bpfilter: match bit size of bpfilter_umh to that of the kernel

bpfilter_umh is built for the default machine bit of the compiler,
which may not match to the bit size of the kernel.

This happens in the scenario below:

You can use biarch GCC that defaults to 64-bit for building the 32-bit
kernel. In this case, Kbuild passes -m32 to teach the compiler to
produce 32-bit kernel space objects. However, it is missing when
building bpfilter_umh. It is built as a 64-bit ELF, and then embedded
into the 32-bit kernel.

The 32-bit kernel and 64-bit umh is a bad combination.

In theory, we can have 32-bit umh running on 64-bit kernel, but we do
not have a good reason to support such a usecase.

The best is to match the bit size between them.

Pass -m32 or -m64 to the umh build command if it is found in
$(KBUILD_CFLAGS). Evaluate CC_CAN_LINK against the kernel bit-size.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

init/Kconfig

@@ -45,7 +45,9 @@ config CLANG_VERSION
 	default $(shell,$(srctree)/scripts/clang-version.sh $(CC))
 
 config CC_CAN_LINK
-	def_bool $(success,$(srctree)/scripts/cc-can-link.sh $(CC))
+	bool
+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(m64-flag)) if 64BIT
+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(m32-flag))
 
 config CC_HAS_ASM_GOTO
 	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))

net/bpfilter/Makefile

@@ -5,14 +5,15 @@
 
 hostprogs := bpfilter_umh
 bpfilter_umh-objs := main.o
-KBUILD_HOSTCFLAGS += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
+KBUILD_HOSTCFLAGS += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
+			$(filter -m32 -m64, $(KBUILD_CFLAGS))
 HOSTCC := $(CC)
 
 ifeq ($(CONFIG_BPFILTER_UMH), y)
 # builtin bpfilter_umh should be compiled with -static
 # since rootfs isn't mounted at the time of __init
 # function is called and do_execv won't find elf interpreter
-KBUILD_HOSTLDFLAGS += -static
+KBUILD_HOSTLDFLAGS += -static $(filter -m32 -m64, $(KBUILD_CFLAGS))
 endif
 
 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh

usr/include/Makefile

@@ -8,6 +8,10 @@
 # We cannot go as far as adding -Wpedantic since it emits too many warnings.
 UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
 
+# In theory, we do not care -m32 or -m64 for header compile tests.
+# It is here just because CONFIG_CC_CAN_LINK is tested with -m32 or -m64.
+UAPI_CFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+
 override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
 
 # The following are excluded for now because they fail to build.