Reduce the statically linked startup code [BZ #23323]

It turns out the startup code in csu/elf-init.c has a perfect pair of
ROP gadgets (see Marco-Gisbert and Ripoll-Ripoll, "return-to-csu: A
New Method to Bypass 64-bit Linux ASLR").  These functions are not
needed in dynamically-linked binaries because DT_INIT/DT_INIT_ARRAY
are already processed by the dynamic linker.  However, the dynamic
linker skipped the main program for some reason.  For maximum
backwards compatibility, this is not changed, and instead, the main
map is consulted from __libc_start_main if the init function argument
is a NULL pointer.

For statically linked binaries, the old approach based on linker
symbols is still used because there is nothing else available.

A new symbol version __libc_start_main@@GLIBC_2.34 is introduced because
new binaries running on an old libc would not run their ELF
constructors, leading to difficult-to-debug issues.

63 files changed, 276 insertions, 315 deletions

diff --git a/csu/Makefile b/csu/Makefile
index c9385df2e9..e587434be8 100644
--- a/csu/Makefile
+++ b/csu/Makefile
@@ -27,10 +27,9 @@ subdir := csu
 include ../Makeconfig
 
 routines = init-first libc-start $(libc-init) sysdep version check_fds \
-	   libc-tls elf-init dso_handle
+	   libc-tls dso_handle
 aux	 = errno
 elide-routines.os = libc-tls
-static-only-routines = elf-init
 csu-dummies = $(filter-out $(start-installed-name),crt1.o Mcrt1.o)
 extra-objs = start.o \
 	     $(start-installed-name) g$(start-installed-name) $(csu-dummies) \
@@ -59,13 +58,6 @@ CFLAGS-.os += $(no-stack-protector)
 # but it does not matter for this source file.
 CFLAGS-static-reloc.os += $(stack-protector)
 
-# This file is not actually part of the startup code in the nonshared
-# case and statically linked into applications.  See
-# <https://sourceware.org/bugzilla/show_bug.cgi?id=23323>,
-# <https://sourceware.org/ml/libc-alpha/2018-06/msg00717.html>.
-# Also see the note above regarding STACK_PROTECTOR_LEVEL.
-CFLAGS-elf-init.oS += $(stack-protector)
-
 ifeq (yes,$(build-shared))
 extra-objs += S$(start-installed-name) gmon-start.os
 ifneq ($(start-installed-name),$(static-start-installed-name))
diff --git a/csu/Versions b/csu/Versions
index 43010c3443..8e1b21948e 100644
--- a/csu/Versions
+++ b/csu/Versions
@@ -7,6 +7,9 @@ libc {
     # New special glibc functions.
     gnu_get_libc_release; gnu_get_libc_version;
   }
+  GLIBC_2.34 {
+    __libc_start_main;
+  }
   GLIBC_PRIVATE {
     errno;
   }
diff --git a/csu/elf-init.c b/csu/elf-init.c
deleted file mode 100644
index 6e96ab7fce..0000000000
--- a/csu/elf-init.c
+++ /dev/null
@@ -1,107 +0,0 @@
-/* Startup support for ELF initializers/finalizers in the main executable.
-   Copyright (C) 2002-2021 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.
-
-   In addition to the permissions in the GNU Lesser General Public
-   License, the Free Software Foundation gives you unlimited
-   permission to link the compiled version of this file with other
-   programs, and to distribute those programs without any restriction
-   coming from the use of this file. (The GNU Lesser General Public
-   License restrictions do apply in other respects; for example, they
-   cover modification of the file, and distribution when not linked
-   into another program.)
-
-   Note that people who make modified versions of this file are not
-   obligated to grant this special exception for their modified
-   versions; it is their choice whether to do so. The GNU Lesser
-   General Public License gives permission to release a modified
-   version without this exception; this exception also makes it
-   possible to release a modified version which carries forward this
-   exception.
-
-   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/>.  */
-
-#include <stddef.h>
-#include <elf-initfini.h>
-
-
-/* These magic symbols are provided by the linker.  */
-extern void (*__preinit_array_start []) (int, char **, char **)
-  attribute_hidden;
-extern void (*__preinit_array_end []) (int, char **, char **)
-  attribute_hidden;
-extern void (*__init_array_start []) (int, char **, char **)
-  attribute_hidden;
-extern void (*__init_array_end []) (int, char **, char **)
-  attribute_hidden;
-extern void (*__fini_array_start []) (void) attribute_hidden;
-extern void (*__fini_array_end []) (void) attribute_hidden;
-
-
-#if ELF_INITFINI
-/* These function symbols are provided for the .init/.fini section entry
-   points automagically by the linker.  */
-extern void _init (void);
-extern void _fini (void);
-#endif
-
-
-/* These functions are passed to __libc_start_main by the startup code.
-   These get statically linked into each program.  For dynamically linked
-   programs, this module will come from libc_nonshared.a and differs from
-   the libc.a module in that it doesn't call the preinit array.  */
-
-
-void
-__libc_csu_init (int argc, char **argv, char **envp)
-{
-  /* For dynamically linked executables the preinit array is executed by
-     the dynamic linker (before initializing any shared object).  */
-
-#ifndef LIBC_NONSHARED
-  /* For static executables, preinit happens right before init.  */
-  {
-    const size_t size = __preinit_array_end - __preinit_array_start;
-    size_t i;
-    for (i = 0; i < size; i++)
-      (*__preinit_array_start [i]) (argc, argv, envp);
-  }
-#endif
-
-#if ELF_INITFINI
-  _init ();
-#endif
-
-  const size_t size = __init_array_end - __init_array_start;
-  for (size_t i = 0; i < size; i++)
-      (*__init_array_start [i]) (argc, argv, envp);
-}
-
-/* This function should not be used anymore.  We run the executable's
-   destructor now just like any other.  We cannot remove the function,
-   though.  */
-void
-__libc_csu_fini (void)
-{
-#ifndef LIBC_NONSHARED
-  size_t i = __fini_array_end - __fini_array_start;
-  while (i-- > 0)
-    (*__fini_array_start [i]) ();
-
-# if ELF_INITFINI
-  _fini ();
-# endif
-#endif
-}
diff --git a/csu/libc-start.c b/csu/libc-start.c
index feb0d7ce11..05ff7afddf 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -1,4 +1,5 @@
-/* Copyright (C) 1998-2021 Free Software Foundation, Inc.
+/* Perform initialization and invoke main.
+   Copyright (C) 1998-2021 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
@@ -15,10 +16,15 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+/* Note: This code is only part of the startup code proper for
+   statically linked binaries.  For dynamically linked binaries, it
+   resides in libc.so.  */
+
 /* Mark symbols hidden in static PIE for early self relocation to work.  */
 #if BUILD_PIE_DEFAULT
 # pragma GCC visibility push(hidden)
 #endif
+
 #include <assert.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -29,6 +35,8 @@
 #include <libc-internal.h>
 #include <elf/libc-early-init.h>
 #include <stdbool.h>
+#include <elf-initfini.h>
+#include <shlib-compat.h>
 
 #include <elf/dl-tunables.h>
 
@@ -95,9 +103,11 @@ apply_irel (void)
 # else
 #  define STATIC static inline __attribute__ ((always_inline))
 # endif
+# define DO_DEFINE_LIBC_START_MAIN_VERSION 0
 #else
 # define STATIC<