[PATCH v2 5/6] bootconfig: add xbc_prepend_embedded_cmdline() helper

[Breno Leitao <leitao@debian.org>]

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.

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

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 include/linux/bootconfig.h |  9 ++++++
 lib/bootconfig.c           | 78 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 87 insertions(+)

diff --git a/include/linux/bootconfig.h b/include/linux/bootconfig.h
index 1c7f3b74ffcf..c186137f87ac 100644
--- a/include/linux/bootconfig.h
+++ b/include/linux/bootconfig.h
@@ -308,4 +308,13 @@ static inline const char *xbc_get_embedded_bootconfig(size_t *size)
 }
 #endif
 
+/* Build-time-rendered bootconfig cmdline prepended in setup_arch() */
+#ifdef CONFIG_BOOT_CONFIG_EMBED_CMDLINE
+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
diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index 926094d97397..f66be0b2dc24 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -19,6 +19,7 @@
 #include <linux/errno.h>
 #include <linux/cache.h>
 #include <linux/compiler.h>
+#include <linux/printk.h>
 #include <linux/sprintf.h>
 #include <linux/memblock.h>
 #include <linux/string.h>
@@ -34,6 +35,83 @@ const char * __init xbc_get_embedded_bootconfig(size_t *size)
 	return (*size) ? embedded_bootconfig_data : NULL;
 }
 #endif
+
+#ifdef CONFIG_BOOT_CONFIG_EMBED_CMDLINE
+/* embedded_kernel_cmdline is defined in embedded-cmdline.S */
+extern __visible const char embedded_kernel_cmdline[];
+extern __visible const char embedded_kernel_cmdline_end[];
+
+/* Set once the embedded cmdline has actually been prepended. */
+static bool xbc_cmdline_applied __initdata;
+
+/*
+ * str_prepend() - Prepend @src in front of the string in @dst, in place
+ * @dst: NUL-terminated destination buffer, currently @dst_len bytes long
+ * @dst_len: length of the current @dst string (excluding its NUL)
+ * @src: bytes to prepend (not NUL-terminated)
+ * @src_len: number of bytes from @src to prepend
+ *
+ * The caller must guarantee @dst has room for src_len + dst_len + 1 bytes.
+ * Moving dst_len + 1 bytes carries @dst's NUL terminator too, so an empty
+ * @dst needs no special case.
+ */
+static void __init str_prepend(char *dst, size_t dst_len,
+			       const char *src, size_t src_len)
+{
+	memmove(dst + src_len, dst, dst_len + 1);
+	memcpy(dst, src, src_len);
+}
+
+/**
+ * xbc_prepend_embedded_cmdline() - Prepend embedded bootconfig cmdline
+ * @dst: cmdline buffer to prepend into (must already contain a NUL byte)
+ * @size: total capacity of @dst in bytes
+ *
+ * Prepend the build-time-rendered "kernel" subtree of the embedded
+ * bootconfig to @dst. The rendered string already ends with a single
+ * space (the xbc_snprint_cmdline() invariant), which serves as the
+ * separator between the embedded keys and any existing content of @dst.
+ * On overflow, log an error and leave @dst untouched rather than
+ * silently truncating: booting without the embedded values is better
+ * than refusing to boot, and the error message tells the user why
+ * their embedded keys are missing.
+ *
+ * Intended to be called from setup_arch() before parse_early_param() so
+ * that early_param() handlers see the embedded values.
+ */
+void __init xbc_prepend_embedded_cmdline(char *dst, size_t size)
+{
+	size_t embed_len = embedded_kernel_cmdline_end - embedded_kernel_cmdline;
+	size_t dst_len;
+
+	if (!size || embed_len <= 1)	/* trailing NUL only */
+		return;
+	embed_len--;			/* exclude trailing NUL byte */
+
+	dst_len = strnlen(dst, size);
+	if (embed_len + dst_len + 1 > size) {
+		pr_err("embedded bootconfig cmdline (%zu bytes) does not fit in COMMAND_LINE_SIZE with %zu bytes already used; ignoring embedded values\n",
+		       embed_len, dst_len);
+		return;
+	}
+
+	str_prepend(dst, dst_len, embedded_kernel_cmdline, embed_len);
+	xbc_cmdline_applied = true;
+}
+
+/**
+ * xbc_embedded_cmdline_applied() - Did the embedded cmdline get prepended?
+ *
+ * Return true if xbc_prepend_embedded_cmdline() actually prepended the
+ * embedded "kernel" subtree. setup_boot_config() uses this to avoid
+ * rendering the same keys a second time.
+ */
+bool __init xbc_embedded_cmdline_applied(void)
+{
+	return xbc_cmdline_applied;
+}
+#endif
+
 #endif
 
 /*

-- 
2.53.0-Meta