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>

diff --git a/Makefile b/Makefile
index 30242e7..0904b73 100644 (file)
--- a/Makefile
+++ b/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))

diff --git a/arch/Kconfig b/arch/Kconfig
index b8a4ff3..101a13f 100644 (file)
--- a/arch/Kconfig
+++ b/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"

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 1924f2d..5aedbd7 100644 (file)
--- a/arch/mips/Kconfig
+++ b/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.

diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index be8cb44..d9057e2 100644 (file)
--- a/arch/mips/Makefile
+++ b/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

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 7612c52..6f58001 100644 (file)
--- a/arch/riscv/Kconfig
+++ b/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

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 13fbc0f..6ef0d10 100644 (file)
--- a/arch/riscv/Makefile
+++ b/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)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 9c9ec08..ea67b73 100644 (file)
--- a/arch/s390/Kconfig
+++ b/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.

diff --git a/arch/s390/Makefile b/arch/s390/Makefile
index 5fae311..d5f4be4 100644 (file)
--- a/arch/s390/Makefile
+++ b/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__

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0e27ebd..57fc0e7 100644 (file)
--- a/arch/x86/Kconfig
+++ b/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"

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b773b3..f65ed6d 100644 (file)
--- a/arch/x86/Makefile
+++ b/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