Merge tag 'kbuild-fixes-7.4-1' of git://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux

Pull Kbuild fixes from Nathan Chancellor:

 - Move probing of '-mfentry' back into Makefile instead of Kconfig
   because it does not interact well with toolchains that support
   multiple bit widths

 - Add a big comment in scripts/Kconfig.toolchain of two pitfalls when
   moving cc-option checks from Kbuild to Kconfig

* tag 'kbuild-fixes-7.4-1' of git://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux:
  scripts/Kconfig.toolchain: Highlight cc-option differences between Kconfig and Kbuild
  kbuild: Move testing of '-mfentry' back to Makefile from Kconfig

2 files changed, 21 insertions, 5 deletions

diff --git a/Makefile b/Makefile
index 7e6baa461b91..451e35112238 100644
--- a/Makefile
+++ b/Makefile
@@ -1059,7 +1059,8 @@ ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
   endif
 endif
 ifdef CONFIG_HAVE_FENTRY
-  ifdef CONFIG_CC_HAS_MFENTRY
+  # s390-linux-gnu-gcc did not support -mfentry until gcc-9.
+  ifeq ($(call cc-option-yn, -mfentry),y)
     CC_FLAGS_FTRACE	+= -mfentry
     CC_FLAGS_USING	+= -DCC_USING_FENTRY
   endif
diff --git a/scripts/Kconfig.toolchain b/scripts/Kconfig.toolchain
index 4b962831d62a..2acbd40a38f1 100644
--- a/scripts/Kconfig.toolchain
+++ b/scripts/Kconfig.toolchain
@@ -144,6 +144,25 @@ config CC_HAS_ASSUME
 config CC_HAS_NO_PROFILE_FN_ATTR
 	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
 
+# Beware of two problems when using cc-option in Kconfig compared to Kbuild
+#
+# 1. cc-option in Kbuild tests the provided flags with all previously added
+#    CFLAGS, whereas cc-option in Kconfig only tests the provided flags with a
+#    small, constant set of additional flags.  If a flag (e.g., '-mfoo')
+#    depends on another (e.g, '-fbar') to function properly, the dependency
+#    must be included in the cc-option call (e.g, '-fbar -mfoo').
+#
+# 2. Bit and endianness flags ('-m32', '-m64', '-mbig-endian', and
+#    '-mlittle-endian') are not present in Kconfig cc-option checks because
+#    they may be included in KBUILD_CFLAGS / KBUILD_CPPFLAGS based on the
+#    finalized configuration result. This can result in inconsistent results
+#    when probing toolchains that support either option, as the toolchain's
+#    configured default tested in Kconfig may not support the tested flag but
+#    the post-configuration result in Kbuild would.
+#
+#    In general, prefer adding truly generic compiler flags to avoid this
+#    pitfall.
+
 config CC_OPT_ZERO_INIT_PADDING_BITS
 	def_string "$(cc-option-str,-fzero-init-padding-bits=all)"
 
@@ -169,10 +188,6 @@ config CC_OPT_NO_ALLOW_STORE_DATA_RACES
 config CC_HAS_MNOP_MCOUNT
 	def_bool $(cc-option,-mnop-mcount)
 
-# s390-linux-gnu-gcc did not support -mfentry until gcc-9.
-config CC_HAS_MFENTRY
-	def_bool $(cc-option,-mfentry)
-
 config LD_OPT_FORCE_GROUP_ALLOCATION
 	def_string "$(ld-option-str,--force-group-allocation)"