initramfs_test: kunit tests for initramfs unpacking

Provide some basic initramfs unpack sanity tests covering:

- simple file / dir extraction
- filename field overrun, as reported and fixed separately via
  https://lore.kernel.org/r/20241030035509.20194-2-ddiss@suse.de
- "070702" cpio data checksums
- hardlinks

Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250304061020.9815-3-ddiss@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: ddiss

init/.kunitconfig

@@ -0,0 +1,3 @@
+CONFIG_KUNIT=y
+CONFIG_BLK_DEV_INITRD=y
+CONFIG_INITRAMFS_TEST=y

init/Kconfig

@@ -1454,6 +1454,13 @@ config INITRAMFS_PRESERVE_MTIME
 
 	  If unsure, say Y.
 
+config INITRAMFS_TEST
+	bool "Test initramfs cpio archive extraction" if !KUNIT_ALL_TESTS
+	depends on BLK_DEV_INITRD && KUNIT=y
+	default KUNIT_ALL_TESTS
+	help
+	  Build KUnit tests for initramfs. See Documentation/dev-tools/kunit
+
 choice
 	prompt "Compiler optimization level"
 	default CC_OPTIMIZE_FOR_PERFORMANCE

init/Makefile

@@ -12,6 +12,7 @@ else
 obj-$(CONFIG_BLK_DEV_INITRD)   += initramfs.o
 endif
 obj-$(CONFIG_GENERIC_CALIBRATE_DELAY) += calibrate.o
+obj-$(CONFIG_INITRAMFS_TEST)   += initramfs_test.o
 
 obj-y                          += init_task.o