arm64: add support for building vmlinux as a relocatable PIE binary

This implements CONFIG_RELOCATABLE, which links the final vmlinux
image with a dynamic relocation section, allowing 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.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index c85c29d660bd..fef3dc6e21b9 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -787,6 +787,17 @@ 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"