[tip: x86/build] x86/build: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)

[tip-bot2 for John Millikin]

The following commit has been merged into the x86/build branch of tip:

Commit-ID:     8abe7fc26ad8f28bfdf78adbed56acd1fa93f82d
Gitweb:        https://git.kernel.org/tip/8abe7fc26ad8f28bfdf78adbed56acd1fa93f82d
Author:        John Millikin <john@xxxxxxxxxxxxxxxxx>
AuthorDate:    Thu, 25 Mar 2021 17:04:33 -07:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 26 Mar 2021 11:32:47 +01:00

x86/build: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)

When cross-compiling with Clang, the `$(CLANG_FLAGS)' variable
contains additional flags needed to build C and assembly sources
for the target platform. Normally this variable is automatically
included in `$(KBUILD_CFLAGS)' via the top-level Makefile.

The x86 real-mode makefile builds `$(REALMODE_CFLAGS)' from a
plain assignment and therefore drops the Clang flags. This causes
Clang to not recognize x86-specific assembler directives:

  arch/x86/realmode/rm/header.S:36:1: error: unknown directive
  .type real_mode_header STT_OBJECT ; .size real_mode_header, .-real_mode_header
  ^

Explicit propagation of `$(CLANG_FLAGS)' to `$(REALMODE_CFLAGS)',
which is inherited by real-mode make rules, fixes cross-compilation
with Clang for x86 targets.

Relevant flags:

* `--target' sets the target architecture when cross-compiling. This
  flag must be set for both compilation and assembly (`KBUILD_AFLAGS')
  to support architecture-specific assembler directives.

* `-no-integrated-as' tells clang to assemble with GNU Assembler
  instead of its built-in LLVM assembler. This flag is set by default
  unless `LLVM_IAS=1' is set, because the LLVM assembler can't yet
  parse certain GNU extensions.

Signed-off-by: John Millikin <john@xxxxxxxxxxxxxxxxx>
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20210326000435.4785-2-nathan@xxxxxxxxxx
---
 arch/x86/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2d6d5a2..9a73e0c 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -33,6 +33,7 @@ REALMODE_CFLAGS += -ffreestanding
 REALMODE_CFLAGS += -fno-stack-protector
 REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
 REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
+REALMODE_CFLAGS += $(CLANG_FLAGS)
 export REALMODE_CFLAGS
 
 # BITS is used as extension for files which are available in a 32 bit