[RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

[Ard Biesheuvel <ardb+git@xxxxxxxxxx>]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

As an intermediate step towards enabling PIE linking for the 64-bit x86
kernel, enable PIE codegen for all objects that are linked into the
kernel proper.

This substantially reduces the number of relocations that need to be
processed when booting a relocatable KASLR kernel.

Before (size in bytes of the reloc table):

  797372 arch/x86/boot/compressed/vmlinux.relocs

After:

  400252 arch/x86/boot/compressed/vmlinux.relocs

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/x86/Makefile                 | 11 ++++++++++-
 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 +
 6 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index b78b7623a4a9..83d20f402535 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -193,13 +193,22 @@ else
         KBUILD_RUSTFLAGS += -Cno-redzone=y
         KBUILD_RUSTFLAGS += -Ccode-model=kernel
 
+        PIE_CFLAGS-y := -fpie -mcmodel=small \
+                        -include $(srctree)/include/linux/hidden.h
+
+        PIE_CFLAGS-$(CONFIG_CC_IS_GCC) += $(call 
cc-option.-mdirect-extern-access)
+        PIE_CFLAGS-$(CONFIG_CC_IS_CLANG) += -fdirect-access-external-data
+
         ifeq ($(CONFIG_STACKPROTECTOR),y)
                 KBUILD_CFLAGS += 
-mstack-protector-guard-symbol=fixed_percpu_data
+
+                # the 'small' C model defaults to %fs
+                PIE_CFLAGS-$(CONFIG_SMP) += -mstack-protector-guard-reg=gs
         endif
 
         # Don't emit relaxable GOTPCREL relocations
         KBUILD_AFLAGS_KERNEL += -Wa,-mrelax-relocations=no
-        KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no
+        KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no $(PIE_CFLAGS-y)
 endif
 
 #
diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 9cc0ff6e9067..4d3ba35cb619 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -57,6 +57,7 @@ KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
 KBUILD_CFLAGS  += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 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 f2051644de94..c362d36b5b69 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -73,7 +73,7 @@ LDFLAGS_vmlinux += -T
 hostprogs      := mkpiggy
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include
 
-sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] 
\(_text\|__start_rodata\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
+sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABbCDdGRSTtVW] 
\(_text\|__start_rodata\|__bss_start\|_end\)$$/\#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 c9216ac4fb1e..7af9fecf9abb 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -141,6 +141,7 @@ 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 2b079f73820f..3a084ac77109 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -349,6 +349,7 @@
        *(DATA_MAIN)                                                    \
        *(.data..decrypted)                                             \
        *(.ref.data)                                                    \
+       *(.data.rel*)                                                   \
        *(.data..shared_aligned) /* percpu related */                   \
        *(.data.unlikely)                                               \
        __start_once = .;                                               \
-- 
2.46.0.792.g87dc391469-goog