Commit 6072b2c4 authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: warn if a different compiler is used for external module builds 

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
risk.

To avoid the locale issue as in commit bcbcf50f ("kbuild: fix
ld-version.sh to not be affected by locale"), pass LC_ALL=C to
"$(CC) --version".

[1] https://lore.kernel.org/linux-hardening/efe6b039a544da8215d5e54aa7c4b6d1986fc2b0.1611607264.git.jpoimboe@redhat.com/
[2] https://lore.kernel.org/lkml/CAHk-=wgjwhDy-y4mQh34L+2aF=n6BjzHdqAW2=8wri5x7O04pA@mail.gmail.com/



Acked-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
parent 0058d07e

Makefile

+12 −2
Original line number Diff line number Diff line
@@ -581,7 +581,7 @@ 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),)
@@ -1739,6 +1739,16 @@ 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

prepare: CC_VERSION_TEXT := $(CC_VERSION_TEXT)

prepare:

	@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

help:

	@echo  '  Building external modules.'
@@ -1750,7 +1760,7 @@ help: 
	@echo  ''



# no-op for external module builds

PHONY += prepare modules_prepare

PHONY += modules_prepare



endif # KBUILD_EXTMOD