From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-x22d.google.com (mail-oi1-x22d.google.com [IPv6:2607:f8b0:4864:20::22d]) by sourceware.org (Postfix) with ESMTPS id CD2603858004 for ; Mon, 22 Mar 2021 20:29:05 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org CD2603858004 Received: by mail-oi1-x22d.google.com with SMTP id m13so14486299oiw.13 for ; Mon, 22 Mar 2021 13:29:05 -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=DYe0JchLgiE4Oqjmto4CJTJGgCRNUoyEW6K1l0ui12M=; b=f/Cegyaj5hVx1vkOvaxTJtXOEFQGG35gsOoN9NV0OVbFsQkHd0AjLf4TXuzEGs7DXf wZknZ6KgvDFDQs2UZC5TjZQmHPpUAIS6V8KLQrasFbavA+u13LjR10jE3Pu399tOBWIb 0fjg2g9WDEN4Wukfctwgr3JTNnUQR9FzFKl5fT+5ap+mLUre0k/JTiPdg4UghKQQpKrS xgXDEEmnepAarZO5pRO7qdZBamEzmUo+d/Tz1p5RmVJ3+mmljUk/CRQE+bL/BBqWfZB9 voUDbPZhPdYPflllr8MhiRRG4SPNno8mYX7GevJ+R4v0AhmwShfKR7ByOy7lZL9HPVN+ yCTg== X-Gm-Message-State: AOAM532mMifDaVw1hUNXql5GYBFpp9CCXcZNGNbLyE3DLG9fJODMlJD+ IcrxOy+a7341HrZAOaDenHhpDuTSuPy+rWlyaBYXbJGy/SM= X-Google-Smtp-Source: ABdhPJzNuN9abj6oOyxVRKCZUfHwVxEaqYtJZGfWjjPr4/UXxhQmQPUxdAWRip3LnSYRFA4Two6DGXVklJ4LKmoE5WE= X-Received: by 2002:a05:6808:94:: with SMTP id s20mr623165oic.25.1616444944643; Mon, 22 Mar 2021 13:29:04 -0700 (PDT) MIME-Version: 1.0 References: <87zgz4ygui.fsf@oldenburg.str.redhat.com> <20210322040044.2559211-1-maskray@google.com> <20210322044031.4goih7otuijx4xwg@google.com> In-Reply-To: <20210322044031.4goih7otuijx4xwg@google.com> From: "H.J. Lu" Date: Mon, 22 Mar 2021 13:28:28 -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=-3035.3 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 20:29:09 -0000 On Sun, Mar 21, 2021 at 9:40 PM Fangrui Song wrote: > > On 2021-03-21, H.J. Lu wrote: > >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 variab= le > >> will not be optimized out; 'retain' prevents section garbage collectio= n > >> 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 m= acro. > >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute r= etain 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 a= bout 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 m= acro. > >> +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 incom= patible 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? > > > >-- > >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) =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 > ? Yes, older linkers just ignore unknown section flags. That is the ELF s= pirit. --=20 H.J.