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Add markdown doc about coredump package metadata
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Luca Boccassi
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docs/COREDUMP_PACKAGE_METADATA.md
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docs/COREDUMP_PACKAGE_METADATA.md
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---
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title: Package Metadata for Core Files
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category: Interfaces
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layout: default
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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 ELF header section, in a key-value JSON
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format. Implementers working on parsing core files should not assume a specific list of
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keys, but parse anything that is included in the section.
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Implementers working on build tools should strive to use the same key names, for
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consistency. The most common will be listed here. When corresponding to the content of
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os-release, the values should match, again for consistency.
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* Section header
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```
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SECTION: `.note.package`
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node-id: `0xcafe1a7e`
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Owner: `FDO` (FreeDesktop.org)
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Value: a JSON string with the structure described below
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```
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* JSON payload
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```json
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{
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"packageType":"rpm", # this provides a namespace for the package+package-version fields
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"packageDistro":"fedora",
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"packageDistroVersion":"33",
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"package":"coreutils",
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"packageVersion": "4711.0815.fc13.arm32",
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"cpe": # A CPE name for the operating system, `CPE_NAME` from os-release is a good default
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}
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```
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A reference implementations of a [build-time tool is provided](https://github.com/keszybz/rpm-version-note/)
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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.fc34
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SECTIONS
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{
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.note.package : ALIGN(4) {
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BYTE(0x04) BYTE(0x00) BYTE(0x00) BYTE(0x00) /* Length of Owner including NUL */
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BYTE(0x73) BYTE(0x00) BYTE(0x00) BYTE(0x00) /* Length of Value including NUL */
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BYTE(0x7e) BYTE(0x1a) BYTE(0xfe) BYTE(0xca) /* 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(0x70) BYTE(0x61) /* Value: '{"packageType":"rpm","package":"systemd","packageVersion":"248~rc2-1.fc34","cpe":"cpe:/o:fedoraproject:fedora:33"}\x00\x00' */
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BYTE(0x63) BYTE(0x6b) BYTE(0x61) BYTE(0x67)
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BYTE(0x65) BYTE(0x54) BYTE(0x79) BYTE(0x70)
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BYTE(0x65) BYTE(0x22) BYTE(0x3a) BYTE(0x22)
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BYTE(0x72) BYTE(0x70) BYTE(0x6d) BYTE(0x22)
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BYTE(0x2c) BYTE(0x22) BYTE(0x70) BYTE(0x61)
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BYTE(0x63) BYTE(0x6b) BYTE(0x61) BYTE(0x67)
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BYTE(0x65) BYTE(0x22) BYTE(0x3a) BYTE(0x22)
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BYTE(0x73) BYTE(0x79) BYTE(0x73) BYTE(0x74)
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BYTE(0x65) BYTE(0x6d) BYTE(0x64) BYTE(0x22)
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BYTE(0x2c) BYTE(0x22) BYTE(0x70) BYTE(0x61)
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BYTE(0x63) BYTE(0x6b) BYTE(0x61) BYTE(0x67)
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BYTE(0x65) BYTE(0x56) BYTE(0x65) BYTE(0x72)
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BYTE(0x73) BYTE(0x69) BYTE(0x6f) BYTE(0x6e)
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BYTE(0x22) BYTE(0x3a) BYTE(0x22) BYTE(0x32)
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BYTE(0x34) BYTE(0x38) BYTE(0x7e) BYTE(0x72)
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BYTE(0x63) BYTE(0x32) BYTE(0x2d) BYTE(0x31)
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BYTE(0x2e) BYTE(0x66) BYTE(0x63) BYTE(0x33)
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BYTE(0x34) BYTE(0x22) BYTE(0x2c) BYTE(0x22)
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BYTE(0x63) 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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