macros that help simplify the process and also +ensure the replacement member's size won't exceed the reserved space. + +4.3.2. Hiding members +===================== + +Predicting which structures will require changes during the support +timeframe isn't always possible, in which case one might have to resort +to placing new members into existing alignment holes:: + + struct s { + int a; + /* a 4-byte alignment hole */ + unsigned long b; + }; + + +While this won't change the size of the data structure, one needs to +be able to hide the added members from symbol versioning. Similarly +to reserved fields, this can be accomplished by wrapping the added +member to a union where one of the fields has a name starting with +`__kabi_ignored`:: + + struct s { + int a; + union { + char __kabi_ignored_0; + int n; + }; + unsigned long b; + }; + +With **--stable**, both versions produce the same symbol version. diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst index cee2f99f734b..e82af05cd652 100644 --- a/Documentation/kbuild/index.rst +++ b/Documentation/kbuild/index.rst @@ -21,6 +21,7 @@ Kernel Build System reproducible-builds gcc-plugins llvm + gendwarfksyms .. only:: subproject and html