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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.
157 lines
8.7 KiB
Markdown
157 lines
8.7 KiB
Markdown
# Linux kernel build config
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`inspect_kernel_build_config` / `inspect-kernel-build-config` reads the
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`Linux version` banner and the embedded `.config` from one selected local
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kernel image without executing or booting it. Accepted inputs are a `vmlinux`
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ELF, a raw arm64 `Image`, an x86 `bzImage` or `zImage` whose payload is gzip,
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and a `configs.ko` module. It does not require a Linux host,
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`extract-ikconfig`, or `/proc/config.gz`.
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```bash
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rea inspect-kernel-build-config /absolute/path/to/vmlinux --json
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rea inspect-kernel-build-config /absolute/path/to/bzImage --name CONFIG_SECURITY_SELINUX --json
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```
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The CLI resolves relative paths against the caller's working directory. MCP
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requires an absolute filesystem path and preserves that exact selection:
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```json
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{
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"name": "inspect_kernel_build_config",
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"arguments": { "path": "/artifacts/vmlinux", "name": "CONFIG_MODULE_SIG" }
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}
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```
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## Observations
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The inline Evidence result contains the artifact path, SHA-256, and byte
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length. `format` is `elf` when the file starts with the ELF magic and `file`
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otherwise.
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`banner` is the first printable string that starts with `Linux version `
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followed by a version token beginning with a digit and ends at a NUL or
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newline within 512 bytes. `version` is that token. `builder` is the first
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parenthesized group (`user@host`). `compiler` is the second group up to the
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last `) #`, so nested parentheses in compiler strings are kept. Since Linux
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6.1, kbuild also leaves the weak banner of `init/version.o`. It is compiled
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before the build number and timestamp exist, so unless `KBUILD_BUILD_VERSION`
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and `KBUILD_BUILD_TIMESTAMP` are both set, it has a space right after `#` or at
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its end. When a later banner repeats that text up to `#` without that gap, the
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later banner is reported; otherwise the first one is. The banner is `absent`
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when no such string was found in the searched bytes and `unknown` when the
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bytes were not searched.
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`config` is `present` only when all of the following hold:
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- `IKCFG_ST` is immediately followed by a gzip member (`1f 8b 08`).
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- The member inflates within `max_config_bytes` and its CRC-32 and ISIZE
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trailer match.
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- The member trailer ends exactly at the first `IKCFG_ED` after the gzip
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header. Inflation reads only the bytes up to that marker.
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- Every non-blank line of the inflated text is a `#` comment,
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`# CONFIG_X is not set`, or `CONFIG_X=value`, and the text is valid UTF-8
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with no control bytes other than tab, CR, and LF.
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An `IKCFG_ST` string that is not followed by gzip magic, or whose complete
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member is not followed by `IKCFG_ED`, is skipped, so an ordinary string is never
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reported as a config. When `IKCFG_ST`, gzip magic, and a later `IKCFG_ED` frame
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a member that does not decode, a deflate stream cut off by that marker, a
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member that fails its CRC-32 or ISIZE check, or text that fails the grammar,
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the config is `malformed` with coverage
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`incomplete`, and `marker`, `gzip`, and `end_marker` locate the blob. A
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malformed blob is never reported as an absent config. A valid config later in
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the searched bytes still wins. `marker`, `gzip`, and `end_marker` give
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the three ranges. `sha256` hashes the inflated text. `option_count` counts
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every `CONFIG_*` line. `matched_count` counts the options that match the
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optional exact `name` filter, and `entries` is the `offset`/`limit` page of
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those matches in file order.
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Each entry has a `name`, a `kind`, and a `value`:
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| `kind` | Source line | `value` |
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| --------- | -------------------------- | ------------------------- |
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| `y`, `m` | `CONFIG_X=y`, `CONFIG_X=m` | `y` or `m` |
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| `number` | decimal or `0x` hex | the text as written |
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| `string` | `CONFIG_X="..."` | decoded text (`\"`, `\\`) |
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| `not_set` | `# CONFIG_X is not set` | `null` |
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Banner text and configuration values are target-derived and untrusted.
