arch: enable relative relocations for arm64, power and x86

Patch series "add support for relative references in special sections", v10.

This adds support for emitting special sections such as initcall arrays,
PCI fixups and tracepoints as relative references rather than absolute
references.  This reduces the size by 50% on 64-bit architectures, but
more importantly, it removes the need for carrying relocation metadata for
these sections in relocatable kernels (e.g., for KASLR) that needs to be
fixed up at boot time.  On arm64, this reduces the vmlinux footprint of
such a reference by 8x (8 byte absolute reference + 24 byte RELA entry vs
4 byte relative reference)

Patch #3 was sent out before as a single patch.  This series supersedes
the previous submission.  This version makes relative ksymtab entries
dependent on the new Kconfig symbol HAVE_ARCH_PREL32_RELOCATIONS rather
than trying to infer from kbuild test robot replies for which
architectures it should be blacklisted.

Patch #1 introduces the new Kconfig symbol HAVE_ARCH_PREL32_RELOCATIONS,
and sets it for the main architectures that are expected to benefit the
most from this feature, i.e., 64-bit architectures or ones that use
runtime relocations.

Patch #2 add support for #define'ing __DISABLE_EXPORTS to get rid of
ksymtab/kcrctab sections in decompressor and EFI stub objects when
rebuilding existing C files to run in a different context.

Patches #4 - #6 implement relative references for initcalls, PCI fixups
and tracepoints, respectively, all of which produce sections with order
~1000 entries on an arm64 defconfig kernel with tracing enabled.  This
means we save about 28 KB of vmlinux space for each of these patches.

[From the v7 series blurb, which included the jump_label patches as well]:

  For the arm64 kernel, all patches combined reduce the memory footprint
  of vmlinux by about 1.3 MB (using a config copied from Ubuntu that has
  KASLR enabled), of which ~1 MB is the size reduction of the RELA section
  in .init, and the remaining 300 KB is reduction of .text/.data.

This patch (of 6):

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.

Link: http://lkml.kernel.org/r/20180704083651.24360-2-ard.biesheuvel@linaro.org
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: James Morris <jmorris@namei.org>
Cc: Nicolas Pitre <nico@linaro.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Cc: James Morris <james.morris@microsoft.com>
Cc: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/Kconfig b/arch/Kconfig
index c614816..4426e96 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -841,6 +841,16 @@ 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"
 
 source "scripts/gcc-plugins/Kconfig"

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index d0a53cc..29e75b4 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -108,6 +108,7 @@ 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_STACKLEAK
        select HAVE_ARCH_THREAD_STRUCT_WHITELIST

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a806692..db0b6ee 100644 (file)
--- 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 HAVE_CBPF_JIT                    if !PPC64

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b0312f8..512003f 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -124,6 +124,7 @@ 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