bootconfig: add xbc_prepend_embedded_cmdline() helper

Add a helper that prepends the build-time-rendered embedded bootconfig
"kernel" subtree (embedded_kernel_cmdline[] from embedded-cmdline.S) to
a cmdline buffer with a separating space. Architectures call this from
setup_arch() before parse_early_param() so early_param() handlers
(mem=, earlycon=, loglevel=, ...) see values supplied via the embedded
bootconfig.

The in-place prepend (shift the existing string right, then drop the
embedded string in front) is factored into a small str_prepend() helper.

On overflow the helper logs an error and leaves the cmdline untouched
rather than panicking. Booting without the embedded values is better
than refusing to boot, and the error tells the user why their embedded
keys are missing.

The helper records whether it actually prepended, exposed via
xbc_embedded_cmdline_applied(). setup_boot_config() uses this to decide
whether the runtime "kernel" render would duplicate keys already folded
into boot_command_line.

Also add bootconfig_cmdline_requested(), a small parse_args() wrapper
that reports whether "bootconfig" was passed on the command line and,
via an optional out-parameter, where the "--" init arguments begin.
setup_arch() and setup_boot_config() share it so the early and late
paths agree on the opt-in. It sits under CONFIG_BOOT_CONFIG rather than
CONFIG_CMDLINE_FROM_BOOTCONFIG because the runtime parser needs it on
every bootconfig build.

When CONFIG_CMDLINE_FROM_BOOTCONFIG=n, the public declaration in
<linux/bootconfig.h> resolves to a no-op stub so callers compile
unchanged.

Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-5-24ab72139c29@debian.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/bootconfig.h b/include/linux/bootconfig.h
index 1c7f3b74ffcf..deda507500da 100644
--- a/include/linux/bootconfig.h
+++ b/include/linux/bootconfig.h
@@ -308,4 +308,18 @@ static inline const char *xbc_get_embedded_bootconfig(size_t *size)
 }
 #endif
 
+/* Bootconfig opt-in detection, shared by setup_arch() and setup_boot_config() */
+#ifdef CONFIG_BOOT_CONFIG
+bool __init bootconfig_cmdline_requested(const char *boot_cmdline, int *end_offset);
+#endif
+
+/* Build-time-rendered bootconfig cmdline prepended in setup_arch() */
+#ifdef CONFIG_CMDLINE_FROM_BOOTCONFIG
+void __init xbc_prepend_embedded_cmdline(char *dst, size_t size);
+bool __init xbc_embedded_cmdline_applied(void);
+#else
+static inline void xbc_prepend_embedded_cmdline(char *dst, size_t size) { }
+static inline bool xbc_embedded_cmdline_applied(void) { return false; }
+#endif
+
 #endif