bootconfig: render embedded bootconfig as a kernel cmdline at build time

Add the build-time pipeline that renders the "kernel" subtree of
CONFIG_BOOT_CONFIG_EMBED_FILE into a flat cmdline string and stashes
it in .init.rodata as embedded_kernel_cmdline[]. A follow-up patch
adds the runtime helper that prepends this string to boot_command_line
during early architecture setup so parse_early_param() sees the values.

The build wires up:
  tools/bootconfig -C kernel - userspace tool already shared with
                               lib/bootconfig.c, used here in -C mode
                               to render a bootconfig file to a cmdline
  lib/embedded-cmdline.S     - .incbin's the rendered text plus a NUL
                               (listed under the EXTRA BOOT CONFIG
                               MAINTAINERS entry)
  lib/Makefile rule          - runs tools/bootconfig at build time
  Makefile prepare dep       - ensures tools/bootconfig is built first,
                               same pattern as tools/objtool and
                               tools/bpf/resolve_btfids

Drop the test target from tools/bootconfig/Makefile's default 'all'
recipe so that hooking the binary into the kernel build does not run
test-bootconfig.sh on every prepare. The tests stay available as
'make -C tools/bootconfig test', matching the convention of
tools/objtool and tools/bpf/resolve_btfids whose 'all' targets only
build the binary.

Require BOOT_CONFIG_EMBED_FILE to be non-empty before the new option
can be enabled, otherwise tools/bootconfig -C runs against an empty
file and prints a parse error on every kernel build.

The feature gates on CONFIG_ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG, a
silent symbol arches select once they've wired the prepend call into
setup_arch(). No arch selects it in this patch, so the user-visible
CONFIG_CMDLINE_FROM_BOOTCONFIG is not yet enableable; when an arch
later opts in, the runtime behavior is added by the follow-up patches.

tools/bootconfig also installs on target systems, so its own Makefile
keeps $(CC) and stays cross-buildable as a standalone tool. The kernel
build, which runs the tool on the build host during prepare, instead
forces CC=$(HOSTCC) from a dedicated tools/bootconfig rule and clears
CROSS_COMPILE= in the sub-make. Without that clear, an LLVM=1 cross
build would inherit CROSS_COMPILE and tools/scripts/Makefile.include
would inject --target=/--sysroot= flags into the host clang invocation,
producing a target binary that fails to exec ("Exec format error").

embedded-cmdline.S places the rendered string in its own .init.rodata
subsection (.init.rodata.embed_cmdline) with the "a" (allocatable,
read-only) flag and %progbits. lib/bootconfig-data.S already places
the embedded bootconfig blob in .init.rodata with the "aw" flag
(xbc_init() rewrites separators in place, so that data must be
writable). Using a distinct subsection name avoids the ld.lld section-
type mismatch that would otherwise arise from mixing "a" and "aw"
under the same name; the linker's "*(.init.rodata .init.rodata.*)"
glob still folds both into the init image and frees them after boot.

A follow-up patch wires the build-time tools/bootconfig into the
top-level clean target.

Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-3-24ab72139c29@debian.org/

Reviewed-by: Nicolas Schier <n.schier@fritz.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index b9c5792c79e0..f3598f2bc0b6 100644
--- a/Makefile
+++ b/Makefile
@@ -1552,6 +1552,22 @@ prepare: tools/bpf/resolve_btfids
 endif
 endif
 
+# tools/bootconfig renders the embedded bootconfig into a cmdline at build time.
+ifdef CONFIG_CMDLINE_FROM_BOOTCONFIG
+prepare: tools/bootconfig
+endif
+
+# tools/bootconfig is run on the build host during prepare, so force a host
+# binary here; its own Makefile keeps $(CC) for standalone and cross builds.
+# CROSS_COMPILE= is cleared so tools/scripts/Makefile.include does not inject
+# the target's --target=/--sysroot= flags into the host clang invocation under
+# LLVM=1 cross builds (which would produce a target binary that fails to exec).
+tools/bootconfig: export CC := $(HOSTCC)
+tools/bootconfig: FORCE
+	$(Q)mkdir -p $(objtree)/tools
+	$(Q)$(MAKE) O=$(abspath $(objtree)) subdir=tools -C $(srctree)/tools/ \
+		bootconfig CROSS_COMPILE=
+
 # The tools build system is not a part of Kbuild and tends to introduce
 # its own unique issues. If you need to integrate a new tool into Kbuild,
 # please consider locating that tool outside the tools/ tree and using the