aarch64: fix pac-ret support in _mcount

Currently gcc -pg -mbranch-protection=pac-ret passes signed return
address to _mcount, so _mcount now has to always strip pac from the
frompc since that's from user code that may be built with pac-ret.

This is gcc PR target/94791: signed pointers should not escape and get
passed across extern call boundaries, since that's an ABI break, but
because existing gcc has this issue we work it around in glibc until
that is resolved. This is compatible with a fixed gcc and it is a nop
on systems without PAuth support. The bug was introduced in gcc-7 with
-msign-return-address=non-leaf|all support which in gcc-9 got renamed
to -mbranch-protection=pac-ret|pac-ret+leaf|standard.

strip_pac uses inline asm instead of __builtin_aarch64_xpaclri since
that is not a documented api and not available in all supported gccs.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

diff --git a/sysdeps/aarch64/machine-gmon.h b/sysdeps/aarch64/machine-gmon.h
index 730a23b..a687298 100644 (file)
--- a/sysdeps/aarch64/machine-gmon.h
+++ b/sysdeps/aarch64/machine-gmon.h
@@ -27,8 +27,9 @@ static void mcount_internal (u_long frompc, u_long selfpc);
 #define _MCOUNT_DECL(frompc, selfpc) \
 static inline void mcount_internal (u_long frompc, u_long selfpc)
 
+/* Note: strip_pac is needed for frompc because of gcc PR target/94791.  */
 #define MCOUNT                                                    \
 void __mcount (void *frompc)                                      \
 {                                                                 \
-  mcount_internal ((u_long) frompc, (u_long) RETURN_ADDRESS (0)); \
+  mcount_internal ((u_long) strip_pac (frompc), (u_long) RETURN_ADDRESS (0)); \
 }

diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
index cd88023..f442506 100644 (file)
--- a/sysdeps/aarch64/sysdep.h
+++ b/sysdeps/aarch64/sysdep.h
@@ -35,6 +35,17 @@
 
 #define PTR_SIZE       (1<<PTR_LOG_SIZE)
 
+#ifndef __ASSEMBLER__
+/* Strip pointer authentication code from pointer p.  */
+static inline void *
+strip_pac (void *p)
+{
+  register void *ra asm ("x30") = (p);
+  asm ("hint 7 // xpaclri" : "+r"(ra));
+  return ra;
+}
+#endif
+
 #ifdef __ASSEMBLER__
 
 /* Syntactic details of assembler.  */