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Subject[PATCH v3 1/2] retpolines: Enable retpoline support only when compiler support it
Since retpoline capable compilers are widely available, make
CONFIG_RETPOLINE hard depend on it. This is achieved by breaking
the build early and showing an error like below:

"arch/x86/Makefile:226: *** You are building kernel with non-retpoline
compiler, please update your compiler.. Stop."

[peterz@infradead.org: initial suggestion]
Link: https://lore.kernel.org/lkml/caeb3213f13e57191e362ca04f2892cfa2ccda65.camel@infradead.org
[dwmw@amazon.co.uk: fail the build with non-retpoline compiler]
Link: https://lore.kernel.org/lkml/alpine.DEB.2.21.1811011317400.1642@nanos.tec.linutronix.de
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
---
arch/x86/Kconfig | 4 ----
arch/x86/Makefile | 5 +++--
arch/x86/include/asm/nospec-branch.h | 10 ++++++----
arch/x86/kernel/cpu/bugs.c | 2 +-
scripts/Makefile.build | 2 --
5 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c51c989..fcb7f37 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -443,10 +443,6 @@ config RETPOLINE
branches. Requires a compiler with -mindirect-branch=thunk-extern
support for full protection. The kernel may run slower.

- Without compiler support, at least indirect branches in assembler
- code are eliminated. Since this includes the syscall entry path,
- it is not entirely pointless.
-
config INTEL_RDT
bool "Intel Resource Director Technology support"
depends on X86 && CPU_SUP_INTEL
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b562e4..6d94fe8 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -222,9 +222,10 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables

# Avoid indirect branches in kernel to deal with Spectre
ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
- KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+ $(error You are building kernel with non-retpoline compiler, please update your compiler.)
endif
+ KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
endif

archscripts: scripts_basic
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 80dc144..8b09cbb 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -162,11 +162,12 @@
_ASM_PTR " 999b\n\t" \
".popsection\n\t"

-#if defined(CONFIG_X86_64) && defined(RETPOLINE)
+#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_X86_64

/*
- * Since the inline asm uses the %V modifier which is only in newer GCC,
- * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
+ * Inline asm uses the %V modifier which is only in newer GCC
+ * which is ensured when CONFIG_RETPOLINE is defined.
*/
# define CALL_NOSPEC \
ANNOTATE_NOSPEC_ALTERNATIVE \
@@ -181,7 +182,7 @@
X86_FEATURE_RETPOLINE_AMD)
# define THUNK_TARGET(addr) [thunk_target] "r" (addr)

-#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
+#else /* CONFIG_X86_32 */
/*
* For i386 we use the original ret-equivalent retpoline, because
* otherwise we'll run out of registers. We don't care about CET
@@ -211,6 +212,7 @@
X86_FEATURE_RETPOLINE_AMD)

# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
+#endif
#else /* No retpoline for C / inline asm */
# define CALL_NOSPEC "call *%[thunk_target]\n"
# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index c37e66e..d0108fb 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -252,7 +252,7 @@ static void __init spec2_print_if_secure(const char *reason)

static inline bool retp_compiler(void)
{
- return __is_defined(RETPOLINE);
+ return __is_defined(CONFIG_RETPOLINE);
}

static inline bool match_option(const char *arg, int arglen, const char *opt)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a8e7ba9..6a6be9f 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -236,10 +236,8 @@ ifdef CONFIG_GCOV_KERNEL
objtool_args += --no-unreachable
endif
ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
objtool_args += --retpoline
endif
-endif


ifdef CONFIG_MODVERSIONS
--
1.8.3.1
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