Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

So that text_set_element/data_set_element/bss_set_element defined
variables will be retained by the linker.

Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
will not be optimized out; 'retain' prevents section garbage collection
if the linker support SHF_GNU_RETAIN.

GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
identifier name sections to be GCed even if there are live
__start_/__stop_ references.

Without the change, there are some static linking problems, e.g.
_IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
stdout is not flushed on exit.

Note: GCC may warning 'retain' attribute ignored while __has_attribute(retain)
is 1 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

config.h.in

@@ -187,6 +187,9 @@
 /* Define if gcc supports attribute ifunc.  */
 #undef HAVE_GCC_IFUNC
 
+/* Define if CC supports attribute retain.  */
+#undef HAVE_GNU_RETAIN
+
 /* Define if the linker defines __ehdr_start.  */
 #undef HAVE_EHDR_START
 

configure

@@ -4105,6 +4105,31 @@ fi
 $as_echo "$libc_cv_textrel_ifunc" >&6; }
 
 
+# Check if CC supports attribute retain as it is used in attribute_used_retain macro.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for GNU attribute retain support" >&5
+$as_echo_n "checking for GNU attribute retain support... " >&6; }
+if ${libc_cv_gnu_retain+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat > conftest.c <<EOF
+static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
+EOF
+libc_cv_gnu_retain=no
+if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
+   2>&5 ; then
+  libc_cv_gnu_retain=yes
+fi
+rm -f conftest*
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gnu_retain" >&5
+$as_echo "$libc_cv_gnu_retain" >&6; }
+if test $libc_cv_gnu_retain = yes; then
+  $as_echo "#define HAVE_GNU_RETAIN 1" >>confdefs.h
+
+fi
+config_vars="$config_vars
+have-gnu-retain = $libc_cv_gnu_retain"
+
 # Check if gcc warns about alias for function with incompatible types.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
 $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
@@ -5871,6 +5896,40 @@ fi
 $as_echo "$libc_linker_feature" >&6; }
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports -z start-stop-gc" >&5
+$as_echo_n "checking for linker that supports -z start-stop-gc... " >&6; }
+libc_linker_feature=no
+if test x"$gnu_ld" = x"yes"; then
+  libc_linker_check=`$LD -v --help 2>/dev/null | grep "\-z start-stop-gc"`
+  if test -n "$libc_linker_check"; then
+    cat > conftest.c <<EOF
+int _start (void) { return 42; }
+EOF
+    if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+				-Wl,-z,start-stop-gc -nostdlib -nostartfiles
+				-fPIC -shared -o conftest.so conftest.c
+				1>&5'
+  { { eval echo "\"\$as_me\":${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; }; }
+    then
+      libc_linker_feature=yes
+    fi
+    rm -f conftest*
+  fi
+fi
+if test $libc_linker_feature = yes; then
+  libc_cv_z_start_stop_gc=yes
+else
+  libc_cv_z_start_stop_gc=no
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5
+$as_echo "$libc_linker_feature" >&6; }
+config_vars="$config_vars
+have-z-start-stop-gc = $libc_cv_z_start_stop_gc"
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
 $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
 libc_linker_feature=no

configure.ac

@@ -707,6 +707,23 @@ fi
 rm -f conftest*])
 AC_SUBST(libc_cv_textrel_ifunc)
 
+# Check if CC supports attribute retain as it is used in attribute_used_retain macro.
+AC_CACHE_CHECK([for GNU attribute retain support],
+	       libc_cv_gnu_retain, [dnl
+cat > conftest.c <<EOF
+static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
+EOF
+libc_cv_gnu_retain=no
+if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
+   2>&AS_MESSAGE_LOG_FD ; then
+  libc_cv_gnu_retain=yes
+fi
+rm -f conftest*])
+if test $libc_cv_gnu_retain = yes; then
+  AC_DEFINE(HAVE_GNU_RETAIN)
+fi
+LIBC_CONFIG_VAR([have-gnu-retain], [$libc_cv_gnu_retain])
+
 # Check if gcc warns about alias for function with incompatible types.
 AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
 	       libc_cv_gcc_incompatible_alias, [dnl
@@ -1317,6 +1334,10 @@ LIBC_LINKER_FEATURE([-z execstack], [-Wl,-z,execstack],
 		    [libc_cv_z_execstack=yes], [libc_cv_z_execstack=no])
 AC_SUBST(libc_cv_z_execstack)
 
