[PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]
H.J. Lu
hjl.tools@gmail.com
Mon Mar 22 20:28:28 GMT 2021
On Sun, Mar 21, 2021 at 9:40 PM Fangrui Song <maskray@google.com> wrote:
>
> 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
__attribute_used_and_retain__ is more flex and can be used
without section.
> # 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.
--
H.J.
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