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Since Linux 6.1, kbuild compiles init/version.o with a temporary UTS_VERSION. Its build number and timestamp stay empty unless KBUILD_BUILD_VERSION or KBUILD_BUILD_TIMESTAMP is set. That weak linux_banner stays in the image ahead of the banner from init/version-timestamp.o, and the first-match scan reported it. The WSL 6.18 kernel showed '# SMP PREEMPT_DYNAMIC ' instead of '#1 SMP PREEMPT_DYNAMIC Tue Oct 6 19:13:07 UTC 2026'. Treat a banner with a space right after '#' or at its end as the placeholder. Keep scanning and report a later banner that repeats its text up to '#' and is not a placeholder. Report the placeholder when no such banner follows.
79 lines
4.1 KiB
Markdown
79 lines
4.1 KiB
Markdown
# Linux kernel build config
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## Problem and outcome
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Analysing a Linux kernel image usually starts with two questions: which kernel
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version is this, and how was it configured. A kernel built with
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`CONFIG_IKCONFIG` embeds its `.config` as a gzip blob between `IKCFG_ST` and
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`IKCFG_ED`. REA reports kernel modules and ELF notes, but it cannot read the
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version banner or the embedded config, so an agent needs a Linux host and
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`scripts/extract-ikconfig`.
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This feature adds `inspect_kernel_build_config` and
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`inspect-kernel-build-config`. They read one selected local kernel image and
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report its `Linux version` banner and embedded build config, without
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executing or booting it.
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[Linux kernel build config](../../../guides/native/kernel-build-config.md) describes the
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current behavior and limitations. This directory is the feature
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specification, implementation plan and task list. The guide does not replace
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these files.
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## Requirements
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1. Accepted inputs are a `vmlinux` ELF, a raw arm64 `Image`, an x86 `bzImage`
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or `zImage` whose payload is gzip, and a `configs.ko` module. The file is
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read through the stable artifact read and never executed.
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2. The banner is the first printable `Linux version ` string followed by a
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version token that starts with a digit and ends at a NUL or newline within
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512 bytes. Version, builder and compiler are split from it. A banner with a
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space right after `#` or at its end is the kbuild placeholder from
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`init/version.o`, built without a build number or timestamp; a later banner
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that repeats its text up to `#` without that gap replaces it. The scan is
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linear in the searched bytes.
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3. A config is `present` only when `IKCFG_ST` is followed by a gzip member that
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ends exactly at the next `IKCFG_ED`, inflates within `max_config_bytes`,
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passes its CRC-32 and ISIZE checks, and parses as `.config` text. Inflation
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reads only the bytes up to that marker.
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4. Framing that fails its checks or grammar, including a framed deflate stream
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that does not decode or is cut off by `IKCFG_ED`, is `malformed`, never
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`absent`. An unframed `IKCFG_ST` string is skipped. A valid config later in the bytes
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still wins.
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5. Entries report name, kind (`y`, `m`, `number`, `string`, `not_set`) and
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value, with an exact `name` filter and `offset`/`limit` paging.
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6. A bzImage setup header from boot protocol 2.08 locates the payload. A gzip
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payload is inflated within `max_payload_bytes` and searched, with locations
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marked `inflated_payload`. Other payload codecs, and a file that is itself a
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compressed stream such as `Image.gz`, are `unsupported` and name the codec,
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using the codec detection shared with the kernel module parser.
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7. Budgets bound file size, config and payload inflate size, option count and
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page size. The config search inflates at most 16 candidates and at most
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four times `max_config_bytes` in total, charging a stream that fails
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partway through its full per-candidate allowance. Exhausting the search budget is
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`incomplete` with `config_search_exhausted`. A member past
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`max_config_bytes` fails the budget only when no later candidate is
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accepted.
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8. Cancellation is checked between candidates, between banner hits, while
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scanning long text and while parsing config lines. Inflation runs off the
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event loop and each scan, including marker searches that find nothing, text
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checks, validation of a single long value and hashing, yields to it at least once per MiB, so a live
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cancellation reaches those checks.
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9. Banner text and config values are marked target-derived and untrusted.
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10. MCP requires an absolute path. The CLI resolves relative paths against the
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working directory.
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## Acceptance
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- Unit tests cover banner parsing, config framing, grammar, malformed and
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truncated members, decoy candidates, search budgets, cancellation, and
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bzImage payload handling.
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- Service tests cover each status and reason.
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- CLI acceptance, filesystem boundary and MCP boundary tests exercise the
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public contract.
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## Non-goals
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- Decompressing xz, lz4, lzma, zstd, bzip2 or lzo payloads.
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- Comparing the config with a source tree or evaluating Kconfig dependencies.
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- Executing or booting the image.
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