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The format described by this document is used not only for coredumps, but also for other purposes, so we've outgrown the old title. A redirect was added based on https://codepo8.github.io/github-redirection-demo/. I tried to use a relative link, but it's hard to test if it works before merging. Co-authored-by: Benjamin Franzke <benjaminfranzke@googlemail.com>
150 lines
7.6 KiB
Markdown
150 lines
7.6 KiB
Markdown
---
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title: Package Metadata for ELF Files
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category: Interfaces
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# Package Metadata for Core Files
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*Intended audience: hackers working on userspace subsystems that create ELF binaries
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or parse ELF core files.*
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## Motivation
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ELF binaries get stamped with a unique, build-time generated hex string identifier called
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`build-id`, [which gets embedded as an ELF note called `.note.gnu.build-id`](https://fedoraproject.org/wiki/Releases/FeatureBuildId).
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In most cases, this allows to associate a stripped binary with its debugging information.
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It is used, for example, to dynamically fetch DWARF symbols from a debuginfo server, or
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to query the local package manager and find out the package metadata or, again, the DWARF
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symbols or program sources.
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However, this usage of the `build-id` requires either local metadata, usually set up by
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the package manager, or access to a remote server over the network. Both of those might
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be unavailable or forbidden.
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Thus it becomes desirable to add additional metadata to a binary at build time, so that
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`systemd-coredump` and other services analyzing core files are able to extract said
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metadata simply from the core file itself, without external dependencies.
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## Implementation
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This document will attempt to define a common metadata format specification, so that
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multiple implementers might use it when building packages, or core file analyzers, and
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so on.
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The metadata will be embedded in a single, new, 4-bytes-aligned, allocated, 0-padded,
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read-only ELF header section, in a name-value JSON object format. Implementers working on parsing
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core files should not assume a specific list of names, but parse anything that is included
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in the section, and should look for the note using the `note type`. Implementers working on
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build tools should strive to use the same names, for consistency. The most common will be
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listed here. When corresponding to the content of os-release, the values should match, again for consistency.
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If available, the metadata should also include the debuginfod server URL that can provide
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the original executable, debuginfo and sources, to further facilitate debugging.
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* Section header
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```
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SECTION: `.note.package`
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note type: `0xcafe1a7e`
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Owner: `FDO` (FreeDesktop.org)
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Value: a single JSON object encoded as a zero-terminated UTF-8 string
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```
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* JSON payload
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```json
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{
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"type":"rpm", # this provides a namespace for the package+package-version fields
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"os":"fedora",
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"osVersion":"33",
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"name":"coreutils",
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"version":"4711.0815.fc13",
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"architecture":"arm32",
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"osCpe": "cpe:/o:fedoraproject:fedora:33", # A CPE name for the operating system, `CPE_NAME` from os-release is a good default
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"debugInfoUrl": "https://debuginfod.fedoraproject.org/"
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}
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```
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The format is a single JSON object, encoded as a zero-terminated `UTF-8` string.
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Each name in the object shall be unique as per recommendations of
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[RFC8259](https://datatracker.ietf.org/doc/html/rfc8259#section-4). Strings shall
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not contain any control character, nor use `\uXXX` escaping.
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When it comes to JSON numbers, this specification assumes that JSON parsers
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processing this information are capable of reproducing the full signed 53bit
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integer range (i.e. -2⁵³+1…+2⁵³-1) as well as the full 64bit IEEE floating
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point number range losslessly (with the exception of NaN/-inf/+inf, since JSON
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cannot encode that), as per recommendations of
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[RFC8259](https://datatracker.ietf.org/doc/html/rfc8259#page-8). Fields in
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these JSON objects are thus permitted to encode numeric values from these
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ranges as JSON numbers, and should not use numeric values not covered by these
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types and ranges.
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A reference implementations of a [build-time tool is provided](https://github.com/systemd/package-notes)
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and can be used to generate a linker script, which can then be used at build time via
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```LDFLAGS="-Wl,-T,/path/to/generated/script"``` to include the note in the binary.
