[PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

[Will Deacon]

When building with LTO, there is an increased risk of the compiler
converting an address dependency headed by a READ_ONCE() invocation
into a control dependency and consequently allowing for harmful
reordering by the CPU.

Ensure that such transformations are harmless by overriding the generic
READ_ONCE() definition with one that provides acquire semantics when
building with LTO.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
 arch/arm64/kernel/vdso/Makefile   |  2 +-
 arch/arm64/kernel/vdso32/Makefile |  2 +-
 3 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm64/include/asm/rwonce.h

diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
new file mode 100644
index 000000000000..515e360b01a1
--- /dev/null
+++ b/arch/arm64/include/asm/rwonce.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2020 Google LLC.
+ */
+#ifndef __ASM_RWONCE_H
+#define __ASM_RWONCE_H
+
+#ifdef CONFIG_CLANG_LTO
+
+#include <linux/compiler_types.h>
+#include <asm/alternative-macros.h>
+
+#ifndef BUILD_VDSO
+
+#ifdef CONFIG_AS_HAS_LDAPR
+#define __LOAD_RCPC(sfx, regs...)					\
+	ALTERNATIVE(							\
+		"ldar"	#sfx "\t" #regs,				\
+		".arch_extension rcpc\n"				\
+		"ldapr"	#sfx "\t" #regs,				\
+	ARM64_HAS_LDAPR)
+#else
+#define __LOAD_RCPC(sfx, regs...)	"ldar" #sfx "\t" #regs
+#endif /* CONFIG_AS_HAS_LDAPR */
+
+#define __READ_ONCE(x)							\
+({									\
+	int atomic = 1;							\
+	union { __unqual_scalar_typeof(x) __val; char __c[1]; } __u;	\
+	typeof(&(x)) __x = &(x);					\
+	switch (sizeof(x)) {						\
+	case 1:								\
+		asm volatile(__LOAD_RCPC(b, %w0, %1)			\
+			: "=r" (*(__u8 *)__u.__c)			\
+			: "Q" (*__x) : "memory");			\
+		break;							\
+	case 2:								\
+		asm volatile(__LOAD_RCPC(h, %w0, %1)			\
+			: "=r" (*(__u16 *)__u.__c)			\
+			: "Q" (*__x) : "memory");			\
+		break;							\
+	case 4:								\
+		asm volatile(__LOAD_RCPC(, %w0, %1)			\
+			: "=r" (*(__u32 *)__u.__c)			\
+			: "Q" (*__x) : "memory");			\
+		break;							\
+	case 8:								\
+		asm volatile(__LOAD_RCPC(, %0, %1)			\
+			: "=r" (*(__u64 *)__u.__c)			\
+			: "Q" (*__x) : "memory");			\
+		break;							\
+	default:							\
+		atomic = 0;						\
+	}								\
+	atomic ? (typeof(x))__u.__val : (*(volatile typeof(x) *)__x);	\
+})
+
+#endif	/* !BUILD_VDSO */
+#endif	/* CONFIG_CLANG_LTO */
+
+#include <asm-generic/rwonce.h>
+
+#endif	/* __ASM_RWONCE_H */
diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index 45d5cfe46429..60df97f2e7de 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -28,7 +28,7 @@ ldflags-y := -shared -nostdlib -soname=linux-vdso.so.1 --hash-style=sysv	\
 	     $(btildflags-y) -T
 
 ccflags-y := -fno-common -fno-builtin -fno-stack-protector -ffixed-x18
-ccflags-y += -DDISABLE_BRANCH_PROFILING
+ccflags-y += -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
 
 CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) $(GCC_PLUGINS_CFLAGS)
 KBUILD_CFLAGS			+= $(DISABLE_LTO)
diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
index d88148bef6b0..4fdf3754a058 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -43,7 +43,7 @@ cc32-as-instr = $(call try-run,\
 # As a result we set our own flags here.
 
 # KBUILD_CPPFLAGS and NOSTDINC_FLAGS from top-level Makefile
-VDSO_CPPFLAGS := -D__KERNEL__ -nostdinc -isystem $(shell $(CC_COMPAT) -print-file-name=include)
+VDSO_CPPFLAGS := -DBUILD_VDSO -D__KERNEL__ -nostdinc -isystem $(shell $(CC_COMPAT) -print-file-name=include)
 VDSO_CPPFLAGS += $(LINUXINCLUDE)
 
 # Common C and assembly flags
-- 
2.27.0.212.ge8ba1cc988-goog