[PATCH v2] kbuild: tools: drop overridden CFLAGS from MAKEOVERRIDES

[Max Filippov <jcmvbkbc@xxxxxxxxx>]

Some Makefiles under tools/ use the 'override CFLAGS += ...' construct
to add a few required options to CFLAGS passed by the user.
Unfortunately that only works when user passes CFLAGS as an environment
variable, i.e.
  CFLAGS=... make ...
and not in case when CFLAGS are passed as make command line arguments:
  make ... CFLAGS=...
It happens because in the latter case CFLAGS=... is recorded in the make
variable MAKEOVERRIDES and this variable is passed in its original form
to all $(MAKE) subcommands, taking precedence over modified CFLAGS value
passed in the environment variable. E.g. this causes build failure for
gpio and iio tools when the build is run with user CFLAGS because of
missing _GNU_SOURCE definition needed for the asprintf().

One way to fix it is by removing overridden variables from the
MAKEOVERRIDES. Add macro 'drop-var-from-overrides' that removes a
definition of a variable passed to it from the MAKEOVERRIDES and use it
to fix CFLAGS passing for tools/gpio and tools/iio.

This implementation tries to be precise in string processing and handle
variables with embedded spaces and backslashes correctly. To achieve
that it replaces every '\\' sequence with '\-' to make sure that every
'\' in the resulting string is an escape character. It then replaces
every '\ ' sequence with '\_' to turn string values with embedded spaces
into single words. After filtering the overridden variable definition
out of the resulting string these two transformations are reversed.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 4ccc98a48958 ("tools gpio: Allow overriding CFLAGS")
Fixes: 572974610273 ("tools iio: Override CFLAGS assignments")
Signed-off-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
---
Changes v1->v2:
- make drop-var-from-overrides-code work correctly with arbitrary
  variables, including thoses ending with '\'.

 tools/gpio/Makefile            | 1 +
 tools/iio/Makefile             | 1 +
 tools/scripts/Makefile.include | 9 +++++++++
 3 files changed, 11 insertions(+)

diff --git a/tools/gpio/Makefile b/tools/gpio/Makefile
index d29c9c49e251..46fc38d51639 100644
--- a/tools/gpio/Makefile
+++ b/tools/gpio/Makefile
@@ -24,6 +24,7 @@ ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))
 all: $(ALL_PROGRAMS)
 
 export srctree OUTPUT CC LD CFLAGS
+$(call drop-var-from-overrides,CFLAGS)
 include $(srctree)/tools/build/Makefile.include
 
 #
diff --git a/tools/iio/Makefile b/tools/iio/Makefile
index fa720f062229..04307588dd3f 100644
--- a/tools/iio/Makefile
+++ b/tools/iio/Makefile
@@ -20,6 +20,7 @@ ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))
 all: $(ALL_PROGRAMS)
 
 export srctree OUTPUT CC LD CFLAGS
+$(call drop-var-from-overrides,CFLAGS)
 include $(srctree)/tools/build/Makefile.include
 
 #
diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 6fba29f3222d..0f68b95cf55c 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -51,6 +51,15 @@ define allow-override
     $(eval $(1) = $(2)))
 endef
 
+# When a Makefile overrides a variable and exports it for the nested $(MAKE)
+# invocations to use its modified value, it must remove that variable definition
+# from the MAKEOVERRIDES variable, otherwise the original definition from the
+# MAKEOVERRIDES takes precedence over the exported value.
+drop-var-from-overrides = $(eval $(drop-var-from-overrides-code))
+define drop-var-from-overrides-code
+MAKEOVERRIDES := $(subst \-,\\,$(subst \_,\ ,$(filter-out $(1)=%,$(subst \ ,\_,$(subst \\,\-,$(MAKEOVERRIDES))))))
+endef
+
 ifneq ($(LLVM),)
 ifneq ($(filter %/,$(LLVM)),)
 LLVM_PREFIX := $(LLVM)
-- 
2.39.2