ld.so: Do not export free/calloc/malloc/realloc functions [BZ #25486]

Exporting functions and relying on symbol interposition from libc.so
makes the choice of implementation dependent on DT_NEEDED order, which
is not what some compiler drivers expect.

This commit replaces one magic mechanism (symbol interposition) with
another one (preprocessor-/compiler-based redirection).  This makes
the hand-over from the minimal malloc to the full malloc more
explicit.

Removing the ABI symbols is backwards-compatible because libc.so is
always in scope, and the dynamic loader will find the malloc-related
symbols there since commit f0b2132b35
("ld.so: Support moving versioned symbols between sonames
[BZ #24741]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

include/rtld-malloc.h

@@ -0,0 +1,85 @@
+/* Redirection of malloc inside the dynamic linker.
+   Copyright (C) 2020 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/>.  */
+
+/* The dynamic linker needs to use its own minimal malloc before libc
+   has been relocated, and the libc malloc afterwards.  The active
+   malloc implementation is reached via the __rtld_* function pointers
+   declared below.  They are initialized to the minimal malloc by
+   __rtld_malloc_init_stubs, and set to the final implementation by
+   __rtld_malloc_init_real.  */
+
+#ifndef _RTLD_MALLOC_H
+#define _RTLD_MALLOC_H
+
+#if IS_IN (rtld)
+
+extern __typeof (calloc) *__rtld_calloc attribute_hidden;
+extern __typeof (free) *__rtld_free attribute_hidden;
+extern __typeof (malloc) *__rtld_malloc attribute_hidden;
+extern __typeof (realloc) *__rtld_realloc attribute_hidden;
+
+/* Wrapper functions which call through the function pointers above.
+   Note that it is not supported to take the address of those
+   functions.  Instead the function pointers must be used
+   directly.  */
+
+__extern_inline void *
+calloc (size_t a, size_t b)
+{
+  return __rtld_calloc (a, b);
+}
+
+__extern_inline void
+free (void *ptr)
+{
+   __rtld_free (ptr);
+}
+
+__extern_inline void *
+malloc (size_t size)
+{
+  return __rtld_malloc (size);
+}
+
+__extern_inline void *
+realloc (void *ptr, size_t size)
+{
+  return __rtld_realloc (ptr, size);
+}
+
+/* Called after the first self-relocation to activate the minimal malloc
+   implementation.  */
+void __rtld_malloc_init_stubs (void) attribute_hidden;
+
+/* Called shortly before the final self-relocation (when RELRO
+   variables are still writable) to activate the real malloc
+   implementation.  MAIN_MAP is the link map of the executable.  */
+struct link_map;
+void __rtld_malloc_init_real (struct link_map *main_map) attribute_hidden;
+
+#else /* !IS_IN (rtld) */
+
+/* This allows static/non-rtld builds to get a pointer to the
+   functions, in the same way that is required inside rtld.  */
+# define __rtld_calloc (&calloc)
+# define __rtld_free (&free)
+# define __rtld_malloc (&malloc)
+# define __rtld_realloc (&realloc)
+
+#endif /* !IS_IN (rtld) */
+#endif /* _RTLD_MALLOC_H */