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diff --git a/docs/ELF_PACKAGE_METADATA.md b/docs/ELF_PACKAGE_METADATA.md new file mode 100644 index 0000000..6cb3f78 --- /dev/null +++ b/docs/ELF_PACKAGE_METADATA.md @@ -0,0 +1,105 @@ +--- +title: Package Metadata for ELF Files +category: Interfaces +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# Package Metadata for Core Files + +*Intended audience: hackers working on userspace subsystems that create ELF binaries +or parse ELF core files.* + +## Motivation + +ELF binaries get stamped with a unique, build-time generated hex string identifier called +`build-id`, [which gets embedded as an ELF note called `.note.gnu.build-id`](https://fedoraproject.org/wiki/Releases/FeatureBuildId). +In most cases, this allows to associate a stripped binary with its debugging information. +It is used, for example, to dynamically fetch DWARF symbols from a debuginfo server, or +to query the local package manager and find out the package metadata or, again, the DWARF +symbols or program sources. + +However, this usage of the `build-id` requires either local metadata, usually set up by +the package manager, or access to a remote server over the network. Both of those might +be unavailable or forbidden. + +Thus it becomes desirable to add additional metadata to a binary at build time, so that +`systemd-coredump` and other services analyzing core files are able to extract said +metadata simply from the core file itself, without external dependencies. + +## Implementation + +This document will attempt to define a common metadata format specification, so that +multiple implementers might use it when building packages, or core file analyzers, and +so on. + +The metadata will be embedded in a single, new, 4-bytes-aligned, allocated, 0-padded, +read-only ELF header section, in a name-value JSON object format. Implementers working on parsing +core 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. When corresponding to the content of os-release, the values should match, again for consistency. + +If available, the metadata should also include the debuginfod server URL that can provide +the original executable, debuginfo and sources, to further facilitate debugging. + +* Section header + +``` +SECTION: `.note.package` +note type: `0xcafe1a7e` +Owner: `FDO` (FreeDesktop.org) +Value: a single JSON object encoded as a zero-terminated UTF-8 string +``` + +* JSON payload + +```json +{ + "type":"rpm", # this provides a namespace for the package+package-version fields + "os":"fedora", + "osVersion":"33", + "name":"coreutils", + "version":"4711.0815.fc13", + "architecture":"arm32", + "osCpe": "cpe:/o:fedoraproject:fedora:33", # A CPE name for the operating system, `CPE_NAME` from os-release is a good default + "debugInfoUrl": "https://debuginfod.fedoraproject.org/" +} +``` + +The format is a single JSON object, encoded as a zero-terminated `UTF-8` string. +Each name in the object shall be unique as per recommendations of +[RFC8259](https://datatracker.ietf.org/doc/html/rfc8259#section-4). Strings shall +not contain any control character, nor use `\uXXX` escaping. + +When it comes to JSON numbers, this specification assumes that JSON parsers +processing this information are capable of reproducing the full signed 53bit +integer range (i.e. -2⁵³+1…+2⁵³-1) as well as the full 64-bit IEEE floating +point number range losslessly (with the exception of NaN/-inf/+inf, since JSON +cannot encode that), as per recommendations of +[RFC8259](https://datatracker.ietf.org/doc/html/rfc8259#page-8). Fields in +these JSON objects are thus permitted to encode numeric values from these +ranges as JSON numbers, and should not use numeric values not covered by these +types and ranges. + +Reference implementations of [packaging tools for .deb and .rpm](https://github.com/systemd/package-notes) +are available, and provide macros/helpers to include the note in binaries built +by the package build system. They make use of the new `--package-metadata` flag that +is available in the bfd, gold, mold and lld linkers (versions 2.39, 1.3.0 and 15.0 +respectively). This linker flag takes a JSON payload as parameter. + +## Well-known keys + +The metadata format is intentionally left open, so that vendors can add their own information. +A set of well-known keys is defined here, and hopefully shared among all vendors. + +| Key name | Key description | Example value | +|--------------|--------------------------------------------------------------------------|---------------------------------------| +| type | The packaging type | rpm | +| os | The OS name, typically corresponding to ID in os-release | fedora | +| osVersion | The OS version, typically corresponding to VERSION_ID in os-release | 33 | +| name | The source package name | coreutils | +| version | The source package version | 4711.0815.fc13 | +| architecture | The binary package architecture | arm32 | +| osCpe | A CPE name for the OS, typically corresponding to CPE_NAME in os-release | cpe:/o:fedoraproject:fedora:33 | +| debugInfoUrl | The debuginfod server url, if available | https://debuginfod.fedoraproject.org/ | |