Introduce <pointer_guard.h>, extracted from <sysdep.h>

This allows us to define a generic no-op version of PTR_MANGLE and
PTR_DEMANGLE.  In the future, we can use PTR_MANGLE and PTR_DEMANGLE
unconditionally in C sources, avoiding an unintended loss of hardening
due to missing include files or unlucky header inclusion ordering.

In i386 and x86_64, we can avoid a <tls.h> dependency in the C
code by using the computed constant from <tcb-offsets.h>.  <sysdep.h>
no longer includes these definitions, so there is no cyclic dependency
anymore when computing the <tcb-offsets.h> constants.

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

1 files changed, 29 insertions, 0 deletions

diff --git a/sysdeps/generic/pointer_guard.h b/sysdeps/generic/pointer_guard.h
new file mode 100644
index 0000000000..58a624e1d5
--- /dev/null
+++ b/sysdeps/generic/pointer_guard.h
@@ -0,0 +1,29 @@
+/* Pointer obfuscation implenentation.  Generic (no-op) version.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef POINTER_GUARD_H
+#define POINTER_GUARD_H
+
+/* Assembler code depends on PTR_MANGLE not being defined for
+   optimization purposes.  */
+#ifndef __ASSEMBLER__
+# define PTR_MANGLE(x) (void) (x)
+# define PTR_DEMANGLE(x) (void) (x)
+#endif
+
+#endif /* POINTER_GUARD_H */