This implements CONFIG_RELOCATABLE, which links the final vmlinux image
with a dynamic relocation section, which allows the early boot code to
perform a relocation to a different virtual address at runtime.
This is a prerequisite for KASLR (CONFIG_RANDOMIZE_BASE).
Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org>
---
arch/arm64/Kconfig | 12 ++++++++
arch/arm64/Makefile | 4 +++
arch/arm64/include/asm/elf.h | 2 ++
arch/arm64/kernel/head.S | 32 ++++++++++++++++++++
arch/arm64/kernel/vmlinux.lds.S | 16 ++++++++++
5 files changed, 66 insertions(+)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 141f65ab0ed5..6aa86f86fd10 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -84,6 +84,7 @@ config ARM64
select IOMMU_DMA if IOMMU_SUPPORT
select IRQ_DOMAIN
select IRQ_FORCED_THREADING
+ select KALLSYMS_TEXT_RELATIVE
select MODULES_USE_ELF_RELA
select NO_BOOTMEM
select OF
@@ -762,6 +763,17 @@ config ARM64_MODULE_PLTS
select ARM64_MODULE_CMODEL_LARGE
select HAVE_MOD_ARCH_SPECIFIC
+config RELOCATABLE
+ bool
+ help
+ This builds the kernel as a Position Independent Executable (PIE),
+ which retains all relocation metadata required to relocate the
+ kernel binary at runtime to a different virtual address than the
+ address it was linked at.
+ Since AArch64 uses the RELA relocation format, this requires a
+ relocation pass at runtime even if the kernel is loaded at the
+ same address it was linked at.
+
endmenu
menu "Boot options"
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index db462980c6be..c3eaa03f9020 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -15,6 +15,10 @@ CPPFLAGS_vmlinux.lds = -DTEXT_OFFSET=$(TEXT_OFFSET)
OBJCOPYFLAGS :=-O binary -R .note -R .note.gnu.build-id -R .comment -S
GZFLAGS :=-9
+ifneq ($(CONFIG_RELOCATABLE),)
+LDFLAGS_vmlinux += -pie
+endif
+
KBUILD_DEFCONFIG := defconfig
# Check for binutils support for specific extensions
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 435f55952e1f..24ed037f09fd 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -77,6 +77,8 @@
#define R_AARCH64_MOVW_PREL_G2_NC 292
#define R_AARCH64_MOVW_PREL_G3 293
+#define R_AARCH64_RELATIVE 1027
+
/*
* These are used to set parameters in the core dumps.
*/
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index b4be53923942..92f9c26632f3 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -29,6 +29,7 @@
#include <asm/asm-offsets.h>
#include <asm/cache.h>
#include <asm/cputype.h>
+#include <asm/elf.h>
#include <asm/kernel-pgtable.h>
#include <asm/memory.h>
#include <asm/pgtable-hwdef.h>
@@ -432,6 +433,37 @@ __mmap_switched:
bl __pi_memset
dsb ishst // Make zero page visible to PTW
+#ifdef CONFIG_RELOCATABLE
+
+ /*
+ * Iterate over each entry in the relocation table, and apply the
+ * relocations in place.
+ */
+ adr_l x8, __dynsym_start // start of symbol table
+ adr_l x9, __reloc_start // start of reloc table
+ adr_l x10, __reloc_end // end of reloc table
+
+0: cmp x9, x10
+ b.hs 2f
+ ldp x11, x12, [x9], #24
+ ldr x13, [x9, #-8]
+ cmp w12, #R_AARCH64_RELATIVE
+ b.ne 1f
+ str x13, [x11]
+ b 0b
+
+1: cmp w12, #R_AARCH64_ABS64
+ b.ne 0b
+ add x12, x12, x12, lsl #1 // symtab offset: 24x top word
+ add x12, x8, x12, lsr #(32 - 3) // ... shifted into bottom word
+ ldr x15, [x12, #8] // Elf64_Sym::st_value
+ add x15, x13, x15
+ str x15, [x11]
+ b 0b
+
+2:
+#endif
+
adr_l sp, initial_sp, x4
mov x4, sp
and x4, x4, #~(THREAD_SIZE - 1)
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 282e3e64a17e..e3f6cd740ea3 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -87,6 +87,7 @@ SECTIONS
EXIT_CALL
*(.discard)
*(.discard.*)
+ *(.interp .dynamic)
}
. = KIMAGE_VADDR + TEXT_OFFSET;
@@ -149,6 +150,21 @@ SECTIONS
.altinstr_replacement : {
*(.altinstr_replacement)
}
+ .rela : ALIGN(8) {
+ __reloc_start = .;
+ *(.rela .rela*)
+ __reloc_end = .;
+ }
+ .dynsym : ALIGN(8) {
+ __dynsym_start = .;
+ *(.dynsym)
+ }
+ .dynstr : {
+ *(.dynstr)
+ }
+ .hash : {
+ *(.hash)
+ }
. = ALIGN(PAGE_SIZE);
__init_end = .;
--
2.5.0
