diff options
authorMasahiro Yamada <>2020-04-29 12:45:13 +0900
committerMasahiro Yamada <>2020-05-17 18:52:01 +0900
commit9371f86ecb60f6f1f120e3d93fe892bbb70d04c0 (patch)
parent59721d4eb7f66f27440ad74f875b97e64133ee3b (diff)
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 <>
3 files changed, 10 insertions, 3 deletions
diff --git a/init/Kconfig b/init/Kconfig
index ed1d82c9f1df..b2ce83c2e84c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -45,7 +45,9 @@ config CLANG_VERSION
default $(shell,$(srctree)/scripts/ $(CC))
config CC_CAN_LINK
- def_bool $(success,$(srctree)/scripts/ $(CC))
+ bool
+ default $(success,$(srctree)/scripts/ $(CC) $(m64-flag)) if 64BIT
+ default $(success,$(srctree)/scripts/ $(CC) $(m32-flag))
def_bool $(success,$(srctree)/scripts/ $(CC))
diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
index 36580301da70..f6209e4827b9 100644
--- a/net/bpfilter/Makefile
+++ b/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))
# 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 $(filter -m32 -m64, $(KBUILD_CFLAGS))
$(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
diff --git a/usr/include/Makefile b/usr/include/Makefile
index 5a7ee3e5ed86..55362f3ab393 100644
--- a/usr/include/Makefile
+++ b/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.