[PATCH v2 3/6] bootconfig: render embedded bootconfig as a kernel cmdline at build time

[Breno Leitao <leitao@debian.org>]

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_BOOT_CONFIG_EMBED_CMDLINE is not yet enableable; when an arch
later opts in, the runtime behavior is added by the follow-up patches.

tools/bootconfig is compiled with $(HOSTCC), not $(CC): Kbuild exports
CC as the target cross-compiler, but the prepare hook runs the tool on
the build host, so $(CC) would produce a binary that fails to exec
("Exec format error") under ARCH=... cross builds. This mirrors
tools/objtool and tools/bpf/resolve_btfids.

embedded-cmdline.S places the rendered string in .init.rodata with the
"a" (allocatable, read-only) flag and %progbits, not "aw": the data is
never written at runtime, so it must not land in a writable section.

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

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 MAINTAINERS               |  1 +
 Makefile                  |  5 +++++
 init/Kconfig              | 33 +++++++++++++++++++++++++++++++++
 lib/Makefile              | 16 ++++++++++++++++
 lib/embedded-cmdline.S    | 16 ++++++++++++++++
 tools/bootconfig/Makefile |  8 ++++++--
 6 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/MAINTAINERS b/MAINTAINERS
index 4087b67bbc69..fb9314cbe344 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9845,6 +9845,7 @@ F:	fs/proc/bootconfig.c
 F:	include/linux/bootconfig.h
 F:	lib/bootconfig-data.S
 F:	lib/bootconfig.c
+F:	lib/embedded-cmdline.S
 F:	tools/bootconfig/*
 F:	tools/bootconfig/scripts/*
 
diff --git a/Makefile b/Makefile
index d59f703f9797..e95992959f44 100644
--- a/Makefile
+++ b/Makefile
@@ -1543,6 +1543,11 @@ prepare: tools/bpf/resolve_btfids
 endif
 endif
 
+# tools/bootconfig renders the embedded bootconfig into a cmdline at build time.
+ifdef CONFIG_BOOT_CONFIG_EMBED_CMDLINE
+prepare: tools/bootconfig
+endif
+
 # 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
diff --git a/init/Kconfig b/init/Kconfig
index ca35184532dc..5f491a5ac4b8 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1569,6 +1569,39 @@ config BOOT_CONFIG_EMBED_FILE
 	  This bootconfig will be used if there is no initrd or no other
 	  bootconfig in the initrd.
 
+config ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG
+	bool
+	help
+	  Selected by architectures whose setup_arch() prepends the
+	  build-time-rendered embedded bootconfig cmdline to
+	  boot_command_line before parse_early_param() runs.
+
+config BOOT_CONFIG_EMBED_CMDLINE
+	bool "Render embedded bootconfig as kernel cmdline at build time"
+	depends on BOOT_CONFIG_EMBED
+	depends on BOOT_CONFIG_EMBED_FILE != ""
+	depends on ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG
+	default n
+	help
+	  Render the "kernel" subtree of the embedded bootconfig file into a
+	  flat cmdline string at kernel build time and prepend it to
+	  boot_command_line during early architecture setup. This makes
+	  early_param() handlers (e.g. mem=, earlycon=, loglevel=) see the
+	  values supplied via the embedded bootconfig.
+
+	  The runtime bootconfig parser is unaffected, so tree-structured
+	  consumers such as ftrace boot-time tracing keep working.
+
+	  Note: when an initrd also carries a bootconfig, its "kernel"
+	  subtree is still parsed at runtime, but the embedded "kernel"
+	  keys remain in boot_command_line for parse_early_param() and
+	  end up later than the initrd keys in saved_command_line, so
+	  parse_args() last-wins favors the embedded values. If you need
+	  initrd to override embedded kernel.* keys, leave this option
+	  off.
+
+	  If unsure, say N.
+
 config CMDLINE_LOG_WRAP_IDEAL_LEN
 	int "Length to try to wrap the cmdline when logged at boot"
 	default 1021
diff --git a/lib/Makefile b/lib/Makefile
index 6e72d2c1cce7..9de0ac7732a2 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -273,6 +273,22 @@ filechk_defbconf = cat $(or $(real-prereqs), /dev/null)
 $(obj)/default.bconf: $(CONFIG_BOOT_CONFIG_EMBED_FILE) FORCE
 	$(call filechk,defbconf)
 
+obj-$(CONFIG_BOOT_CONFIG_EMBED_CMDLINE) += embedded-cmdline.o
+$(obj)/embedded-cmdline.o: $(obj)/embedded_cmdline.bin
+
+# Render the bootconfig "kernel" subtree to a flat cmdline string using
+# the userspace tools/bootconfig parser (-C mode). The runtime prepend
+# helper enforces COMMAND_LINE_SIZE at boot, so no build-time size
+# check is performed here (COMMAND_LINE_SIZE is an arch header
+# constant, not a Kconfig value).
+quiet_cmd_render_cmdline = BCONF2C $@
+      cmd_render_cmdline = \
+	$(objtree)/tools/bootconfig/bootconfig -C $< > $@
+
+targets += embedded_cmdline.bin
+$(obj)/embedded_cmdline.bin: $(obj)/default.bconf $(objtree)/tools/bootconfig/bootconfig FORCE
+	$(call if_changed,render_cmdline)
+
 obj-$(CONFIG_RBTREE_TEST) += rbtree_test.o
 obj-$(CONFIG_INTERVAL_TREE_TEST) += interval_tree_test.o
 
diff --git a/lib/embedded-cmdline.S b/lib/embedded-cmdline.S
new file mode 100644
index 000000000000..740d7ad2dc01
--- /dev/null
+++ b/lib/embedded-cmdline.S
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Embed the build-time-rendered bootconfig "kernel" subtree as a flat
+ * cmdline string. setup_arch() prepends this to boot_command_line on
+ * architectures that select ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG.
+ *
+ * Copyright (c) 2026 Meta Platforms, Inc. and affiliates
+ * Copyright (c) 2026 Breno Leitao <leitao@debian.org>
+ */
+	.section .init.rodata, "a", %progbits
+	.global embedded_kernel_cmdline
+embedded_kernel_cmdline:
+	.incbin "lib/embedded_cmdline.bin"
+	.byte 0
+	.global embedded_kernel_cmdline_end
+embedded_kernel_cmdline_end:
diff --git a/tools/bootconfig/Makefile b/tools/bootconfig/Makefile
index 90eb47c9d8de..aa75a7828685 100644
--- a/tools/bootconfig/Makefile
+++ b/tools/bootconfig/Makefile
@@ -15,10 +15,14 @@ override CFLAGS += -Wall -g -I$(CURDIR)/include
 ALL_TARGETS := bootconfig
 ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))
 
-all: $(ALL_PROGRAMS) test
+all: $(ALL_PROGRAMS)
 
+# bootconfig is a build host tool: Kbuild's prepare hook runs it on the
+# build machine to render the embedded cmdline, so always compile it with
+# $(HOSTCC). Using $(CC) would cross-compile it under ARCH=... builds and
+# fail to exec on the host ("Exec format error").
 $(OUTPUT)bootconfig: main.c include/linux/bootconfig.h $(LIBSRC)
-	$(CC) $(filter %.c,$^) $(CFLAGS) $(LDFLAGS) -o $@
+	$(HOSTCC) $(filter %.c,$^) $(CFLAGS) $(LDFLAGS) -o $@
 
 test: $(ALL_PROGRAMS) test-bootconfig.sh
 	./test-bootconfig.sh $(OUTPUT)

-- 
2.53.0-Meta