[PATCH v4] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

[Fangrui Song <maskray@google.com>]

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).
---
Changes in v1 -> v2:
* Define attribute_used_retain_section
Changes in v2 -> v3:
* Use attribute_used_retain instead attribute_used_retain_section
Changes in v3 -> v4:
* Add LIBC_LINKER_FEATURE detection for -z start-stop-gc and add tst-cleanup* tests
---
 config.h.in            |  3 +++
 configure              | 58 ++++++++++++++++++++++++++++++++++++++++++
 configure.ac           | 20 +++++++++++++++
 include/libc-symbols.h | 14 +++++++---
 libio/Makefile         | 32 +++++++++++++++++++++++
 libio/tst-cleanup.c    | 33 ++++++++++++++++++++++++
 libio/tst-cleanup.exp  |  1 +
 7 files changed, 157 insertions(+), 4 deletions(-)
 create mode 100644 libio/tst-cleanup.c
 create mode 100644 libio/tst-cleanup.exp

diff --git a/config.h.in b/config.h.in
index ca1547ae67..95af1ef229 100644
--- a/config.h.in
+++ b/config.h.in
@@ -187,6 +187,9 @@
 /* Define if gcc supports attribute ifunc.  */
 #undef HAVE_GCC_IFUNC
 
+/* Define if gcc supports attribute retain.  */
+#undef HAVE_GCC_RETAIN
+
 /* Define if the linker defines __ehdr_start.  */
 #undef HAVE_EHDR_START
 
diff --git a/configure b/configure
index fcf43bf7de..fa86348342 100755
--- a/configure
+++ b/configure
@@ -626,6 +626,7 @@ fno_unit_at_a_time
 libc_cv_has_glob_dat
 libc_cv_hashstyle
 libc_cv_fpie
+libc_cv_z_start_stop_gc
 libc_cv_z_execstack
 libc_cv_z_combreloc
 ASFLAGS_config
@@ -4105,6 +4106,29 @@ fi
 $as_echo "$libc_cv_textrel_ifunc" >&6; }
 
 
+# Check if gcc supports attribute retain as it is used in attribute_used_retain macro.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
+$as_echo_n "checking for gcc attribute retain support... " >&6; }
+if ${libc_cv_gcc_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_gcc_retain=no
+if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
+   2>&5 ; then
+  libc_cv_gcc_retain=yes
+fi
+rm -f conftest*
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
+$as_echo "$libc_cv_gcc_retain" >&6; }
+if test $libc_cv_gcc_retain = yes; then
+  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
+
+fi
+
 # 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 +5895,40 @@ 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
diff --git a/configure.ac b/configure.ac
index fce967f2c2..4e2907da92 100644
--- a/configure.ac
+++ b/configure.ac
@@ -707,6 +707,22 @@ fi
 rm -f conftest*])
 AC_SUBST(libc_cv_textrel_ifunc)
 
+# Check if gcc supports attribute retain as it is used in attribute_used_retain macro.
+AC_CACHE_CHECK([for gcc attribute retain support],
+	       libc_cv_gcc_retain, [dnl
+cat > conftest.c <<EOF
+static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
+EOF
+libc_cv_gcc_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_gcc_retain=yes
+fi
+rm -f conftest*])
+if test $libc_cv_gcc_retain = yes; then
+  AC_DEFINE(HAVE_GCC_RETAIN)
+fi
+
 # 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 +1333,10 @@ 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],
diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index 546fc26a7b..b311c90f0c 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -352,6 +352,12 @@ for linking")
 \f
 */
 
+#ifdef HAVE_GCC_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.  */
@@ -367,12 +373,12 @@ for linking")
 /* When building a shared library, make the set section writable,
    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)
+    static const void *__elf_set_##set##_element_##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)
+    static const void *const __elf_set_##set##_element_##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
diff --git a/libio/Makefile b/libio/Makefile
index 12ce41038f..439d22e057 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -195,6 +195,20 @@ 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
+tests-static += tst-cleanup-default
+tests-special += $(objpfx)tst-cleanup-default-cmp.out
+LDFLAGS-tst-cleanup-default = -Wl,--gc-sections
+
+ifeq ($(have-z-start-stop-gc),yes)
+tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \
+		$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out
+LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
+endif
 endif
 
 include ../Rules
@@ -224,6 +238,24 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
 $(objpfx)tst-wfile-sync.out: $(gen-locales)
 endif
 
+$(objpfx)tst-cleanup-default-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-default.out
+	cmp $^ > $@; \
+	$(evaluate-test)
+$(objpfx)tst-cleanup-default.o: tst-cleanup.c
+	$(compile.c) -o $@
+
+$(objpfx)tst-cleanup-start-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-start-stop-gc.out
+	cmp $^ > $@; \
+	$(evaluate-test)
+$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
+$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-nostart-stop-gc.out
+	cmp $^ > $@; \
+	$(evaluate-test)
+$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
 $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
 	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'	\
 	$(common-objpfx)libio/; \
diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
new file mode 100644
index 0000000000..7f0a34a91e
--- /dev/null
+++ b/libio/tst-cleanup.c
@@ -0,0 +1,33 @@
+/* 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.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+hook (void)
+{
+  puts ("hello");
+}
+
+int
+main (void)
+{
+  atexit (hook);
+}
diff --git a/libio/tst-cleanup.exp b/libio/tst-cleanup.exp
new file mode 100644
index 0000000000..ce01362503
--- /dev/null
+++ b/libio/tst-cleanup.exp
@@ -0,0 +1 @@
+hello
-- 
2.31.0.291.g576ba9dcdaf-goog