[glibc] aarch64: redefine RETURN_ADDRESS to strip PAC

[Szabolcs Nagy <nsz@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=d174ec248d5b6bba3842f425f036495b682c313f

commit d174ec248d5b6bba3842f425f036495b682c313f
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Wed Apr 15 17:40:45 2020 +0100

    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 it may contain a
    pointer authentication code (PAC), so its definition is adjusted.
    
    This is gcc PR target/94891: __builtin_return_address should not
    expose signed pointers to user code where it can cause ABI issues.
    In glibc RETURN_ADDRESS is only changed if it is built with pac-ret.
    There is no detection for the specific gcc issue because it is
    hard to test and the additional xpac does not cause problems.
    
    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Diff:
---
 sysdeps/aarch64/sysdep.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
index f442506952..ae269146e3 100644
--- a/sysdeps/aarch64/sysdep.h
+++ b/sysdeps/aarch64/sysdep.h
@@ -44,6 +44,13 @@ strip_pac (void *p)
   asm ("hint 7 // xpaclri" : "+r"(ra));
   return ra;
 }
+
+/* This is needed when glibc is built with -mbranch-protection=pac-ret
+   with a gcc that is affected by PR target/94891.  */
+# if HAVE_AARCH64_PAC_RET
+#  undef RETURN_ADDRESS
+#  define RETURN_ADDRESS(n) strip_pac (__builtin_return_address (n))
+# endif
 #endif
 
 #ifdef	__ASSEMBLER__