Documentation/kbuild: Add DWARF module versioning

=======================

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. To generate

symtypes files during a kernel build, set `KBUILD_SYMTYPES=1`.

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. To use stable features during a kernel

build, set `KBUILD_GENDWARFKSYMS_STABLE=1`.

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 definition visibility

=====================================

A 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 type even if the ABI remains unchanged. As

it may not be possible to drop includes without breaking the build, the

`declonly` rule can be used to specify a type as declaration-only, even

if the debugging information contains the full definition.

The rule fields are expected to be as follows:

- `type`: "declonly"

- `target`: The fully qualified name of the target data structure

  (as shown in **--dump-dies** output).

- `value`: This field is ignored.

Using the `__KABI_RULE` macro, this rule can be defined as::

	#define KABI_DECLONLY(fqn) __KABI_RULE(declonly, fqn, )

Example usage::

	struct s {

		/* definition */

	};

	KABI_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) and the name of the

  enumerator field separated by a space.

- `value`: This field is ignored.

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);

If the enum additionally includes an end marker and new values must

be added in the middle, we may need to use the old value for the last

enumerator when calculating versions. The `enumerator_value` rule allows

us to override the value of an enumerator for version calculation:

- `type`: "enumerator_value"

- `target`: The fully qualified name of the target enum

  (as shown in **--dump-dies** output) and the name of the

  enumerator field separated by a space.

- `value`: Integer value used for the field.

Using the `__KABI_RULE` macro, this rule can be defined as::

	#define KABI_ENUMERATOR_VALUE(fqn, field, value) \

		__KABI_RULE(enumerator_value, fqn field, value)

Example usage::

	enum e {

		A, B, C, LAST,

	};

	KABI_ENUMERATOR_IGNORE(e, C);

	KABI_ENUMERATOR_VALUE(e, LAST, 2);

4.3. Adding structure members

=============================

Perhaps the most common ABI compatible change is 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

============================================

Space is typically reserved for later use by appending integer types, or

arrays, to the end of the data structure, but any type can be used. Each

reserved member needs a unique name, but as the actual purpose is usually

not known at the time the space is reserved, for convenience, names that

start with `__kabi_` are left out when calculating symbol versions::

        struct s {

                long a;

                long __kabi_reserved_0; /* reserved for future use */

        };

The reserved space can be taken into use by wrapping the member in a

union, which includes the original type and the replacement member::

        struct s {

                long a;

                union {

                        long __kabi_reserved_0; /* original type */

                        struct b b; /* replaced field */

                };

        };

If the `__kabi_` naming scheme was used when reserving space, the name

of the first member of the union must start with `__kabi_reserved`. This

ensures the original type is used when calculating versions, but the name

is again left out. The rest of the union is ignored.

If we're replacing a member that doesn't follow this naming convention,

we also need to preserve the original name to avoid changing versions,

which we can do by changing the first union member's name to start with

`__kabi_renamed` followed by the original name.

The examples include `KABI_(RESERVE|USE|REPLACE)*` macros that help

simplify the process and also ensure the replacement member is correctly

aligned and its 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.

    reproducible-builds

    gcc-plugins

    llvm

    gendwarfksyms

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