Default to --with-default-link=no (bug 25812)

This is necessary to place the libio vtables into the RELRO segment.
New tests elf/tst-relro-ldso and elf/tst-relro-libc are added to
verify that this is what actually happens.

The new tests fail on ia64 due to lack of (default) RELRO support
inbutils, so they are XFAILed there.

INSTALL

@@ -90,6 +90,12 @@ if 'CFLAGS' is specified it must enable optimization.  For example:
      library will still be usable, but functionality may be lost--for
      example, you can't build a shared libc with old binutils.
 
+'--with-default-link=FLAG'
+     With '--with-default-link=yes', the build system does not use a
+     custom linker script for linking shared objects.  The default for
+     FLAG is the opposite, 'no', because the custom linker script is
+     needed for full RELRO protection.
+
 '--with-nonshared-cflags=CFLAGS'
      Use additional compiler flags CFLAGS to build the parts of the
      library which are always statically linked into applications and

configure

@@ -3375,7 +3375,7 @@ fi
 if test "${with_default_link+set}" = set; then :
   withval=$with_default_link; use_default_link=$withval
 else
-  use_default_link=default
+  use_default_link=no
 fi
 
 
@@ -6184,69 +6184,6 @@ fi
 $as_echo "$libc_cv_hashstyle" >&6; }
 
 
-# The linker's default -shared behavior is good enough if it
-# does these things that our custom linker scripts ensure that
-# all allocated NOTE sections come first.
-if test "$use_default_link" = default; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for sufficient default -shared layout" >&5
-$as_echo_n "checking for sufficient default -shared layout... " >&6; }
-if ${libc_cv_use_default_link+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-    libc_cv_use_default_link=no
-  cat > conftest.s <<\EOF
-	  .section .note.a,"a",%note
-	  .balign 4
-	  .long 4,4,9
-	  .string "GNU"
-	  .string "foo"
-	  .section .note.b,"a",%note
-	  .balign 4
-	  .long 4,4,9
-	  .string "GNU"
-	  .string "bar"
-EOF
-  if { ac_try='  ${CC-cc} $ASFLAGS -shared -o conftest.so conftest.s 1>&5'
-  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
-  (eval $ac_try) 2>&5
-  ac_status=$?
-  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
-  test $ac_status = 0; }; } &&
-       ac_try=`$READELF -S conftest.so | sed -n \
-	 '${x;p;}
-	  s/^ *\[ *[1-9][0-9]*\]  *\([^ ][^ ]*\)  *\([^ ][^ ]*\) .*$/\2 \1/
-	  t a
-	  b
-	  : a
-	  H'`
-  then
-    libc_seen_a=no libc_seen_b=no
-    set -- $ac_try
-    while test $# -ge 2 -a "$1" = NOTE; do
-      case "$2" in
-      .note.a) libc_seen_a=yes ;;
-      .note.b) libc_seen_b=yes ;;
-      esac
-      shift 2
-    done
-    case "$libc_seen_a$libc_seen_b" in
-    yesyes)
-      libc_cv_use_default_link=yes
-      ;;
-    *)
-      echo >&5 "\
-$libc_seen_a$libc_seen_b from:
-$ac_try"
-      ;;
-    esac
-  fi
-  rm -f conftest*
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_use_default_link" >&5
-$as_echo "$libc_cv_use_default_link" >&6; }
-  use_default_link=$libc_cv_use_default_link
-fi
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for GLOB_DAT reloc" >&5
 $as_echo_n "checking for GLOB_DAT reloc... " >&6; }
 if ${libc_cv_has_glob_dat+:} false; then :

configure.ac

@@ -153,7 +153,7 @@ AC_ARG_WITH([default-link],
 	    AS_HELP_STRING([--with-default-link],
 			   [do not use explicit linker scripts]),
 	    [use_default_link=$withval],
-	    [use_default_link=default])
+	    [use_default_link=no])
 
 dnl Additional build flags injection.
 AC_ARG_WITH([nonshared-cflags],
@@ -1371,59 +1371,6 @@ fi
 rm -f conftest*])
 AC_SUBST(libc_cv_hashstyle)
 
