kbuild: Introduce Kconfig symbol for linking vmlinux with relocations

Some architectures build vmlinux with static relocations preserved, but
strip them again from the final vmlinux image. Arch specific tools
consume these static relocations in order to construct relocation tables
for KASLR.

The fact that vmlinux is created, consumed and subsequently updated goes
against the typical, declarative paradigm used by Make, which is based
on rules and dependencies. So as a first step towards cleaning this up,
introduce a Kconfig symbol to declare that the arch wants to consume the
static relocations emitted into vmlinux. This will be wired up further
in subsequent patches.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: ardbiesheuvel

Makefile

@@ -1120,6 +1120,10 @@ ifdef CONFIG_LD_ORPHAN_WARN
 LDFLAGS_vmlinux += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 endif
 
+ifneq ($(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS),)
+LDFLAGS_vmlinux	+= --emit-relocs --discard-none
+endif
+
 # Align the bit size of userspace programs with the kernel
 KBUILD_USERCFLAGS  += $(filter -m32 -m64 --target=%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
 KBUILD_USERLDFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))

arch/Kconfig

@@ -1695,6 +1695,13 @@ config ARCH_HAS_KERNEL_FPU_SUPPORT
 	  Architectures that select this option can run floating-point code in
 	  the kernel, as described in Documentation/core-api/floating-point.rst.
 
+config ARCH_VMLINUX_NEEDS_RELOCS
+	bool
+	help
+	  Whether the architecture needs vmlinux to be built with static
+	  relocations preserved. This is used by some architectures to
+	  construct bespoke relocation tables for KASLR.
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"

arch/mips/Kconfig

@@ -2617,6 +2617,7 @@ config RELOCATABLE
 		   CPU_MIPS32_R6 || CPU_MIPS64_R6 || \
 		   CPU_P5600 || CAVIUM_OCTEON_SOC || \
 		   CPU_LOONGSON64
+	select ARCH_VMLINUX_NEEDS_RELOCS
 	help
 	  This builds a kernel image that retains relocation information
 	  so it can be loaded someplace besides the default 1MB.

arch/mips/Makefile

@@ -100,10 +100,6 @@ LDFLAGS_vmlinux			+= -G 0 -static -n -nostdlib
 KBUILD_AFLAGS_MODULE		+= -mlong-calls
 KBUILD_CFLAGS_MODULE		+= -mlong-calls
 
-ifeq ($(CONFIG_RELOCATABLE),y)
-LDFLAGS_vmlinux			+= --emit-relocs
-endif
-
 cflags-y += -ffreestanding
 
 cflags-$(CONFIG_CPU_BIG_ENDIAN)		+= -EB

arch/riscv/Kconfig

@@ -1077,6 +1077,7 @@ config RELOCATABLE
 	bool "Build a relocatable kernel"
 	depends on MMU && 64BIT && !XIP_KERNEL
 	select MODULE_SECTIONS if MODULES
+	select ARCH_VMLINUX_NEEDS_RELOCS
 	help
           This builds a kernel as a Position Independent Executable (PIE),
           which retains all relocation metadata required to relocate the

arch/riscv/Makefile

@@ -8,7 +8,7 @@
 
 LDFLAGS_vmlinux := -z norelro
 ifeq ($(CONFIG_RELOCATABLE),y)
-	LDFLAGS_vmlinux += -shared -Bsymbolic -z notext --emit-relocs
+	LDFLAGS_vmlinux += -shared -Bsymbolic -z notext
 	KBUILD_CFLAGS += -fPIE
 endif
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)

arch/s390/Kconfig

@@ -630,6 +630,7 @@ endchoice
 
 config RELOCATABLE
 	def_bool y
+	select ARCH_VMLINUX_NEEDS_RELOCS
 	help
 	  This builds a kernel image that retains relocation information
 	  so it can be loaded at an arbitrary address.

arch/s390/Makefile

@@ -15,7 +15,7 @@ KBUILD_CFLAGS_MODULE += -fPIC
 KBUILD_AFLAGS	+= -m64
 KBUILD_CFLAGS	+= -m64
 KBUILD_CFLAGS	+= -fPIC
-LDFLAGS_vmlinux	:= -no-pie --emit-relocs --discard-none
+LDFLAGS_vmlinux	:= -no-pie
 extra_tools	:= relocs
 aflags_dwarf	:= -Wa,-gdwarf-2
 KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__

arch/x86/Kconfig

@@ -2200,6 +2200,7 @@ config RANDOMIZE_BASE
 config X86_NEED_RELOCS
 	def_bool y
 	depends on RANDOMIZE_BASE || (X86_32 && RELOCATABLE)
+	select ARCH_VMLINUX_NEEDS_RELOCS
 
 config PHYSICAL_ALIGN
 	hex "Alignment value to which kernel should be aligned"

arch/x86/Makefile

@@ -251,12 +251,6 @@ endif
 
 KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)
 
-ifdef CONFIG_X86_NEED_RELOCS
-LDFLAGS_vmlinux := --emit-relocs --discard-none
-else
-LDFLAGS_vmlinux :=
-endif
-
 #
 # The 64-bit kernel must be aligned to 2MB.  Pass -z max-page-size=0x200000 to
 # the linker to force 2MB page size regardless of the default page size used