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Re: [PATCH] x86/build: fix compiler support check for CONFIG_RETPOLINE





On 2018/12/5 11:00, Masahiro Yamada wrote:
It is wrong to add CONFIG option diagnostic to the Makefile parse
stage.

Once you are hit by the error about non-retpoline compiler, the
compilation still breaks even after disabling CONFIG_RETPOLINE.

The easiest fix is to move this check to the "archprepare" like commit
829fe4aa9ac1 ("x86: Allow generating user-space headers without a
compiler") did.

Link: https://lkml.org/lkml/2018/12/4/206
Reported-by: Meelis Roos <mroos@xxxxxxxx>
Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

  arch/x86/Makefile | 14 ++++++++------
  1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index f5d7f41..ae0148b 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -219,12 +219,7 @@ KBUILD_CFLAGS += -Wno-sign-compare
  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
# Avoid indirect branches in kernel to deal with Spectre
-ifdef CONFIG_RETPOLINE
-ifeq ($(RETPOLINE_CFLAGS),)
-  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
-endif
-  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
-endif
+KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)

Is it better to also move "# Avoid indirect branches in kernel to deal with Spectre"

and "KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)"?

Seems unconditionaly using "KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)" will have compiler

using "__x86_indirect_thunk_\reg" call even ifCONFIG_RETPOLINE is disabled. I guess

link process will fail?

Thanks

Zhenzhong

archscripts: scripts_basic
  	$(Q)$(MAKE) $(build)=arch/x86/tools relocs
@@ -307,6 +302,13 @@ ifndef CC_HAVE_ASM_GOTO
  	@echo Compiler lacks asm-goto support.
  	@exit 1
  endif
+ifdef CONFIG_RETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+	@echo "You are building kernel with non-retpoline compiler." >&2
+	@echo "Please update your compiler." >&2
+	@false
+endif
+endif
archclean:
  	$(Q)rm -rf $(objtree)/arch/i386