-# The linker's default -shared behavior is good enough if it
-# does these things that our custom linker scripts ensure that
-# all allocated NOTE sections come first.
-if test "$use_default_link" = default; then
-  AC_CACHE_CHECK([for sufficient default -shared layout],
-		  libc_cv_use_default_link, [dnl
-  libc_cv_use_default_link=no
-  cat > conftest.s <<\EOF
-	  .section .note.a,"a",%note
-	  .balign 4
-	  .long 4,4,9
-	  .string "GNU"
-	  .string "foo"
-	  .section .note.b,"a",%note
-	  .balign 4
-	  .long 4,4,9
-	  .string "GNU"
-	  .string "bar"
-EOF
-  if AC_TRY_COMMAND([dnl
-  ${CC-cc} $ASFLAGS -shared -o conftest.so conftest.s 1>&AS_MESSAGE_LOG_FD]) &&
-       ac_try=`$READELF -S conftest.so | sed -n \
-	 ['${x;p;}
-	  s/^ *\[ *[1-9][0-9]*\]  *\([^ ][^ ]*\)  *\([^ ][^ ]*\) .*$/\2 \1/
-	  t a
-	  b
-	  : a
-	  H']`
-  then
-    libc_seen_a=no libc_seen_b=no
-    set -- $ac_try
-    while test $# -ge 2 -a "$1" = NOTE; do
-      case "$2" in
-      .note.a) libc_seen_a=yes ;;
-      .note.b) libc_seen_b=yes ;;
-      esac
-      shift 2
-    done
-    case "$libc_seen_a$libc_seen_b" in
-    yesyes)
-      libc_cv_use_default_link=yes
-      ;;
-    *)
-      echo >&AS_MESSAGE_LOG_FD "\
-$libc_seen_a$libc_seen_b from:
-$ac_try"
-      ;;
-    esac
-  fi
-  rm -f conftest*])
-  use_default_link=$libc_cv_use_default_link
-fi
-
 AC_CACHE_CHECK(for GLOB_DAT reloc,
 	       libc_cv_has_glob_dat, [dnl
 cat > conftest.c <<EOF

elf/Makefile

@@ -543,6 +543,39 @@ endif
 endif
 endif
 
+tests-special += $(objpfx)tst-relro-ldso.out $(objpfx)tst-relro-libc.out
+$(objpfx)tst-relro-ldso.out: tst-relro-symbols.py $(..)/scripts/glibcelf.py \
+  $(objpfx)ld.so
+	$(PYTHON) tst-relro-symbols.py $(objpfx)ld.so \
+	  --required=_rtld_global_ro \
+	  > $@ 2>&1; $(evaluate-test)
+# The optional symbols are present in libc only if the architecture has
+# the GLIBC_2.0 symbol set in libc.
+$(objpfx)tst-relro-libc.out: tst-relro-symbols.py $(..)/scripts/glibcelf.py \
+  $(common-objpfx)libc.so
+	$(PYTHON) tst-relro-symbols.py $(common-objpfx)libc.so \
+	    --required=_IO_cookie_jumps \
+	    --required=_IO_file_jumps \
+	    --required=_IO_file_jumps_maybe_mmap \
+	    --required=_IO_file_jumps_mmap \
+	    --required=_IO_helper_jumps \
+	    --required=_IO_mem_jumps \
+	    --required=_IO_obstack_jumps \
+	    --required=_IO_proc_jumps \
+	    --required=_IO_str_chk_jumps \
+	    --required=_IO_str_jumps \
+	    --required=_IO_strn_jumps \
+	    --required=_IO_wfile_jumps \
+	    --required=_IO_wfile_jumps_maybe_mmap \
+	    --required=_IO_wfile_jumps_mmap \
+	    --required=_IO_wmem_jumps \
+	    --required=_IO_wstr_jumps \
+	    --required=_IO_wstrn_jumps \
+	    --optional=_IO_old_cookie_jumps \
+	    --optional=_IO_old_file_jumps \
+	    --optional=_IO_old_proc_jumps \
+	  > $@ 2>&1; $(evaluate-test)
+
 ifeq ($(run-built-tests),yes)
 tests-special += $(objpfx)tst-valgrind-smoke.out
 endif

