[PATCH v8 1/6] arch: enable relative relocations for arm64, power and x86

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

Before updating certain subsystems to use place relative 32-bit
relocations in special sections, to save space and reduce the
number of absolute relocations that need to be processed at runtime
by relocatable kernels, introduce the Kconfig symbol and define it
for some architectures that should be able to support and benefit
from it.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: x86 at kernel.org
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/Kconfig         | 10 ++++++++++
 arch/arm64/Kconfig   |  1 +
 arch/powerpc/Kconfig |  1 +
 arch/x86/Kconfig     |  1 +
 4 files changed, 13 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 76c0b54443b1..4e624f75823a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -975,4 +975,14 @@ config REFCOUNT_FULL
 	  against various use-after-free conditions that can be used in
 	  security flaw exploits.
 
+config HAVE_ARCH_PREL32_RELOCATIONS
+	bool
+	help
+	  May be selected by an architecture if it supports place-relative
+	  32-bit relocations, both in the toolchain and in the module loader,
+	  in which case relative references can be used in special sections
+	  for PCI fixup, initcalls etc which are only half the size on 64 bit
+	  architectures, and don't require runtime relocation on relocatable
+	  kernels.
+
 source "kernel/gcov/Kconfig"
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7381eeb7ef8e..7c543667703e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -91,6 +91,7 @@ config ARM64
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
+	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 73ce5dd07642..21d5ad1608d8 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -181,6 +181,7 @@ config PPC
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
+	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_CBPF_JIT			if !PPC64
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c1236b187824..1c6e2ddd2fdc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -117,6 +117,7 @@ config X86
 	select HAVE_ARCH_MMAP_RND_BITS		if MMU
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT
 	select HAVE_ARCH_COMPAT_MMAP_BASES	if MMU && COMPAT
+	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK
-- 
2.15.1