systemd/docs/ELF_DLOPEN_METADATA.md

title	category	layout	SPDX-License-Identifier
Dlopen Metadata for ELF Files	Interfaces	default	LGPL-2.1-or-later

dlopen() Metadata for ELF Files

Intended audience: hackers working on packaging ELF files that use dlopen to load libraries.

Motivation

Using dlopen() to load optional dependencies brings several advantages: programs can gracefully downgrade a feature when a library is not available, and the shared library is only loaded into the process (and its ELF constructors are run) only when the requested feature is actually used. But it also has some drawbacks, and the main one is that it is harder to track a program's dependencies, since unlike build-time dynamic linking there will not be a mention in the ELF metadata. This specification aims to solve this problem by providing a standardized specification for a custom ELF note that can be used to list dlopen() dependencies.

Implementation

This document will attempt to define a common metadata format specification, so that multiple implementers might use it when coding upstream software, and packagers might use it when building packages and setting dependencies.

The metadata will be embedded in a series of new, 4-byte-aligned, allocated, 0-padded, read-only ELF header sections, in a JSON array containing name-value objects, either one ELF note per dependency or as a single note listing multiple dependencies in the top-level array. Implementers working on parsing ELF files should not assume a specific list of names, but parse anything that is included in the section, and should look for the note using the note type. Implementers working on build tools should strive to use the same names, for consistency. The most common will be listed here.

• Section header

SECTION: `.note.dlopen`
note type: `0x407c0c0a`
Owner: `FDO` (FreeDesktop.org)
Value: an array of JSON objects encoded as a zero-terminated UTF-8 string

• JSON payload

[
    {
        "soname":      ["libfoo.so.1"],
        "feature":     "foo",
        "description": "Enables the foo feature",
        "priority":    "recommended"
    }
]

The format is a single JSON array containing objects, encoded as a zero-terminated UTF-8 string. Each key in each object shall be unique as per recommendations of RFC8259. Strings shall not contain any control characters or use \uXXX escaping.

Reference implementations of packaging tools for .deb and .rpm are available, and provide macros/helpers to parse the note when building packages and adding dependencies.

Well-known keys

The metadata format is intentionally extensible, so that upstreams and later revisions of this spec can add their own information. The 'soname' array is required, with at least one element, everything else is optional. If alternative soname versions for the same library are supported at the same time, an array can be used, listing the most preferred first, and parsers are expected to select only the first one that is available on the system, as it is a mechanism to specify alternatives. If the priority field is used, it must follow the specification and use one of the values specified in the table. If it is not specified, a parser should assume 'recommended' if a priority is needed. If the feature field is used, it will identify an individual feature, and multiple entries using the same feature denote functionality that requires all of the libraries they specify in order to be enabled.

Key name	Key type	Mandatory	Key description	Example value
soname	array of strings	yes	The library names loaded by dlopen()	[ "libfoo.so.1", "libfoo.so.0" ]
feature	string	no	A keyword identifying the feature that the library contributes to enable	"foo"
description	string	no	A human-readable text string describing the feature	"Enables the foo feature"
priority	string	no	The priority of the feature, one of: required, recommended, suggested	"recommended"

Priority definition

Priority	Semantics
required	Core functionality needs the dependency, the binary will not work if it cannot be found
recommended	Important functionality needs the dependency, the binary will work but in most cases the dependency should be provided
suggested	Secondary functionality needs the dependency, the binary will work and the dependency is only needed for full-featured installations