[PATCH] 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.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 4ccc98a48958 ("tools gpio: Allow overriding CFLAGS")
Fixes: 572974610273 ("tools iio: Override CFLAGS assignments")
Signed-off-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
---
 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..43e1fbb8329f 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 := $(subst -escaped-space-,\ ,$(filter-out $(1)=%,$(subst \ ,-escaped-space-,$(MAKEOVERRIDES))))
+endef
+
 ifneq ($(LLVM),)
 ifneq ($(filter %/,$(LLVM)),)
 LLVM_PREFIX := $(LLVM)
-- 
2.39.2