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## bzImage payloads
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A file with `HdrS` at offset `0x202` is an x86 setup header. From boot
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protocol 2.08, `payload_offset` and `payload_length` locate the compressed
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kernel after the `(setup_sects + 1) * 512` setup code. `payload` reports the
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container, codec, file range, and inflated length. A gzip payload is inflated
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within `max_payload_bytes` and the inflated bytes are searched. Locations in
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those bytes have `space: "inflated_payload"` and are offsets in the inflated
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payload, not file offsets. Only the leading gzip member is inflated, and its
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own ISIZE trailer gives the inflated length. Kbuild appends a separate 4-byte
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size only for the non-gzip codecs. Any bytes after the gzip member inside the
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payload are not read.
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xz, lz4, lzma, zstd, bzip2, and lzo payloads are `unsupported`. The result
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names the codec and `external_decompression` as the next probe. This is an
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external step, not a REA operation: decompress the payload with the matching
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utility into a separate file, then call `inspect_kernel_build_config` on that
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file. A file that is itself a compressed stream, including a whole-file gzip
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such as `Image.gz`, is reported as `unsupported` with its codec and is not
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inflated. Only a gzip payload inside a bzImage is inflated.
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## Limits and failures
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The selected file is limited to 64 MiB by default (512 MiB maximum). An IKCFG
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member may inflate to 4 MiB by default (32 MiB maximum) and a bzImage payload
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to 128 MiB by default (512 MiB maximum). At most 16384 `CONFIG_*` options are
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examined by default (65536 maximum). A page holds 256 entries by default
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(4096 maximum). Exceeding any budget fails with no partial success and names
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the option to raise. An IKCFG member past `max_config_bytes` fails that way only
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when no later candidate is accepted, so an oversized decoy cannot hide a valid
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config.
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One search inflates at most 16 `IKCFG_ST` gzip candidates and at most four
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times `max_config_bytes` of output across them. A candidate whose stream fails
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partway through is charged its full per-candidate allowance, since its partial
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output is not measured. When that search budget runs
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out before a config is accepted, the config is `incomplete` with reason
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`config_search_exhausted` instead of failing or reporting an absent config.
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Inflation runs off the event loop. Every scan, including marker
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searches that find nothing, config text checks, validation of one long value,
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and hashing, yields to it at least once
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per MiB, so a cancellation that arrives while the image is being parsed is honored
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at the next check. One inflate call runs to its output bound once started.
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Results that could not search or read the kernel bytes stay `incomplete`,
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`malformed`, or `unsupported` and are never
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reported as an absent config:
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| `config.reason` | Meaning |
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| ---------------------------------- | ---------------------------------------------------------------------------- |
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| `config_gzip_invalid` | a framed `IKCFG_ST` gzip member does not decode or fails its CRC-32 or ISIZE |
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| `config_text_invalid` | a framed `IKCFG_ST` gzip member is not `.config` text |
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| `config_gzip_truncated` | an `IKCFG_ST` gzip member runs past the searched bytes |
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| `config_search_exhausted` | decoy candidates used up the config search budget |
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| `payload_truncated` | the declared bzImage payload or its gzip member ends early |
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| `payload_gzip_invalid` | the bzImage gzip payload fails to inflate or its trailer |
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| `payload_codec_unsupported` | a compressed payload or whole-file stream was not inflated |
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| `unrecognized_payload_codec` | the bzImage payload starts with no known codec magic |
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| `setup_header_payload_unavailable` | the setup header predates 2.08 or has no payload length |
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A kernel built without `CONFIG_IKCONFIG` carries no embedded config, so an
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`absent` config describes only the bytes that were searched. A kernel built
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with `CONFIG_IKCONFIG=m` keeps the config in `configs.ko`; inspect that module
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instead of the image. This workflow does not execute the image, compare the
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config with a source tree, or evaluate Kconfig dependencies.
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## Format references
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The markers follow Linux v6.12's
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[`kernel/configs.c`](https://github.com/torvalds/linux/blob/v6.12/kernel/configs.c)
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and
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[`scripts/extract-ikconfig`](https://github.com/torvalds/linux/blob/v6.12/scripts/extract-ikconfig).
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The setup header fields follow the
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[x86 boot protocol](https://docs.kernel.org/arch/x86/boot.html). The banner
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format follows
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[`init/version-timestamp.c`](https://github.com/torvalds/linux/blob/v6.12/init/version-timestamp.c).
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These references establish the decoded layout, not runtime behavior.
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