[PATCH v4 0/7] 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 v4:
- Patch 3 (build pipeline): clear CROSS_COMPILE= in the kernel-side
  tools/bootconfig sub-make. Without it, an LLVM=1 cross build
  inherits CROSS_COMPILE and tools/scripts/Makefile.include injects
  --target=/--sysroot= into the host clang, producing a target
  binary that fails to exec.
- Patch 3 (build pipeline): place embedded-cmdline.S in its own
  .init.rodata.embed_cmdline subsection ("a") so ld.lld does not
  see a section-type mismatch against lib/bootconfig-data.S's
  writable .init.rodata ("aw"). The linker's *(.init.rodata
  .init.rodata.*) glob still folds it into the init image.
- Patch 6 (x86/setup): also accept the bootconfig=<anything> form
  via cmdline_find_option(), matching the runtime parse_args() loop.
  Without it, bootconfig=0/=off would skip the early prepend but
  still trigger the late runtime apply -- a split-brain state.
- New patch 7: document CONFIG_BOOT_CONFIG_EMBED_CMDLINE in
  Documentation/admin-guide/bootconfig.rst (semantics, opt-in,
  precedence, overflow behavior, example).
- Link to v3: https://lore.kernel.org/r/20260608-bootconfig_using_tools-v3-0-4ddd079a0696@debian.org

Changes in v3:
- Patch 3: Move HOSTCC override to the kernel-side rule; tool keeps
  $(CC) for standalone/cross builds.
- Patch 6: Drop the false fail-safe wording; document the
  BOOT_CONFIG_FORCE=y default interaction.
- Link to v2:
  https://lore.kernel.org/r/20260605-bootconfig_using_tools-v2-0-d309f544b5f7@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 (7):
      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
      Documentation: bootconfig: document build-time cmdline rendering
      x86/setup: prepend embedded bootconfig cmdline before parse_early_param

 Documentation/admin-guide/bootconfig.rst |  46 +++++++++++++
 MAINTAINERS                              |   1 +
 Makefile                                 |  28 +++++++-
 arch/x86/Kconfig                         |   1 +
 arch/x86/kernel/setup.c                  |  27 ++++++++
 include/linux/bootconfig.h               |   9 +++
 init/Kconfig                             |  36 ++++++++++
 init/main.c                              |  25 ++++++-
 lib/Makefile                             |  16 +++++
 lib/bootconfig.c                         | 112 +++++++++++++++++++++++++++++--
 lib/embedded-cmdline.S                   |  16 +++++
 tools/bootconfig/Makefile                |   4 +-
 12 files changed, 308 insertions(+), 13 deletions(-)
---
base-commit: a87737435cfa134f9cdcc696ba3080759d04cf72
change-id: 20260508-bootconfig_using_tools-cfa7aa9d6a5a

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