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* [PATCH] kbuild: warn if a different compiler is used for external module builds
@ 2021-08-01  2:53 Masahiro Yamada
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From: Masahiro Yamada @ 2021-08-01  2:53 UTC (permalink / raw)
  To: linux-kbuild; +Cc: Masahiro Yamada, Josh Poimboeuf, Michal Marek, linux-kernel

It is always safe to use the same compiler for the kernel and external
modules, but in reality, some distributions such as Fedora release a
different version of GCC from the one used for building the kernel.

There was a long discussion about mixing different compilers [1].

I do not repeat it here, but at least, showing a heads up in that
case is better than nothing.

Linus suggested [2]:
  And a warning might be more palatable even if different compiler
  version work fine together. Just a heads up on "it looks like you
  might be mixing compiler versions" is a valid note, and isn't
  necessarily wrong. Even when they work well together, maybe you want
  to have people at least _aware_ of it.

This commit shows a warning unless the compiler is exactly the same.

  warning: the compiler differs from the one used to build the kernel
    The kernel was built by: gcc (GCC) 11.1.1 20210531 (Red Hat 11.1.1-3)
    You are using:           gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1)

Check the difference, and if it is OK with you, please proceed at your

To avoid the locale issue as in commit bcbcf50f5218 ("kbuild: fix to not be affected by locale"), pass LC_ALL=C to
"$(CC) --version".


Acked-by: Josh Poimboeuf <>
Signed-off-by: Masahiro Yamada <>

 Makefile | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 6b555f64df06..f4cc77a10413 100644
--- a/Makefile
+++ b/Makefile
@@ -583,7 +583,7 @@ endif
 # Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
 # CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
 # and from include/config/auto.conf.cmd to detect the compiler upgrade.
-CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) --version 2>/dev/null | head -n 1))
+CC_VERSION_TEXT = $(subst $(pound),,$(shell LC_ALL=C $(CC) --version 2>/dev/null | head -n 1))
 ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 ifneq ($(CROSS_COMPILE),)
@@ -1731,6 +1731,16 @@ clean-dirs := $(KBUILD_EXTMOD)
 clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
 	$(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
+PHONY += prepare
+# now expand this into a simple variable to reduce the cost of shell evaluations
+	@if [ "$(CC_VERSION_TEXT)" != $(CONFIG_CC_VERSION_TEXT) ]; then \
+		echo >&2 "warning: the compiler differs from the one used to build the kernel"; \
+		echo >&2 "  The kernel was built by: "$(CONFIG_CC_VERSION_TEXT); \
+		echo >&2 "  You are using:           $(CC_VERSION_TEXT)"; \
+	fi
 PHONY += help
 	@echo  '  Building external modules.'
@@ -1742,7 +1752,7 @@ help:
 	@echo  ''
 # no-op for external module builds
-PHONY += prepare modules_prepare
+PHONY += modules_prepare

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2021-08-01  2:53 [PATCH] kbuild: warn if a different compiler is used for external module builds Masahiro Yamada

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