elf/tst-relro-symbols.py

@@ -0,0 +1,137 @@
+#!/usr/bin/python3
+# Verify that certain symbols are covered by RELRO.
+# 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/>.
+
+"""Analyze a (shared) object to verify that certain symbols are
+present and covered by the PT_GNU_RELRO segment.
+
+"""
+
+import argparse
+import os.path
+import sys
+
+# Make available glibc Python modules.
+sys.path.append(os.path.join(
+    os.path.dirname(os.path.realpath(__file__)), os.path.pardir, 'scripts'))
+
+import glibcelf
+
+def find_relro(path: str, img: glibcelf.Image) -> (int, int):
+    """Discover the address range of the PT_GNU_RELRO segment."""
+    for phdr in img.phdrs():
+        if phdr.p_type == glibcelf.Pt.PT_GNU_RELRO:
+            # The computation is not entirely accurate because
+            # _dl_protect_relro in elf/dl-reloc.c rounds both the
+            # start end and downwards using the run-time page size.
+            return phdr.p_vaddr, phdr.p_vaddr + phdr.p_memsz
+    sys.stdout.write('{}: error: no PT_GNU_RELRO segment\n'.format(path))
+    sys.exit(1)
+
+def check_in_relro(kind, relro_begin, relro_end, name, start, size, error):
+    """Check if a section or symbol falls within in the RELRO segment."""
+    end = start + size - 1
+    if not (relro_begin <= start < end < relro_end):
+        error(
+            '{} {!r} of size {} at 0x{:x} is not in RELRO range [0x{:x}, 0x{:x})'.format(
+                kind, name.decode('UTF-8'), start, size,
+                relro_begin, relro_end))
+
+def get_parser():
+    """Return an argument parser for this script."""
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('object', help='path to object file to check')
+    parser.add_argument('--required', metavar='NAME', default=(),
+                        help='required symbol names', nargs='*')
+    parser.add_argument('--optional', metavar='NAME', default=(),
+                        help='required symbol names', nargs='*')
+    return parser
+
+def main(argv):
+    """The main entry point."""
+    parser = get_parser()
+    opts = parser.parse_args(argv)
+    img = glibcelf.Image.readfile(opts.object)
+
+    required_symbols = frozenset([sym.encode('UTF-8')
+                                  for sym in opts.required])
+    optional_symbols = frozenset([sym.encode('UTF-8')
+                                  for sym in opts.optional])
+    check_symbols = required_symbols | optional_symbols
+
+    # Tracks the symbols in check_symbols that have been found.
+    symbols_found = set()
+
+    # Discover the extent of the RELRO segment.
+    relro_begin, relro_end = find_relro(opts.object, img)
+    symbol_table_found = False
+
+    errors = False
+    def error(msg: str) -> None:
+        """Record an error condition and write a message to standard output."""
+        nonlocal errors
+        errors = True
+        sys.stdout.write('{}: error: {}\n'.format(opts.object, msg))
+
+    # Iterate over section headers to find the symbol table.
+    for shdr in img.shdrs():
+        if shdr.sh_type == glibcelf.Sht.SHT_SYMTAB:
+            symbol_table_found = True
+            for sym in img.syms(shdr):
+                if sym.st_name in check_symbols:
+                    symbols_found.add(sym.st_name)
+
+                    # Validate symbol type, section, and size.
+                    if sym.st_info.type != glibcelf.Stt.STT_OBJECT:
+                        error('symbol {!r} has wrong type {}'.format(
+                            sym.st_name.decode('UTF-8'), sym.st_info.type))
+                    if sym.st_shndx in glibcelf.Shn:
+                        error('symbol {!r} has reserved section {}'.format(
+                            sym.st_name.decode('UTF-8'), sym.st_shndx))
+                        continue
+                    if sym.st_size == 0:
+                        error('symbol {!r} has size zero'.format(
+                            sym.st_name.decode('UTF-8')))
+                        continue
+
+                    check_in_relro('symbol', relro_begin, relro_end,
+                                   sym.st_name, sym.st_value, sym.st_size,
+                                   error)
+            continue # SHT_SYMTAB
+        if shdr.sh_name == b'.data.rel.ro' \
+           or shdr.sh_name.startswith(b'.data.rel.ro.'):
+            check_in_relro('section', relro_begin, relro_end,
+                           shdr.sh_name, shdr.sh_addr, shdr.sh_size,
+                           error)
+            continue
+
+    if required_symbols - symbols_found:
+        for sym in sorted(required_symbols - symbols_found):
+            error('symbol {!r} not found'.format(sym.decode('UTF-8')))
+
+    if errors:
+        sys.exit(1)
+
+    if not symbol_table_found:
+        sys.stdout.write(
+            '{}: warning: no symbol table found (stripped object)\n'.format(
+                opts.object))
+        sys.exit(77)
+
+if __name__ == '__main__':
+    main(sys.argv[1:])

manual/install.texi

@@ -117,6 +117,12 @@ problem and suppress these constructs, so that the library will still be
 usable, but functionality may be lost---for example, you can't build a
 shared libc with old binutils.
 
+@item --with-default-link=@var{FLAG}
+With @code{--with-default-link=yes}, the build system does not use a
+custom linker script for linking shared objects.  The default for
+@var{FLAG} is the opposite, @samp{no}, because the custom linker script
+is needed for full RELRO protection.
+
 @item --with-nonshared-cflags=@var{cflags}
 Use additional compiler flags @var{cflags} to build the parts of the
 library which are always statically linked into applications and

sysdeps/unix/sysv/linux/ia64/Makefile

@@ -1,3 +1,9 @@
+ifeq ($(subdir),elf)
+# ia64 does not support PT_GNU_RELRO.
+test-xfail-tst-relro-ldso = yes
+test-xfail-tst-relro-libc = yes
+endif
+
 ifeq ($(subdir),misc)
 sysdep_headers += sys/rse.h
 endif