kbuild: allow architectures to override CC_CAN_LINK

The generic test for CC_CAN_LINK assumes that all architectures use -m32
and -m64 to switch between 32-bit and 64-bit compilation. This is overly
simplistic. Architectures may use other flags (-mabi, -m31, etc.) or may
also require byte order handling (-mlittle-endian, -EL). Expressing all
of the different possibilities will be very complicated and brittle.
Instead allow architectures to supply their own logic which will be
easy to understand and evolve.

Both the boolean ARCH_HAS_CC_CAN_LINK and the string ARCH_USERFLAGS need
to be implemented as kconfig does not allow the reuse of string options.

Signed-off-by: Thomas Weißschuh <[email protected]>
Reviewed-by: Nicolas Schier <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Nicolas Schier <[email protected]>

Author: t-8ch

Makefile

@@ -1137,8 +1137,17 @@ ifneq ($(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS),)
 LDFLAGS_vmlinux	+= --emit-relocs --discard-none
 endif
 
-# Align the bit size of userspace programs with the kernel
-USERFLAGS_FROM_KERNEL := -m32 -m64 --target=%
+# Align the architecture of userspace programs with the kernel
+USERFLAGS_FROM_KERNEL := --target=%
+
+ifdef CONFIG_ARCH_USERFLAGS
+KBUILD_USERCFLAGS += $(CONFIG_ARCH_USERFLAGS)
+KBUILD_USERLDFLAGS += $(CONFIG_ARCH_USERFLAGS)
+else
+# If not overridden also inherit the bit size
+USERFLAGS_FROM_KERNEL += -m32 -m64
+endif
+
 KBUILD_USERCFLAGS  += $(filter $(USERFLAGS_FROM_KERNEL), $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
 KBUILD_USERLDFLAGS += $(filter $(USERFLAGS_FROM_KERNEL), $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
 

init/Kconfig

@@ -82,8 +82,12 @@ config RUSTC_LLVM_VERSION
 	int
 	default $(rustc-llvm-version)
 
+config ARCH_HAS_CC_CAN_LINK
+	bool
+
 config CC_CAN_LINK
 	bool
+	default ARCH_CC_CAN_LINK if ARCH_HAS_CC_CAN_LINK
 	default $(cc_can_link_user,$(m64-flag)) if 64BIT
 	default $(cc_can_link_user,$(m32-flag))