[PATCH v2 00/28] Add support for Clang LTO

[Sami Tolvanen]

This patch series adds support for building x86_64 and arm64 kernels
with Clang's Link Time Optimization (LTO).

In addition to performance, the primary motivation for LTO is
to allow Clang's Control-Flow Integrity (CFI) to be used in the
kernel. Google has shipped millions of Pixel devices running three
major kernel versions with LTO+CFI since 2018.

Most of the patches are build system changes for handling LLVM
bitcode, which Clang produces with LTO instead of ELF object files,
postponing ELF processing until a later stage, and ensuring initcall
ordering.

Note that patches 1-4 are not directly related to LTO, but are
needed to compile LTO kernels with ToT Clang, so I'm including them
in the series for your convenience:

 - Patches 1-3 are required for building the kernel with ToT Clang,
   and IAS, and patch 4 is needed to build allmodconfig with LTO.

 - Patches 3-4 are already in linux-next, but not yet in 5.9-rc.

---
Changes in v2:

  - Fixed -Wmissing-prototypes warnings with W=1.

  - Dropped cc-option from -fsplit-lto-unit and added .thinlto-cache
    scrubbing to make distclean.

  - Added a comment about Clang >=11 being required.

  - Added a patch to disable LTO for the arm64 KVM nVHE code.

  - Disabled objtool's noinstr validation with LTO unless enabled.

  - Included Peter's proposed objtool mcount patch in the series
    and replaced recordmcount with the objtool pass to avoid
    whitelisting relocations that are not calls.

  - Updated several commit messages with better explanations.


Arvind Sankar (2):
  x86/boot/compressed: Disable relocation relaxation
  x86/asm: Replace __force_order with memory clobber

Luca Stefani (1):
  RAS/CEC: Fix cec_init() prototype

Nick Desaulniers (1):
  lib/string.c: implement stpcpy

Peter Zijlstra (1):
  objtool: Add a pass for generating __mcount_loc

Sami Tolvanen (23):
  objtool: Don't autodetect vmlinux.o
  kbuild: add support for objtool mcount
  x86, build: use objtool mcount
  kbuild: add support for Clang LTO
  kbuild: lto: fix module versioning
  kbuild: lto: postpone objtool
  kbuild: lto: limit inlining
  kbuild: lto: merge module sections
  kbuild: lto: remove duplicate dependencies from .mod files
  init: lto: ensure initcall ordering
  init: lto: fix PREL32 relocations
  PCI: Fix PREL32 relocations for LTO
  modpost: lto: strip .lto from module names
  scripts/mod: disable LTO for empty.c
  efi/libstub: disable LTO
  drivers/misc/lkdtm: disable LTO for rodata.o
  arm64: export CC_USING_PATCHABLE_FUNCTION_ENTRY
  arm64: vdso: disable LTO
  KVM: arm64: disable LTO for the nVHE directory
  arm64: allow LTO_CLANG and THINLTO to be selected
  x86, vdso: disable LTO only for vDSO
  x86, relocs: Ignore L4_PAGE_OFFSET relocations
  x86, build: allow LTO_CLANG and THINLTO to be selected

 .gitignore                            |   1 +
 Makefile                              |  65 ++++++-
 arch/Kconfig                          |  67 +++++++
 arch/arm64/Kconfig                    |   2 +
 arch/arm64/Makefile                   |   1 +
 arch/arm64/kernel/vdso/Makefile       |   4 +-
 arch/arm64/kvm/hyp/nvhe/Makefile      |   4 +-
 arch/x86/Kconfig                      |   3 +
 arch/x86/Makefile                     |   5 +
 arch/x86/boot/compressed/Makefile     |   2 +
 arch/x86/boot/compressed/pgtable_64.c |   9 -
 arch/x86/entry/vdso/Makefile          |   5 +-
 arch/x86/include/asm/special_insns.h  |  28 +--
 arch/x86/kernel/cpu/common.c          |   4 +-
 arch/x86/tools/relocs.c               |   1 +
 drivers/firmware/efi/libstub/Makefile |   2 +
 drivers/misc/lkdtm/Makefile           |   1 +
 drivers/ras/cec.c                     |   9 +-
 include/asm-generic/vmlinux.lds.h     |  11 +-
 include/linux/init.h                  |  79 +++++++-
 include/linux/pci.h                   |  19 +-
 kernel/trace/Kconfig                  |   5 +
 lib/string.c                          |  24 +++
 scripts/Makefile.build                |  55 +++++-
 scripts/Makefile.lib                  |   6 +-
 scripts/Makefile.modfinal             |  31 ++-
 scripts/Makefile.modpost              |  26 ++-
 scripts/generate_initcall_order.pl    | 270 ++++++++++++++++++++++++++
 scripts/link-vmlinux.sh               |  94 ++++++++-
 scripts/mod/Makefile                  |   1 +
 scripts/mod/modpost.c                 |  16 +-
 scripts/mod/modpost.h                 |   9 +
 scripts/mod/sumversion.c              |   6 +-
 scripts/module-lto.lds                |  26 +++
 tools/objtool/builtin-check.c         |  13 +-
 tools/objtool/builtin.h               |   2 +-
 tools/objtool/check.c                 |  83 ++++++++
 tools/objtool/check.h                 |   1 +
 tools/objtool/objtool.h               |   1 +
 39 files changed, 883 insertions(+), 108 deletions(-)
 create mode 100755 scripts/generate_initcall_order.pl
 create mode 100644 scripts/module-lto.lds


base-commit: e28f0104343d0c132fa37f479870c9e43355fee4
-- 
2.28.0.402.g5ffc5be6b7-goog