Improve verification of custom rootfs skeletons and overlays

index 8c57531782..b1cddd9146 100644
@@ -23,32 +23,11 @@ $(error No path specified for the custom skeleton)
 endif
 endif
 
-# Extract the inode numbers for all of those directories. In case any is
-# a symlink, we want to get the inode of the pointed-to directory, so we
-# append '/.' to be sure we get the target directory. Since the symlinks
-# can be anyway (/bin -> /usr/bin or /usr/bin -> /bin), we do that for
-# all of them.
-#
-SKELETON_CUSTOM_LIB_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/lib/. 2>/dev/null)
-SKELETON_CUSTOM_BIN_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/bin/. 2>/dev/null)
-SKELETON_CUSTOM_SBIN_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/sbin/. 2>/dev/null)
-SKELETON_CUSTOM_USR_LIB_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/usr/lib/. 2>/dev/null)
-SKELETON_CUSTOM_USR_BIN_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/usr/bin/. 2>/dev/null)
-SKELETON_CUSTOM_USR_SBIN_INODE = $(shell stat -c '%i' $(SKELETON_CUSTOM_PATH)/usr/sbin/. 2>/dev/null)
-
 # For a merged /usr, ensure that /lib, /bin and /sbin and their /usr
 # counterparts are appropriately setup as symlinks ones to the others.
 ifeq ($(BR2_ROOTFS_MERGED_USR),y)
 
-ifneq ($(SKELETON_CUSTOM_LIB_INODE),$(SKELETON_CUSTOM_USR_LIB_INODE))
-SKELETON_CUSTOM_NOT_MERGED_USR_DIRS += /lib
-endif
-ifneq ($(SKELETON_CUSTOM_BIN_INODE),$(SKELETON_CUSTOM_USR_BIN_INODE))
-SKELETON_CUSTOM_NOT_MERGED_USR_DIRS += /bin
-endif
-ifneq ($(SKELETON_CUSTOM_SBIN_INODE),$(SKELETON_CUSTOM_USR_SBIN_INODE))
-SKELETON_CUSTOM_NOT_MERGED_USR_DIRS += /sbin
-endif
+SKELETON_CUSTOM_NOT_MERGED_USR_DIRS = $(shell support/scripts/check-merged-usr.sh $(SKELETON_CUSTOM_PATH))
 
 endif # merged /usr
 

new file mode 100755
index 0000000000..74c43c89fd
@@ -0,0 +1,39 @@
+#!/bin/sh
+#
+# Check if a given custom skeleton or overlay complies to the merged /usr
+# requirements:
+# /
+# /bin -> usr/bin
+# /lib -> usr/lib
+# /sbin -> usr/sbin
+# /usr/bin/
+# /usr/lib/
+# /usr/sbin/
+#
+# Output: the list non-compliant paths (empty if compliant).
+#
+
+# Extract the inode numbers for all of those directories. In case any is
+# a symlink, we want to get the inode of the pointed-to directory, so we
+# append '/.' to be sure we get the target directory. Since the symlinks
+# can be anyway (/bin -> /usr/bin or /usr/bin -> /bin), we do that for
+# all of them.
+#
+lib_inode=$(stat -c '%i' "${1}/lib/." 2>/dev/null)
+bin_inode=$(stat -c '%i' "${1}/bin/." 2>/dev/null)
+sbin_inode=$(stat -c '%i' "${1}/sbin/." 2>/dev/null)
+usr_lib_inode=$(stat -c '%i' "${1}/usr/lib/." 2>/dev/null)
+usr_bin_inode=$(stat -c '%i' "${1}/usr/bin/." 2>/dev/null)
+usr_sbin_inode=$(stat -c '%i' "${1}/usr/sbin/." 2>/dev/null)
+
+not_merged_dirs=""
+test -z "$lib_inode" || \
+	test "$lib_inode" = "$usr_lib_inode" || \
+		not_merged_dirs="/lib"
+test -z "$bin_inode" || \
+	test "$bin_inode" = "$usr_bin_inode" || \
+		not_merged_dirs="$not_merged_dirs /bin"
+test -z "$sbin_inode" || \
+	test "$sbin_inode" = "$usr_sbin_inode" || \
+		not_merged_dirs="$not_merged_dirs /sbin"
+echo "${not_merged_dirs# }"

index 44fc460670..9d3a62ddaf 100644
@@ -100,6 +100,15 @@ To enable this feature, enable config option
   +System configuration+ menu. If you specify a relative path, it will
   be relative to the root of the Buildroot tree.
 +
+Custom skeletons don't need to contain the '/bin', '/lib' or '/sbin'
+  directories, since they are created automatically during the build.
+  When +BR2_ROOTFS_MERGED_USR+ is enabled, then the custom skeleton must
+  not contain the '/bin', '/lib' or '/sbin' directories, as Buildroot
+  will create them as symbolic links to the relevant folders in '/usr'.
+  In such a situation, should the skeleton have any programs or
+  libraries, they should be placed in '/usr/bin', '/usr/sbin' and
+  '/usr/lib'.
++
 This method is not recommended because it duplicates the entire
   skeleton, which prevents taking advantage of the fixes or improvements
   brought to the default skeleton in later Buildroot releases.

