[Buildroot] [PATCH v2 1/1] overlay: Add archive-based (tar) rootfs overlays

[Cam Hutchison]

Add a feature to extract tar archives into the target rootfs when
building filesystem images. The archives are extracted inside the
fakeroot used to build the filesystem images so that the ownership and
permissions of files in the archive are preserved.

This is useful when an external build process produces rootfs archives
or software is supplied as an archive that need to be incorporated the
generated rootfs.

Configuration options are added to configure a list of paths to archives
to be extracted, and to configure how many path components to strip from
paths in those archives. It is not possible to configure a different
number of components for different archives.

Signed-off-by: Cam Hutchison <camh at xdna.net>
---
Changes v1 -> v2:
 - Use suitable-extractor instead of the tar -a option
 - Split config option from BR2_ROOTFS_OVERLAY
 - Add configurable --strip-components
 - Move archive extraction to fakeroot from target-finalize target
---
 fs/common.mk     |  9 +++++++++
 system/Config.in | 21 +++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/fs/common.mk b/fs/common.mk
index 3a424f6..414e5e6 100644
--- a/fs/common.mk
+++ b/fs/common.mk
@@ -38,6 +38,10 @@ ROOTFS_DEVICE_TABLES = $(call qstrip,$(BR2_ROOTFS_DEVICE_TABLE) \
 USERS_TABLE = $(BUILD_DIR)/_users_table.txt
 ROOTFS_USERS_TABLES = $(call qstrip,$(BR2_ROOTFS_USERS_TABLES))
 
+define rootfs-extract-overlay
+$(call suitable-extractor,$(1)) $(1) | tar -xf - -C $(TARGET_DIR) --strip-components=$(call qstrip,$(BR2_ROOTFS_OVERLAY_ARCHIVES_STRIP_COMPONENTS))
+endef
+
 # Since this function will be called from within an $(eval ...)
 # all variable references except the arguments must be $$-quoted.
 define ROOTFS_TARGET_INTERNAL
@@ -91,6 +95,11 @@ endif
 	printf '$$(subst $$(sep),\n,$$(PACKAGES_PERMISSIONS_TABLE))' >> $$(FULL_DEVICE_TABLE)
 	echo "$$(HOST_DIR)/usr/bin/makedevs -d $$(FULL_DEVICE_TABLE) $$(TARGET_DIR)" >> $$(FAKEROOT_SCRIPT)
 endif
+ifneq ($$(BR2_ROOTFS_OVERLAY_ARCHIVES),)
+	$$(foreach archive,$$(call qstrip,$$(BR2_ROOTFS_OVERLAY_ARCHIVES)),\
+		echo "$$(call rootfs-extract-overlay,$$(archive))" >> $$(FAKEROOT_SCRIPT) \
+		$$(sep))
+endif
 	echo "$$(ROOTFS_$(2)_CMD)" >> $$(FAKEROOT_SCRIPT)
 	chmod a+x $$(FAKEROOT_SCRIPT)
 	PATH=$$(BR_PATH) $$(HOST_DIR)/usr/bin/fakeroot -- $$(FAKEROOT_SCRIPT)
diff --git a/system/Config.in b/system/Config.in
index 2e14971..df23ea6 100644
--- a/system/Config.in
+++ b/system/Config.in
@@ -429,6 +429,27 @@ config BR2_ROOTFS_OVERLAY
 	  They are copied as-is into the rootfs, excluding files ending with
 	  ~ and .git, .svn and .hg directories.
 
+config BR2_ROOTFS_OVERLAY_ARCHIVES
+	string "Root filesystem overlay archives"
+	default ""
+	help
+	  Specify a list of tar archives that are extracted into the
+	  output filesystem images as they are built. The archives will
+	  be extracted while running under the fakeroot environment
+	  used to build the images so they may contain files that only
+	  root may extract (such as setuid binaries), and permissions
+	  and ownerships are maintained.
+
+config BR2_ROOTFS_OVERLAY_ARCHIVES_STRIP_COMPONENTS
+	string "Leading path components to strip from overlay archives"
+	default "0"
+	help
+	  The number of leading path components to strip from each
+	  archive specified under Root filesystem overlay archives.
+	  The same number of components must be stripped from all
+	  archives - it is not possible to have a different number of
+	  components stripped from different archives.
+
 config BR2_ROOTFS_POST_BUILD_SCRIPT
 	string "Custom scripts to run before creating filesystem images"
 	default ""
-- 
2.1.4