Enable '-Werror' by default for all kernel builds

... but make it a config option so that broken environments can disable
it when required.

We really should always have a clean build, and will disable specific
over-eager warnings as required, if we can't fix them.  But while I
fairly religiously enforce that in my own tree, it doesn't get enforced
by various build robots that don't necessarily report warnings.

So this just makes '-Werror' a default compiler flag, but allows people
to disable it for their configuration if they have some particular
issues.

Occasionally, new compiler versions end up enabling new warnings, and it
can take a while before we have them fixed (or the warnings disabled if
that is what it takes), so the config option allows for that situation.

Hopefully this will mean that I get fewer pull requests that have new
warnings that were not noticed by various automation we have in place.

Knock wood.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Makefile

@@ -785,6 +785,9 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 
 KBUILD_CFLAGS += $(stackp-flags-y)
 
+KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror
+KBUILD_CFLAGS += $(KBUILD_CFLAGS-y)
+
 ifdef CONFIG_CC_IS_CLANG
 KBUILD_CPPFLAGS += -Qunused-arguments
 # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.

init/Kconfig

@@ -137,6 +137,20 @@ config COMPILE_TEST
 	  here. If you are a user/distributor, say N here to exclude useless
 	  drivers to be distributed.
 
+config WERROR
+	bool "Compile the kernel with warnings as errors"
+	default y
+	help
+	  A kernel build should not cause any compiler warnings, and this
+	  enables the '-Werror' flag to enforce that rule by default.
+
+	  However, if you have a new (or very old) compiler with odd and
+	  unusual warnings, or you have some architecture with problems,
+	  you may need to disable this config option in order to
+	  successfully build the kernel.
+
+	  If in doubt, say Y.
+
 config UAPI_HEADER_TEST
 	bool "Compile test UAPI headers"
 	depends on HEADERS_INSTALL && CC_CAN_LINK