[PATCH v2 0/6] bootconfig: embed kernel.* cmdline at build time

[Breno Leitao <leitao@debian.org>]

The userspace pieces (xbc_snprint_cmdline() in lib/, tools/bootconfig -C)
already landed; this series wires the rendered cmdline into the kernel.

Motivation: today the embedded bootconfig is parsed at runtime, after
parse_early_param() has already run, so early_param() handlers can't
see embedded values. Folding the kernel.* subtree into the cmdline at
build time gives a CONFIG_CMDLINE-equivalent for embedded-bootconfig
users without forcing them to maintain two cmdline sources.

Behaviorally, the "kernel" subtree is rendered to a flat string at
build time and stashed in .init.rodata. setup_arch() prepends it to
boot_command_line before parse_early_param() runs. Overflow is a soft
error: the helper logs and leaves boot_command_line untouched rather
than panicking, so an oversized embedded bconf cannot brick a boot.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2 (addressing review of v1):
- Split out a standalone fix for the NULL-pointer arithmetic in
  xbc_snprint_cmdline() so the build-time render cannot trip host
  UBSan/FORTIFY_SOURCE.
- Rework the leaf-root handling: instead of returning early, skip @root
  inside the loop so a root carrying both a value and subkeys
  (kernel = x together with kernel.foo = bar) still renders its
  descendant keys.
- Build tools/bootconfig with $(HOSTCC) so cross-compiled (ARCH=...)
  builds render the cmdline on the build host instead of failing with
  "Exec format error".
- Mark the embedded cmdline section read-only (drop the "w" flag from
  .init.rodata).
- Add a make-clean hook so tools/bootconfig artifacts are removed by
  make clean.
- Gate the x86 prepend on "bootconfig" being present on the command
  line (or CONFIG_BOOT_CONFIG_FORCE), matching the init.* opt-in
  semantics documented in bootconfig.rst and preserving fail-safe
  recovery: dropping "bootconfig" from the bootloader cmdline now also
  disables the embedded kernel.* keys.
- Link to v1: https://patch.msgid.link/20260527-bootconfig_using_tools-v1-0-b6906a86e7d5@debian.org

---
Breno Leitao (6):
      bootconfig: fix NULL-pointer arithmetic in xbc_snprint_cmdline()
      bootconfig: render descendant keys when xbc_snprint_cmdline() root has a value
      bootconfig: render embedded bootconfig as a kernel cmdline at build time
      bootconfig: clean build-time tools/bootconfig from make clean
      bootconfig: add xbc_prepend_embedded_cmdline() helper
      x86/setup: prepend embedded bootconfig cmdline before parse_early_param

 MAINTAINERS                |   1 +
 Makefile                   |  18 +++++++-
 arch/x86/Kconfig           |   1 +
 arch/x86/kernel/setup.c    |  16 +++++++
 include/linux/bootconfig.h |   9 ++++
 init/Kconfig               |  33 +++++++++++++
 init/main.c                |  18 ++++++--
 lib/Makefile               |  16 +++++++
 lib/bootconfig.c           | 112 ++++++++++++++++++++++++++++++++++++++++++---
 lib/embedded-cmdline.S     |  16 +++++++
 tools/bootconfig/Makefile  |  10 ++--
 11 files changed, 236 insertions(+), 14 deletions(-)
---
base-commit: e7e28506af98ce4e1059e5ec59334b335c00a246
change-id: 20260508-bootconfig_using_tools-cfa7aa9d6a5a

Best regards,
-- 
Breno Leitao <leitao@debian.org>