[Buildroot] [PATCH v2 3/4] Makefile: check rootfs overlays with BR2_ROOTFS_MERGED_USR enabled

[Carlos Santos]

Since commit 0db34529f48 we use rsync with the --keep-dirlinks option to
prevent overlays from accidentally overwriding /{usr,bin,sbin,lib} links
when BR2_ROOTFS_MERGED_USR option is enabled. Unfortunately this also
prevents replacing a symlink by a directory on purpose (e.g. /var/log,
to persist system logs).

Steps to reproduce:

- enable BR2_ROOTFS_MERGED_USR and BR2_PACKAGE_SKELETON_INIT_SYSV
- mkdir some_path/rootfs-overlay/var/log
- enable BR2_ROOTFS_OVERLAY="some_path/rootfs-overlay"
- run 'make'
- 'target/var/log' is still a symlink to '../tmp', not a directory

Fix the problem by adding a step in target-finalize that checks each
overlay, using the same criteria used in skeleton-custom.mk.

Add a paragraph to the documentation clarifying that rootfs overlays
don't need to contain /bin, /lib or /sbin and must not contain them when
BR2_ROOTFS_MERGED_USR is enabled.

Signed-off-by: Carlos Santos <casantos at datacom.ind.br>
---
Changes v1->v2:

- Rebase series to HEAD of master branch
- Rework commit message and documentation, as suggested by Thomas
  Petazzoni
---
 Makefile                         | 20 +++++++++++++++++---
 docs/manual/customize-rootfs.txt |  8 ++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index c024c65f78..dc51269143 100644
--- a/Makefile
+++ b/Makefile
@@ -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)"); \
diff --git a/docs/manual/customize-rootfs.txt b/docs/manual/customize-rootfs.txt
index 9d3a62ddaf..bb6d8da6bf 100644
--- a/docs/manual/customize-rootfs.txt
+++ b/docs/manual/customize-rootfs.txt
@@ -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+.
 
-- 
2.14.3