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From: Ard Biesheuvel <ardb@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Cc: Ard Biesheuvel <ardb@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Quentin Perret <qperret@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>, Will Deacon <will@kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Kees Cook <keescook@chromium.org>
Subject: [PATCH v5 1/2] static_call: force symbol references with external linkage for CFI/LTO
Date: Thu, 28 Oct 2021 01:34:08 +0200	[thread overview]
Message-ID: <20211027233409.902331-2-ardb@kernel.org> (raw)
In-Reply-To: <20211027233409.902331-1-ardb@kernel.org>

When building with Clang with CFI or LTO enabled, the linker may decide
not to emit function symbols with static linkage at all, or emit them
under a different symbol name. This breaks static calls, given that we
refer to such functions both from C code and from assembler, and we
expect the names to be the same.

So let's force the use of an alias with external linkage in a way that
is visible to the compiler. This ensures that the C name and the asm
name are identical.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 include/linux/static_call.h | 21 ++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 3e56a9751c06..19dc210214c0 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -327,10 +327,27 @@ static inline int static_call_text_reserved(void *start, void *end)
 
 #endif /* CONFIG_HAVE_STATIC_CALL */
 
+#ifdef CONFIG_LTO
+/*
+ * DEFINE_STATIC_CALL() accepts any function symbol reference for its _func
+ * argument, but this may cause problems under Clang LTO/CFI if the function
+ * symbol has static linkage, because the symbol names exposed at the
+ * asm/object level may deviate from the C names. So let's force the reference
+ * to go via an alias with external linkage instead.
+ */
+#define _DEFINE_STATIC_CALL(name, _func, _init, _alias)			\
+	extern typeof(_func) _alias __alias(_init);			\
+	__DEFINE_STATIC_CALL(name, _func, _alias)
+#else
+#define _DEFINE_STATIC_CALL(name, _func, _init, _alias)			\
+	__DEFINE_STATIC_CALL(name, _func, _init)
+#endif
+
 #define DEFINE_STATIC_CALL(name, _func)					\
-	__DEFINE_STATIC_CALL(name, _func, _func)
+	_DEFINE_STATIC_CALL(name, _func, _func, __UNIQUE_ID(_func))
 
 #define DEFINE_STATIC_CALL_RET0(name, _func)				\
-	__DEFINE_STATIC_CALL(name, _func, __static_call_return0)
+	_DEFINE_STATIC_CALL(name, _func, __static_call_return0,		\
+			    __UNIQUE_ID(_func))
 
 #endif /* _LINUX_STATIC_CALL_H */
-- 
2.30.2


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  reply	other threads:[~2021-10-27 23:35 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-27 23:34 [PATCH v5 0/2] static call support for arm64 Ard Biesheuvel
2021-10-27 23:34 ` Ard Biesheuvel [this message]
2021-10-27 23:34 ` [PATCH v5 2/2] arm64: implement support for static call trampolines Ard Biesheuvel

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