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 730a23b78107304ad83bec6b3f6c1f870f2b18e8..a687298b1c4b532950264f5d512046d9a20a053c 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 cd88023163009630bea1612fd0deec6a02de9d69..f442506952a9466bf924939085e34524c933394d 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.  */