[PATCH v4 19/19] Documentation/kbuild: Add DWARF module versioning

Sami Tolvanen
2024-10-08T18:39:12Z

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 <samitolvanen@google.com>
Acked-by: Neal Gompa <neal@gompa.dev>
---
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#<type> = enum
+ s#<type> = struct
+ t#<type> = typedef
+ u#<type> = union
+
+Type names with spaces in them are wrapped in single quotes, e.g.::
+
+ s#'core::result::Result<u8, core::num::error::ParseIntError>'
+
+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