Re: [PATCH v3] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

["Fāng-ruì Sòng" <maskray@google.com>]

On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Mar 29, 2021 at 4:24 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
> > ---
> >  config.h.in            |  3 +++
> >  configure              | 23 +++++++++++++++++++++++
> >  configure.ac           | 16 ++++++++++++++++
> >  include/libc-symbols.h | 14 ++++++++++----
> >  4 files changed, 52 insertions(+), 4 deletions(-)
> >
> > 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..6a6b8ac264 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 fce967f2c2..031b8c3fb3 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 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
> > --
> > 2.31.0.291.g576ba9dcdaf-goog
> >
>
> Need gc-sections tests for linkers with and without -z start-stop-gc.
>
> --
> H.J.

I don't know how to add such a test... It needs to check flush at ext
time but I cannot find a template in libio/tst-* ...