+LIBC_LINKER_FEATURE([-z start-stop-gc], [-Wl,-z,start-stop-gc],
+		    [libc_cv_z_start_stop_gc=yes], [libc_cv_z_start_stop_gc=no])
+LIBC_CONFIG_VAR([have-z-start-stop-gc], [$libc_cv_z_start_stop_gc])
+
 LIBC_LINKER_FEATURE([--no-dynamic-linker],
 		    [-Wl,--no-dynamic-linker],
 		    [libc_cv_no_dynamic_linker=yes],

include/libc-symbols.h

@@ -352,6 +352,12 @@ for linking")
 
 */
 
+#ifdef HAVE_GNU_RETAIN
+# define attribute_used_retain __attribute__ ((__used__, __retain__))
+#else
+# define attribute_used_retain __attribute__ ((__used__))
+#endif
+
 /* Symbol set support macros.  */
 
 /* Make SYMBOL, which is in the text segment, an element of SET.  */
@@ -368,11 +374,11 @@ for linking")
    because it will need to be relocated at run time anyway.  */
 # define _elf_set_element(set, symbol) \
     static const void *__elf_set_##set##_element_##symbol##__ \
-    __attribute__ ((used, section (#set))) = &(symbol)
+      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
 #else
 # define _elf_set_element(set, symbol) \
     static const void *const __elf_set_##set##_element_##symbol##__ \
-    __attribute__ ((used, section (#set))) = &(symbol)
+      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
 #endif
 
 /* Define SET as a symbol set.  This may be required (it is in a.out) to

libio/Makefile

@@ -195,6 +195,26 @@ ifeq (yes,$(build-shared))
 tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
 		 $(objpfx)tst-bz24228-mem.out
 endif
+
+tests += tst-cleanup-default tst-cleanup-default-static
+tests-static += tst-cleanup-default-static
+tests-special += $(objpfx)tst-cleanup-default-cmp.out $(objpfx)tst-cleanup-default-static-cmp.out
+LDFLAGS-tst-cleanup-default = -Wl,--gc-sections
+LDFLAGS-tst-cleanup-default-static = -Wl,--gc-sections
+
+ifeq ($(have-gnu-retain)$(have-z-start-stop-gc),yesyes)
+tests += tst-cleanup-start-stop-gc tst-cleanup-start-stop-gc-static \
+		tst-cleanup-nostart-stop-gc tst-cleanup-nostart-stop-gc-static
+tests-static += tst-cleanup-start-stop-gc-static tst-cleanup-nostart-stop-gc-static
+tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \
+		$(objpfx)tst-cleanup-start-stop-gc-static-cmp.out \
+		$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out \
+		$(objpfx)tst-cleanup-nostart-stop-gc-static-cmp.out
+LDFLAGS-tst-cleanup-start-stop-gc := -Wl,--gc-sections,-z,start-stop-gc
+LDFLAGS-tst-cleanup-start-stop-gc-static := -Wl,--gc-sections,-z,start-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc := -Wl,--gc-sections,-z,nostart-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc-static := -Wl,--gc-sections,-z,nostart-stop-gc
+endif
 endif
 
 include ../Rules
@@ -224,6 +244,14 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
 $(objpfx)tst-wfile-sync.out: $(gen-locales)
 endif
 
+define gen-tst-cleanup
+$(objpfx)tst-cleanup-$1-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-$1.out
+	cmp $$^ > $$@; $$(evaluate-test)
+endef
+
+$(foreach t,default default-static start-stop-gc start-stop-gc-static nostart-stop-gc nostart-stop-gc-static, \
+  $(eval $(call gen-tst-cleanup,$(t))))
+
 $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
 	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'	\
 	$(common-objpfx)libio/; \

libio/tst-cleanup-default-static.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup-default.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup-nostart-stop-gc-static.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup-nostart-stop-gc.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup-start-stop-gc-static.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup-start-stop-gc.c

@@ -0,0 +1 @@
+#include "tst-cleanup.c"

libio/tst-cleanup.c

@@ -0,0 +1,34 @@
+/* Copyright (C) 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.
+
+   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/>.  */
+
+/* Test that stdout is flushed after atexit callbacks were run, even if the
+ * executable is linked with --gc-sections.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+hook (void)
+{
+  puts ("hello");
+}
+
+int
+main (void)
+{
+  atexit (hook);
+}

libio/tst-cleanup.exp

@@ -0,0 +1 @@
+hello