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

["H.J. Lu" <hjl.tools@gmail.com>]

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.