index b1cddd9146..01cd62794d 100644
@@ -43,6 +43,7 @@ endif
 # things we customise in the custom skeleton.
 define SKELETON_CUSTOM_INSTALL_TARGET_CMDS
 	$(call SYSTEM_RSYNC,$(SKELETON_CUSTOM_PATH),$(TARGET_DIR))
+	$(call SYSTEM_USR_SYMLINKS_OR_DIRS,$(TARGET_DIR))
 	$(call SYSTEM_LIB_SYMLINK,$(TARGET_DIR))
 	$(INSTALL) -m 0644 support/misc/target-dir-warning.txt \
 		$(TARGET_DIR_WARNING_FILE)

@@ -54,6 +55,7 @@ endef
 # skeleton to staging.
 define SKELETON_CUSTOM_INSTALL_STAGING_CMDS
 	$(call SYSTEM_RSYNC,$(SKELETON_CUSTOM_PATH),$(STAGING_DIR))
+	$(call SYSTEM_USR_SYMLINKS_OR_DIRS,$(STAGING_DIR))
 	$(call SYSTEM_LIB_SYMLINK,$(STAGING_DIR))
 endef
 

index c024c65f78..dc51269143 100644
@@ -746,11 +746,25 @@ endif
 	@$(call MESSAGE,"Sanitizing RPATH in target tree")
 	$(TOPDIR)/support/scripts/fix-rpath target
 
+# For a merged /usr, ensure that /lib, /bin and /sbin and their /usr
+# counterparts are appropriately setup as symlinks ones to the others.
+ifeq ($(BR2_ROOTFS_MERGED_USR),y)
+
+	@$(foreach d, $(call qstrip,$(BR2_ROOTFS_OVERLAY)), \
+		$(call MESSAGE,"Sanity check in overlay $(d)"); \
+		not_merged_dirs="$$(support/scripts/check-merged-usr.sh $(d))"; \
+		test -n "$$not_merged_dirs" && { \
+			echo "ERROR: The overlay in $(d) is not" \
+				"using a merged /usr for the following directories:" \
+				$$not_merged_dirs; \
+			exit 1; \
+		} || true$(sep))
+
+endif # merged /usr
+
 	@$(foreach d, $(call qstrip,$(BR2_ROOTFS_OVERLAY)), \
 		$(call MESSAGE,"Copying overlay $(d)"); \
-		rsync -a --ignore-times --keep-dirlinks $(RSYNC_VCS_EXCLUSIONS) \
-			--chmod=u=rwX,go=rX --exclude .empty --exclude '*~' \
-			$(d)/ $(TARGET_DIR)$(sep))
+		$(call SYSTEM_RSYNC,$(d),$(TARGET_DIR))$(sep))
 
 	@$(foreach s, $(call qstrip,$(BR2_ROOTFS_POST_BUILD_SCRIPT)), \
 		$(call MESSAGE,"Executing post-build script $(s)"); \

index 9d3a62ddaf..bb6d8da6bf 100644
@@ -22,6 +22,14 @@ A filesystem overlay is a tree of files that is copied directly
   etc., files called +.empty+ and files ending in +~+ are excluded from
   the copy.
 +
+Filesystem overlays don't need to contain the '/bin', '/lib' or '/sbin'
+  directories, since they are created automatically during the build.
+  When +BR2_ROOTFS_MERGED_USR+ is enabled, then the overlay must not
+  contain the '/bin', '/lib' or '/sbin' directories, as Buildroot will
+  create them as symbolic links to the relevant folders in '/usr'.  In
+  such a situation, should the overlay have any programs or libraries,
+  they should be placed in '/usr/bin', '/usr/sbin' and '/usr/lib'.
++
 As shown in xref:customize-dir-structure[], the recommended path for
   this overlay is +board/<company>/<boardname>/rootfs-overlay+.
 

index d14a864ca5..911cd424ba 100644
@@ -31,10 +31,6 @@ config BR2_ROOTFS_SKELETON_CUSTOM_PATH
 	help
 	  Path to custom target skeleton.
 
-# dummy config so merged /usr workarounds can also be activated for
-# custom rootfs skeleton
-config BR2_ROOTFS_MERGED_USR
-
 endif
 
 if BR2_ROOTFS_SKELETON_DEFAULT

@@ -209,8 +205,6 @@ config BR2_ROOTFS_DEVICE_TABLE_SUPPORTS_EXTENDED_ATTRIBUTES
 	help
 	  Support extended attributes handling in device tables
 
-if BR2_ROOTFS_SKELETON_DEFAULT
-
 config BR2_ROOTFS_MERGED_USR
 	bool "Use symlinks to /usr for /bin, /sbin and /lib"
 	help

@@ -223,6 +217,8 @@ config BR2_ROOTFS_MERGED_USR
 	  symlinks to their counterparts in /usr. In this case, /usr can
 	  not be a separate filesystem.
 
+if BR2_ROOTFS_SKELETON_DEFAULT
+
 config BR2_TARGET_ENABLE_ROOT_LOGIN
 	bool "Enable root login with password"
 	default y