From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-x329.google.com (mail-ot1-x329.google.com [IPv6:2607:f8b0:4864:20::329]) by sourceware.org (Postfix) with ESMTPS id B52F73858D29 for ; Mon, 22 Mar 2021 04:25:09 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org B52F73858D29 Received: by mail-ot1-x329.google.com with SMTP id v24-20020a9d69d80000b02901b9aec33371so14677922oto.2 for ; Sun, 21 Mar 2021 21:25:09 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=l08p3RGV6GlMbdWu1zLiXOF7PnDMncZbQ0qF7lo9qpA=; b=hr3Mpy7vIoXgC96ACYBzRuzJwk0g1DpvcgkJAw0/WWIPW9YHs0yF2PmYbsrGiuNyxu tAsOVLqYfB9M1CZ66i9BR8H7gNznKsFbX1hS0dMHkb63IlroWXwxxl/XfM32t06ChPnC jogQrFT7d3dXv5niANH3Ui8wYOs2mWUqnkNutbF806chq4uIqbhg+ItRDjKJsKYLQXlP 8KAu37Uybkl2+n9awQB7Q65nutnKWJ97ECiNWdzP2qtj2Dg/zLqqJpvyxvx9ToDMhq8B OtqpEpN0tUYOUlcFzCszAKYGbHjaEiEZj5cKNBFKfeSEeRrvHErF3L+wQ+Y7kgqi2e8G SglA== X-Gm-Message-State: AOAM530Xa4R4qDkWJDpoWBQPkvjmVasA8TN0ZzVmIQ1cxP0BdTiwDPVB gv+jFRH9i5eyb2iv8TaFwXMaIv4iq6igOuz7KPc= X-Google-Smtp-Source: ABdhPJx58NNcDMnGFvOueEWRFg2kSMHFausqZhnnlVbUHry5fb4nO/QGkK3jlndDntgFwQ+zBXLZnV8J0OWHpo+IDmA= X-Received: by 2002:a05:6830:1515:: with SMTP id k21mr9932782otp.269.1616387109177; Sun, 21 Mar 2021 21:25:09 -0700 (PDT) MIME-Version: 1.0 References: <87zgz4ygui.fsf@oldenburg.str.redhat.com> <20210322040044.2559211-1-maskray@google.com> In-Reply-To: <20210322040044.2559211-1-maskray@google.com> From: "H.J. Lu" Date: Sun, 21 Mar 2021 21:24:33 -0700 Message-ID: Subject: Re: [PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492] To: Fangrui Song Cc: GNU C Library , Florian Weimer Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-3035.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 22 Mar 2021 04:25:12 -0000 On Sun, Mar 21, 2021 at 9:01 PM Fangrui Song via Libc-alpha 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 =E2=80=98retain=E2=80=99 attribute ignored while __= has_attribute(retain) is 1 > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99587). > --- > 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 macr= o. > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute reta= in 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 < +static int var __attribute__ ((used, retain, section ("__libc_atexit"))= ); > +EOF > +libc_cv_gcc_retain=3Dno > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \ > + 2>&5 ; then > + libc_cv_gcc_retain=3Dyes > +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 =3D 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 abou= t alias for function with incompatible types" >&5 > $as_echo_n "checking if compiler warns about alias for function with inc= ompatible 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 macr= o. > +AC_CACHE_CHECK([for gcc attribute retain support], > + libc_cv_gcc_retain, [dnl > +cat > conftest.c < +static int var __attribute__ ((used, retain, section ("__libc_atexit"))= ); > +EOF > +libc_cv_gcc_retain=3Dno > +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=3Dyes > +fi > +rm -f conftest*]) > +if test $libc_cv_gcc_retain =3D 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 incompat= ible 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))) =3D &(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))) =3D &(symbol) > +# else > +# define _elf_set_element(set, symbol) \ > + static const void *const __elf_set_##set##_element_##symbol##__ \ > + __attribute__ ((used, retain, section (#set))) =3D &(symbol) > +# endif > #else > -# define _elf_set_element(set, symbol) \ > - static const void *const __elf_set_##set##_element_##symbol##__ \ > - __attribute__ ((used, section (#set))) =3D &(symbol) > -#endif > +# ifdef SHARED > +# define _elf_set_element(set, symbol) \ > + static const void *__elf_set_##set##_element_##symbol##__ \ > + __attribute__ ((used, section (#set))) =3D &(symbol) > +# else > +# define _elf_set_element(set, symbol) \ > + static const void *const __elf_set_##set##_element_##symbol##__ \ > + __attribute__ ((used, section (#set))) =3D &(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? --=20 H.J.