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Generator:
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```console
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$ ./generate-package-notes.py --rpm systemd-248~rc2-1.fc33.arm32 --cpe cpe:/o:fedoraproject:fedora:33
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SECTIONS
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{
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.note.package (READONLY) : ALIGN(4) {
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LONG(0x0004) /* Length of Owner including NUL */
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LONG(0x007b) /* Length of Value including NUL */
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LONG(0xcafe1a7e) /* Note ID */
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BYTE(0x46) BYTE(0x44) BYTE(0x4f) BYTE(0x00) /* Owner: 'FDO\x00' */
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BYTE(0x7b) BYTE(0x22) BYTE(0x74) BYTE(0x79) /* Value: '{"type":"rpm","name":"systemd","version":"248~rc2-1.fc33","architecture":"arm32","osCpe":"cpe:/o:fedoraproject:fedora:33"}\x00\x00' */
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BYTE(0x70) BYTE(0x65) BYTE(0x22) BYTE(0x3a)
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BYTE(0x22) BYTE(0x72) BYTE(0x70) BYTE(0x6d)
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BYTE(0x22) BYTE(0x2c) BYTE(0x22) BYTE(0x6e)
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BYTE(0x61) BYTE(0x6d) BYTE(0x65) BYTE(0x22)
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BYTE(0x3a) BYTE(0x22) BYTE(0x73) BYTE(0x79)
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BYTE(0x73) BYTE(0x74) BYTE(0x65) BYTE(0x6d)
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BYTE(0x64) BYTE(0x22) BYTE(0x2c) BYTE(0x22)
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BYTE(0x76) BYTE(0x65) BYTE(0x72) BYTE(0x73)
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BYTE(0x69) BYTE(0x6f) BYTE(0x6e) BYTE(0x22)
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BYTE(0x3a) BYTE(0x22) BYTE(0x32) BYTE(0x34)
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BYTE(0x38) BYTE(0x7e) BYTE(0x72) BYTE(0x63)
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BYTE(0x32) BYTE(0x2d) BYTE(0x31) BYTE(0x2e)
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BYTE(0x66) BYTE(0x63) BYTE(0x33) BYTE(0x33)
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BYTE(0x22) BYTE(0x2c) BYTE(0x22) BYTE(0x61)
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BYTE(0x72) BYTE(0x63) BYTE(0x68) BYTE(0x69)
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BYTE(0x74) BYTE(0x65) BYTE(0x63) BYTE(0x74)
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BYTE(0x75) BYTE(0x72) BYTE(0x65) BYTE(0x22)
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BYTE(0x3a) BYTE(0x22) BYTE(0x61) BYTE(0x72)
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BYTE(0x6d) BYTE(0x33) BYTE(0x32) BYTE(0x22)
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BYTE(0x2c) BYTE(0x22) BYTE(0x6f) BYTE(0x73)
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BYTE(0x43) BYTE(0x70) BYTE(0x65) BYTE(0x22)
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BYTE(0x3a) BYTE(0x22) BYTE(0x63) BYTE(0x70)
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BYTE(0x65) BYTE(0x3a) BYTE(0x2f) BYTE(0x6f)
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BYTE(0x3a) BYTE(0x66) BYTE(0x65) BYTE(0x64)
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BYTE(0x6f) BYTE(0x72) BYTE(0x61) BYTE(0x70)
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BYTE(0x72) BYTE(0x6f) BYTE(0x6a) BYTE(0x65)
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BYTE(0x63) BYTE(0x74) BYTE(0x3a) BYTE(0x66)
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BYTE(0x65) BYTE(0x64) BYTE(0x6f) BYTE(0x72)
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BYTE(0x61) BYTE(0x3a) BYTE(0x33) BYTE(0x33)
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BYTE(0x22) BYTE(0x7d) BYTE(0x00) BYTE(0x00)
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}
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}
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INSERT AFTER .note.gnu.build-id;
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```
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## Well-known keys
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The metadata format is intentionally left open, so that vendors can add their own information.
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A set of well-known keys is defined here, and hopefully shared among all vendors.
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| Key name | Key description | Example value |
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|--------------|--------------------------------------------------------------------------|---------------------------------------|
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| type | The packaging type | rpm |
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| os | The OS name, typically corresponding to ID in os-release | fedora |
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| osVersion | The OS version, typically corresponding to VERSION_ID in os-release | 33 |
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| name | The source package name | coreutils |
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| version | The source package version | 4711.0815.fc13 |
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| architecture | The binary package architecture | arm32 |
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| osCpe | A CPE name for the OS, typically corresponding to CPE_NAME in os-release | cpe:/o:fedoraproject:fedora:33 |
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| debugInfoUrl | The debuginfod server url, if available | https://debuginfod.fedoraproject.org/ |
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