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

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

On Tue, Mar 30, 2021 at 9:34 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
> On 2021-03-30, H.J. Lu wrote:
> >On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
> >>
> >> 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-* ...
> >
> >You can add a special rule to check for linker output with
> >--print-gc-sections.
>
> I can come up runtime tests. ( https://github.com/MaskRay/glibc/tree/retain)
> Runtime tests are direct.
> (Unsure how to test --print-gc-sections with tests-static...)
>
>
> diff --git a/libio/Makefile b/libio/Makefile
> index 12ce41038f..5e0e7f00cd 100644
> --- a/libio/Makefile
> +++ b/libio/Makefile
> @@ -195,6 +195,13 @@ 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-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.out \
> +               $(objpfx)tst-cleanup-nostart-stop-gc.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

1. Need to check if linker supports -z start-stop-gc.
2. Need to test with only -Wl,--gc-sections.

>   endif
>
>   include ../Rules
> @@ -224,6 +231,20 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
>   $(objpfx)tst-wfile-sync.out: $(gen-locales)
>   endif
>
> +$(objpfx)tst-cleanup-start-stop-gc.out: $(objpfx)tst-cleanup-start-stop-gc
> +       $< > $@T
> +       grep hello $@T; $(evaluate-test)
> +       rm -f $@T

tst-cleanup-start-stop-gc.out isn't generated.  You should add a ".exp" file
and compare the .out file against it.


> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
> +       $(compile.c) -o $@
> +
> +$(objpfx)tst-cleanup-nostart-stop-gc.out: $(objpfx)tst-cleanup-nostart-stop-gc
> +       $< > $@T
> +       grep hello $@T; $(evaluate-test)
> +       rm -f $@T
> +$(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..de8130a482
> --- /dev/null
> +++ b/libio/tst-cleanup.c
> @@ -0,0 +1,14 @@

Missing comments and copyright notice.

> +#include <stdio.h>
> +#include <stdlib.h>
> +
> +void
> +hook (void)
> +{
> +  puts ("hello");
> +}
> +
> +int
> +main (void)
> +{
> +  atexit (hook);
> +}



--
H.J.