arm64: Add support for relocating the kernel with RELR relocations

RELR is a relocation packing format for relative relocations.
The format is described in a generic-abi proposal:
https://groups.google.com/d/topic/generic-abi/bX460iggiKg/discussion

The LLD linker can be instructed to pack relocations in the RELR
format by passing the flag --pack-dyn-relocs=relr.

This patch adds a new config option, CONFIG_RELR. Enabling this option
instructs the linker to pack vmlinux's relative relocations in the RELR
format, and causes the kernel to apply the relocations at startup along
with the RELA relocations. RELA relocations still need to be applied
because the linker will emit RELA relative relocations if they are
unrepresentable in the RELR format (i.e. address not a multiple of 2).

Enabling CONFIG_RELR reduces the size of a defconfig kernel image
with CONFIG_RANDOMIZE_BASE by 3.5MB/16% uncompressed, or 550KB/5%
compressed (lz4).

Signed-off-by: Peter Collingbourne <pcc@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index a7b57dd42c26..aa6bdb3df5c1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -925,6 +925,20 @@ config LOCK_EVENT_COUNTS
           the chance of application behavior change because of timing
           differences. The counts are reported via debugfs.
 
+# Select if the architecture has support for applying RELR relocations.
+config ARCH_HAS_RELR
+        bool
+
+config RELR
+        bool "Use RELR relocation packing"
+        depends on ARCH_HAS_RELR && TOOLS_SUPPORT_RELR
+        default y
+        help
+          Store the kernel's dynamic relocations in the RELR relocation packing
+          format. Requires a compatible linker (LLD supports this feature), as
+          well as compatible NM and OBJCOPY utilities (llvm-nm and llvm-objcopy
+          are compatible).
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"