[PATCH v5 0/2] Support kexec/kdump for clang built kernel

[Nick Desaulniers]

1. Reuse the implementation of memcpy and memset instead of relying on
__builtin_memcpy and __builtin_memset as it causes infinite recursion
in Clang (at any opt level) or GCC at -O2.
2. Don't reset KBUILD_CFLAGS, rather filter CONFIG_FUNCTION_TRACER,
CONFIG_STACKPROTECTOR, CONFIG_STACKPROTECTOR_STRONG, and
CONFIG_RETPOLINE flags via `CFLAGS_REMOVE_<file>.o'.

A good test of this series (besides boot testing a kexec kernel):
* There should be no undefined symbols in arch/x86/purgatory/purgatory.ro:
$ nm arch/x86/purgatory/purgatory.ro
  particularly `warn`, `bcmp`, `__stack_chk_fail`, `memcpy` or `memset`.
* `-pg`, `-fstack-protector`, `-fstack-protector-strong`, and
  $(RETPOLINE_CFLAGS) should not be added to the command line for the C
  source files under arch/x86/purgatory/ when compiling with
  CONFIG_FUNCTION_TRACER=y, CONFIG_STACKPROTECTOR=y,
  CONFIG_STACKPROTECTOR_STRONG=y, and CONFIG_RETPOLINE=y.

V5 of: https://lkml.org/lkml/2019/7/25/1276

Nick Desaulniers (2):
  x86/purgatory: do not use __builtin_memcpy and __builtin_memset
  x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

 arch/x86/boot/string.c         |  7 +++++++
 arch/x86/purgatory/Makefile    | 29 ++++++++++++++++++++++++-----
 arch/x86/purgatory/purgatory.c |  6 ++++++
 arch/x86/purgatory/string.c    | 23 -----------------------
 4 files changed, 37 insertions(+), 28 deletions(-)
 delete mode 100644 arch/x86/purgatory/string.c

-- 
2.22.0.709.g102302147b-goog