[PATCH v7 01/10] arch: enable relative relocations for arm64, power and x86

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@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 400b9e1b2f27..dbc036a7bd1b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -959,4 +959,14 @@ 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 c9a7e9e1414f..66c7b9ab2a3d 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -89,6 +89,7 @@ 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_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c51e6ce42e7a..e172478e2ae7 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -177,6 +177,7 @@ 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 ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d4fc98c50378..9f2bb853aedb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -115,6 +115,7 @@ 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_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
-- 
2.11.0