On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote:
quoted hunk ↗ jump to hunk
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@kernel.org>
---
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, \
^ Should this be here? It seems that READ_ONCE() will actually read
twice... even if that doesn't actually conflict with the required
semantics of READ_ONCE(), it looks odd.
Making a direct link between LTO and the memory model also seems highly
spurious (as discussed in the other subthread) so can we have a comment
explaining the reasoning?
quoted hunk ↗ jump to hunk
+ ".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 */
[...]
Cheers
---Dave
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