[RFC/RFT PATCH 08/19] x86: Use PIE codegen for the relocatable 64-bit kernel

Ard Biesheuvel
2026-01-08T09:28:19Z

As an intermediate step towards enabling PIE linking for the x86_64
KASLR kernel, enable PIE codegen for all C and Rust objects that are
linked into the kernel proper. Add a Kconfig option RELOCATABLE_PIE for
this, depending on RELR support in the linker, as the relocation tables
will blow up the kernel image otherwise.

This results in a code size increase of between 0.2% (clang) and 0.5%
(gcc). Performance (hackbench) appears to be unaffected across several
different uarchs.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/Kconfig | 4 ++++
arch/x86/Makefile | 19 ++++++++++++++++++-
arch/x86/boot/Makefile | 1 +
arch/x86/boot/compressed/Makefile | 2 +-
arch/x86/entry/vdso/Makefile | 1 +
arch/x86/realmode/rm/Makefile | 1 +
include/asm-generic/vmlinux.lds.h | 1 +
include/linux/hidden.h | 2 ++
8 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bf51e17d5813..b3a64cfe04cf 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2067,6 +2067,7 @@ config PHYSICAL_START
config RELOCATABLE
bool "Build a relocatable kernel" if X86_32
default X86_32
+ select RELOCATABLE_PIE if TOOLS_SUPPORT_RELR
help
This builds a kernel image that retains relocation information so it
can be placed someplace besides the default PAGE_OFFSET + 1MB. This
@@ -2087,6 +2088,9 @@ config RELOCATABLE
it has been loaded at and the compile time physical address
(CONFIG_PHYSICAL_START) is used as the minimum location.

+config RELOCATABLE_PIE
+ bool
+
config RANDOMIZE_BASE
bool "Randomize the address of the kernel image (KASLR)"
select RELOCATABLE
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1d403a3612ea..b211d6c950aa 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -89,6 +89,8 @@ ifdef CONFIG_CC_IS_GCC
CC_FLAGS_FPU += -mhard-float
endif

+rustflags-nojumptables := $(if $(call rustc-min-version,109300),-Cjump-tables=n,-Zno-jump-tables)
+
ifeq ($(CONFIG_X86_KERNEL_IBT),y)
#
# Kernel IBT has S_CET.NOTRACK_EN=0, as such the compilers must not generate
@@ -100,7 +102,7 @@ ifeq ($(CONFIG_X86_KERNEL_IBT),y)
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104816
#
KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch -fno-jump-tables)
-KBUILD_RUSTFLAGS += -Zcf-protection=branch $(if $(call rustc-min-version,109300),-Cjump-tables=n,-Zno-jump-tables)
+KBUILD_RUSTFLAGS += -Zcf-protection=branch $(rustflags-nojumptables)
else
KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
endif
@@ -178,6 +180,21 @@ endif
KBUILD_RUSTFLAGS += -Ccode-model=kernel

percpu_seg := gs
+
+ pie-ccflags-$(CONFIG_CC_IS_GCC) += $(call cc-option.-mdirect-extern-access)
+ pie-ccflags-$(CONFIG_CC_IS_CLANG) += -fdirect-access-external-data
+
+ # objtool gets confused by unannotated PIC flavor jump tables
+ pie-ccflags-y += $(call cc-option,-fannotate-jump-tables,-fno-jump-tables)
+
+ pie-cflags-$(CONFIG_RELOCATABLE_PIE) := $(pie-ccflags-y) -fpie -mcmodel=small \
+ -include $(srctree)/include/linux/hidden.h
+ pie-rustflags-$(CONFIG_RELOCATABLE_PIE) := -Crelocation-model=pie \
+ -Ccode-model=small -Zdirect-access-external-data=yes \
+ $(rustflags-nojumptables)
+
+ KBUILD_CFLAGS_KERNEL += $(pie-cflags-y)
+ KBUILD_RUSTFLAGS_KERNEL += $(pie-rustflags-y)
endif

ifeq ($(CONFIG_STACKPROTECTOR),y)
diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 3f9fb3698d66..491b3b2a9a02 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -55,6 +55,7 @@ KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP
KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
KBUILD_CFLAGS += $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
+KBUILD_CFLAGS_KERNEL :=

$(obj)/bzImage: asflags-y := $(SVGA_MODE)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index bc071bdcd11e..96099b5d1064 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -76,7 +76,7 @@ LDFLAGS_vmlinux += -T
hostprogs := mkpiggy
HOST_EXTRACFLAGS += -I$(srctree)/tools/include

-sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABbCDGRSTtVW] \(_text\|__data_segment_start\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
+sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABbCDdGRSTtVW] \(_text\|__data_segment_start\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'

quiet_cmd_voffset = VOFFSET $@
cmd_voffset = $(NM) $< | sed -n $(sed-voffset) > $@
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index f247f5f5cb44..bf4221a0fc08 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -143,6 +143,7 @@ endif
endif

$(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)
+$(obj)/vdso32.so.dbg: KBUILD_CFLAGS_KERNEL :=

$(obj)/vdso32.so.dbg: $(obj)/vdso32/vdso32.lds $(vobjs32) FORCE
$(call if_changed,vdso_and_check)
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index a0fb39abc5c8..70bf0a26da91 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -67,3 +67,4 @@ KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
-I$(srctree)/arch/x86/boot
KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS_KERNEL :=
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 8ca130af301f..1782b6b87b2d 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -373,6 +373,7 @@
*(DATA_MAIN) \
*(.data..decrypted) \
*(.ref.data) \
+ *(.data.rel*) \
*(.data..shared_aligned) /* percpu related */ \
*(.data..unlikely) \
__start_once = .; \
diff --git a/include/linux/hidden.h b/include/linux/hidden.h
index 49a17b6b5962..2ad764c0ca18 100644
--- a/include/linux/hidden.h
+++ b/include/linux/hidden.h
@@ -16,4 +16,6 @@
* giving them 'hidden' visibility.
*/

+#ifndef __BINDGEN__
#pragma GCC visibility push(hidden)
+#endif
--
2.47.3