[RFC PATCH 5/5] x86: Build the core kernel with position independent codegen

Ard Biesheuvel
2024-01-22T09:12:55Z

Pass the -fpie flag to the compiler when building objects that are
intended for the core kernel. This ensures that all implicit symbol
references are emitted using RIP-relative relocations, allowing the code
to be executed correctly even if it runs from a different virtual
address than the address it was linked/loaded/relocated to run at.

This is necessary to ensure that all C code that gets pulled in by the
early startup code runs correctly without the need for unpalatable hacks
in the code to force RIP-relative references.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/Makefile | 7 +++++--
arch/x86/entry/vdso/Makefile | 2 +-
arch/x86/kernel/Makefile | 4 ----
arch/x86/realmode/rm/Makefile | 1 +
include/asm-generic/vmlinux.lds.h | 2 ++
5 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index bed0850d91b0..0382a9534099 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -165,11 +165,13 @@ else
KBUILD_RUSTFLAGS += $(rustflags-y)

KBUILD_CFLAGS += -mno-red-zone
- KBUILD_CFLAGS += -mcmodel=kernel
+ KBUILD_CFLAGS_MODULE += -mcmodel=kernel
KBUILD_RUSTFLAGS += -Cno-redzone=y
- KBUILD_RUSTFLAGS += -Ccode-model=kernel
+ KBUILD_RUSTFLAGS_KERNEL += -Ccode-model=small
+ KBUILD_RUSTFLAGS_MODULE += -Ccode-model=kernel

PIE_CFLAGS := -fpie -mcmodel=small \
+ $(call cc-option,-Wa$(comma)-mrelax-relocations=no) \
-include $(srctree)/include/linux/hidden.h

ifeq ($(CONFIG_STACKPROTECTOR),y)
@@ -178,6 +180,7 @@ else
endif
endif

+ KBUILD_CFLAGS_KERNEL += $(PIE_CFLAGS)
export PIE_CFLAGS
endif

diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index b1b8dd1608f7..e2c79d4c1417 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -149,7 +149,7 @@ $(obj)/vdso32.so.dbg: KBUILD_AFLAGS = $(KBUILD_AFLAGS_32)
$(obj)/vdso32.so.dbg: asflags-$(CONFIG_X86_64) += -m32

KBUILD_CFLAGS_32 := $(filter-out -m64,$(KBUILD_CFLAGS))
-KBUILD_CFLAGS_32 := $(filter-out -mcmodel=kernel,$(KBUILD_CFLAGS_32))
+KBUILD_CFLAGS_32 := $(filter-out -mcmodel=small,$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out -fno-pic,$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out -mfentry,$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(RANDSTRUCT_CFLAGS),$(KBUILD_CFLAGS_32))
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 65194ca79b5c..0000325ab98f 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -21,10 +21,6 @@ CFLAGS_REMOVE_sev.o = -pg
CFLAGS_REMOVE_rethook.o = -pg
endif

-# head64.c contains C code that may execute from a different virtual address
-# than it was linked at, so we always build it using PIE codegen
-CFLAGS_head64.o += $(PIE_CFLAGS)
-
KASAN_SANITIZE_head$(BITS).o := n
KASAN_SANITIZE_dumpstack.o := n
KASAN_SANITIZE_dumpstack_$(BITS).o := n
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index f614009d3e4e..fdb8e780f903 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -76,5 +76,6 @@ 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 := $(filter-out $(PIE_CFLAGS),$(KBUILD_CFLAGS_KERNEL))
GCOV_PROFILE := n
UBSAN_SANITIZE := n
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index ef45331fb043..9518b87207e8 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -355,6 +355,7 @@
*(.data..decrypted) \
*(.ref.data) \
*(.data..shared_aligned) /* percpu related */ \
+ *(.data.rel .data.rel.*) \
MEM_KEEP(init.data*) \
*(.data.unlikely) \
__start_once = .; \
@@ -477,6 +478,7 @@
.rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \
__start_rodata = .; \
*(.rodata) *(.rodata.*) \
+ *(.data.rel.ro*) \
SCHED_DATA \
RO_AFTER_INIT_DATA /* Read only after init */ \
. = ALIGN(8); \
--
2.43.0.429.g432eaa2c6b-goog