core: re-enter make if $(CURDIR) or $(O) are not canonical paths

When $(CURDIR) and/or $(O) contain symlinks in their paths, they can be
resolved differently, depending on each package build-system (whether it
uses the given paths or get the absolute canonical ones).

Using absolute canonical paths will help achieving reproducible builds and
will make easier tracking down host machine paths leaking into the host,
target or staging trees.
So, this change ensures the build takes place with the CURDIR and O
variables are set to their absolute canonical paths.

In order to recall the toplevel makefile with absolute canonical paths
for $(CURDIR) and $(O), we need to:
1- Compute the absolute canonical paths for $(CURDIR) and $(O) that will
   be passed to the sub-make. This is achieved using the 'realpath' make
   primitive. However, some care must be taken when manipulating O:
   - the out-of-tree makefile wrapper happens a trailing "/.", we need
     to strip this part away to not break the comparison driving the
     sub-make call;
   - the user can leave a trailing '/' to $(O);
   - according to [1,2], realpath returns an empty string in case of
     non-existing entry. So, to avoid passing an empty O= variable to
     sub-make, it is necessary to define the output directory and create
     it prior to call realpath on it (because on the first invocation,
     $(O) usually does not yet exists), hence the trick doing the mkdir
     right before calling realpath.
2- Update EXTRAMAKEARGS with the absolute canonical $(O) and use it
   when call recalling the top-level makefile with umask and paths
   correctly set.
3- Lastly, update the condition for setting the CONFIG_DIR and
   NEED_WRAPPER variables.

Note:
* This change takes care of the makefile wrapper installed in $(O) to
  avoid unneeded make recursion.

[1] https://www.gnu.org/software/make/manual/html_node/File-Name-Functions.html
[2] http://man7.org/linux/man-pages/man3/realpath.3.html

Reported-by: Matthew Weber <matt@thewebers.ws>
Cc: Matthew Weber <matt@thewebers.ws>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -33,7 +33,7 @@ SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 # or avoid confusing packages that can use the O=<dir> syntax for out-of-tree
 # build by preventing it from being forwarded to sub-make calls.
 ifneq ("$(origin O)", "command line")
-O := output
+O := $(CURDIR)/output
 else
 # Other packages might also support Linux-style out of tree builds
 # with the O=<dir> syntax (E.G. BusyBox does). As make automatically
@@ -50,27 +50,46 @@ endif
 
 # Check if the current Buildroot execution meets all the pre-requisites.
 # If they are not met, Buildroot will actually do its job in a sub-make meeting
-# its pre-requisites, which is:
+# its pre-requisites, which are:
 #  1- Permissive enough umask:
 #       Wrong or too restrictive umask will prevent Buildroot and packages from
 #       creating files and directories.
+#  2- Absolute canonical CWD (i.e. $(CURDIR)):
+#       Otherwise, some packages will use CWD as-is, others will compute its
+#       absolute canonical path. This makes harder tracking and fixing host
+#       machine path leaks.
+#  3- Absolute canonical output location (i.e. $(O)):
+#       For the same reason as the one for CWD.
+
+# Remove the trailing '/.' from $(O) as it can be added by the makefile wrapper
+# installed in the $(O) directory.
+# Also remove the trailing '/' the user can set when on the command line.
+override O := $(patsubst %/,%,$(patsubst %.,%,$(O)))
+# Make sure $(O) actually exists before calling realpath on it; this is to
+# avoid empty CANONICAL_O in case on non-existing entry.
+CANONICAL_O := $(shell mkdir -p $(O) >/dev/null 2>&1)$(realpath $(O))
+
+CANONICAL_CURDIR = $(realpath $(CURDIR))
 
 REQ_UMASK = 0022
 
-# we need to pass O= everywhere we call back into the toplevel makefile
-EXTRAMAKEARGS = O=$(O)
+# Make sure O= is passed (with its absolute canonical path) everywhere the
+# toplevel makefile is called back.
+EXTRAMAKEARGS := O=$(CANONICAL_O)
 
 # Check Buildroot execution pre-requisites here.
-ifneq ($(shell umask),$(REQ_UMASK))
+ifneq ($(shell umask):$(CURDIR):$(O),$(REQ_UMASK):$(CANONICAL_CURDIR):$(CANONICAL_O))
 .PHONY: _all $(MAKECMDGOALS)
 
 $(MAKECMDGOALS): _all
 	@:
 
 _all:
-	@umask $(REQ_UMASK) && $(MAKE) --no-print-directory $(MAKECMDGOALS)
+	@umask $(REQ_UMASK) && \
+		$(MAKE) -C $(CANONICAL_CURDIR) --no-print-directory \
+			$(MAKECMDGOALS) $(EXTRAMAKEARGS)
 
-else # umask
+else # umask / $(CURDIR) / $(O)
 
 # This is our default rule, so must come first
 all:
@@ -140,8 +159,9 @@ endif
 include support/misc/utils.mk
 
 # Set variables related to in-tree or out-of-tree build.
-ifeq ($(O),output)
-CONFIG_DIR := $(TOPDIR)
+# Here, both $(O) and $(CURDIR) are absolute canonical paths.
+ifeq ($(O),$(CURDIR)/output)
+CONFIG_DIR := $(CURDIR)
 NEED_WRAPPER =
 else
 CONFIG_DIR := $(O)
@@ -1038,4 +1058,4 @@ include docs/manual/manual.mk
 
 .PHONY: $(noconfig_targets)
 
-endif #umask
+endif #umask / $(CURDIR) / $(O)