[PATCH 11/12] aarch64: redefine RETURN_ADDRESS to strip PAC

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

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From 2223e5ed1d78634ef59f0a7efbd3a9885a8da53f Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Wed, 15 Apr 2020 17:40:45 +0100
Subject: [PATCH 11/12] aarch64: redefine RETURN_ADDRESS to strip PAC

RETURN_ADDRESS is used at several places in glibc to mean a valid
code address of the call site, but with pac-ret that includes a
pointer authentication code, so the definition is adjusted.

XPAC is added unconditionally for now, but it's only needed if
glibc is compiled with -mbranch-protection=pac-ret. Inline asm
is used instead of __builtin_aarch64_xpaclri since that's an
undocumented builtin and not available in all supported gccs.
---
 sysdeps/aarch64/sysdep.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
index 63a04a70cd..87f19b9bef 100644
--- a/sysdeps/aarch64/sysdep.h
+++ b/sysdeps/aarch64/sysdep.h
@@ -35,6 +35,16 @@
 
 #define PTR_SIZE	(1<<PTR_LOG_SIZE)
 
+/* Strip pointer authentication code from pointer p.  */
+#define XPAC(p) ({					\
+  register void *__ra asm ("x30") = (p);		\
+  asm ("hint 7 // xpaclri" : "+r"(__ra));		\
+  __ra;})
+
+/* This is needed when glibc is built with -mbranch-protection=pac-ret.  */
+#undef RETURN_ADDRESS
+#define RETURN_ADDRESS(n) XPAC(__builtin_return_address(n))
+
 #ifdef	__ASSEMBLER__
 
 /* Syntactic details of assembler.  */
-- 
2.17.1