[v4] 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).
---
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

On Wed, Mar 31, 2021 at 12:39 PM Fangrui Song <maskray@google.com> wrote:
>
> 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")
>
>  */
>
> +#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

Won't this test fail if linker supports -z start-stop-gc and GCC doesn't support
retain attribute?

> +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
>

On 2021-03-31, H.J. Lu wrote:
>On Wed, Mar 31, 2021 at 12:39 PM Fangrui Song <maskray@google.com> wrote:
>>
>> 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")
>>
>>  */
>>
>> +#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
>
>Won't this test fail if linker supports -z start-stop-gc and GCC doesn't support
>retain attribute?

Yes, but that doesn't happen in practice. If ld supports -z start-stop-gc,
the GNU as should be new and the GCC should support 'retain'.
But perhaps it is good to prevent the issue, I can change

+ifeq ($(have-z-start-stop-gc),yes)

to

+ifeq ($(have-gnu-retain)$(have-z-start-stop-gc),yesyes)

and add a LIBC_CONFIG_VAR in configure.ac.

>> +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
>>
>
>
>-- 
>H.J.

On Wed, Mar 31, 2021 at 5:02 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2021-03-31, H.J. Lu wrote:
> >On Wed, Mar 31, 2021 at 12:39 PM Fangrui Song <maskray@google.com> wrote:
> >>
> >> 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")
> >>
> >>  */
> >>
> >> +#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
> >
> >Won't this test fail if linker supports -z start-stop-gc and GCC doesn't support
> >retain attribute?
>
> Yes, but that doesn't happen in practice. If ld supports -z start-stop-gc,
> the GNU as should be new and the GCC should support 'retain'.

I am using GCC 10 and binutils 2.37.

> But perhaps it is good to prevent the issue, I can change
>
> +ifeq ($(have-z-start-stop-gc),yes)
>
> to
>
> +ifeq ($(have-gnu-retain)$(have-z-start-stop-gc),yesyes)
>
> and add a LIBC_CONFIG_VAR in configure.ac.
>

Yes, please.

Thanks.