init: add initramfs_internal.h

The new header only exports a single unpack function and a CPIO_HDRLEN
constant for future test use.

Signed-off-by: David Disseldorp <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

init/initramfs.c

···
 #include <linux/security.h>
 
 #include "do_mounts.h"
+#include "initramfs_internal.h"
 
 static __initdata bool csum_present;
 static __initdata u32 io_csum;
···
 
 static int __init do_start(void)
 {
-	read_into(header_buf, 110, GotHeader);
+	read_into(header_buf, CPIO_HDRLEN, GotHeader);
 	return 0;
 }
 
···
 
 #include <linux/decompress/generic.h>
 
-static char * __init unpack_to_rootfs(char *buf, unsigned long len)
+/**
+ * unpack_to_rootfs - decompress and extract an initramfs archive
+ * @buf: input initramfs archive to extract
+ * @len: length of initramfs data to process
+ *
+ * Returns: NULL for success or an error message string
+ *
+ * This symbol shouldn't be used externally. It's available for unit tests.
+ */
+char * __init unpack_to_rootfs(char *buf, unsigned long len)
 {
 	long written;
 	decompress_fn decompress;
 	const char *compress_name;
 	static __initdata char msg_buf[64];
 
-	header_buf = kmalloc(110, GFP_KERNEL);
+	header_buf = kmalloc(CPIO_HDRLEN, GFP_KERNEL);
 	symlink_buf = kmalloc(PATH_MAX + N_ALIGN(PATH_MAX) + 1, GFP_KERNEL);
 	name_buf = kmalloc(N_ALIGN(PATH_MAX), GFP_KERNEL);
 

init/initramfs_internal.h

···
+// SPDX-License-Identifier: GPL-2.0
+#ifndef __INITRAMFS_INTERNAL_H__
+#define __INITRAMFS_INTERNAL_H__
+
+char *unpack_to_rootfs(char *buf, unsigned long len);
+#define CPIO_HDRLEN 110
+
+#endif