[PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

[Fangrui Song]

On 2021-03-21, H.J. Lu wrote:
>On Sun, Mar 21, 2021 at 9:01 PM Fangrui Song via Libc-alpha
><libc-alpha@sourceware.org> 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).
>> ---
>>  config.h.in            |  3 +++
>>  configure              | 23 +++++++++++++++++++++++
>>  configure.ac           | 16 ++++++++++++++++
>>  include/libc-symbols.h | 30 +++++++++++++++++++++---------
>>  4 files changed, 63 insertions(+), 9 deletions(-)
>>
>> diff --git a/config.h.in b/config.h.in
>> index f21bf04e47..cc875e90f9 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 37cef37413..28494de748 100755
>> --- a/configure
>> +++ b/configure
>> @@ -4105,6 +4105,29 @@ fi
>>  $as_echo "$libc_cv_textrel_ifunc" >&6; }
>>
>>
>> +# Check if gcc supports attribute ifunc as it is used in libc_ifunc 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; }
>> diff --git a/configure.ac b/configure.ac
>> index 16b15b6f90..94ad713820 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 ifunc as it is used in libc_ifunc 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
>> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
>> index c83e550b03..f8eafcf1f3 100644
>> --- a/include/libc-symbols.h
>> +++ b/include/libc-symbols.h
>> @@ -373,17 +373,29 @@ for linking")
>>
>>  /* These are all done the same way in ELF.
>>     There is a new section created for each set.  */
>> -#ifdef SHARED
>> +#ifdef HAVE_GCC_RETAIN
>> +# ifdef SHARED
>>  /* 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)
>
>You should define __attribute_used_and_retain__ instead.
>
>> +  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, retain, section (#set))) = &(symbol)
>> +# else
>> +#  define _elf_set_element(set, symbol) \
>> +     static const void *const __elf_set_##set##_element_##symbol##__ \
>> +       __attribute__ ((used, retain, section (#set))) = &(symbol)
>> +# endif
>>  #else
>> -# define _elf_set_element(set, symbol) \
>> -  static const void *const __elf_set_##set##_element_##symbol##__ \
>> -    __attribute__ ((used, section (#set))) = &(symbol)
>> -#endif
>> +# ifdef SHARED
>> +#  define _elf_set_element(set, symbol) \
>> +     static const void *__elf_set_##set##_element_##symbol##__ \
>> +       __attribute__ ((used, section (#set))) = &(symbol)
>> +# else
>> +#  define _elf_set_element(set, symbol) \
>> +     static const void *const __elf_set_##set##_element_##symbol##__ \
>> +       __attribute__ ((used, section (#set))) = &(symbol)
>> +# endif
>> +#endif /* HAVE_GCC_RETAIN  */
>>
>>  /* Define SET as a symbol set.  This may be required (it is in a.out) to
>>     be able to use the set's contents.  */
>> --
>> 2.31.0.rc2.261.g7f71774620-goog
>>
>
>libc.a is an installed file.  Does this work with older linkers?
>
>-- 
>H.J.

How about 

   /* These are all done the same way in ELF.
      There is a new section created for each set.  */
   #ifdef HAVE_GCC_RETAIN
   # define attribute_used_retain_section(sec) \
       __attribute__ ((used, retain, section (sec)))
   #else
   # define attribute_used_retain_section(sec) \
       __attribute__ ((used, section (sec)))
   #endif
   #ifdef SHARED
   /* 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_retain_section(#set) = &(symbol)
   #else
   # define _elf_set_element(set, symbol) \
       static const void *const __elf_set_##set##_element_##symbol##__ \
         attribute_used_retain_section(#set) = &(symbol)
   #endif

? Yes, older linkers just ignore unknown section flags. That is the ELF spirit.