From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x102e.google.com (mail-pj1-x102e.google.com [IPv6:2607:f8b0:4864:20::102e]) by sourceware.org (Postfix) with ESMTPS id AF04C3858D29 for ; Mon, 22 Mar 2021 04:40:36 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org AF04C3858D29 Received: by mail-pj1-x102e.google.com with SMTP id j6-20020a17090adc86b02900cbfe6f2c96so7854820pjv.1 for ; Sun, 21 Mar 2021 21:40:36 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=BRf6PEvGizPuc+PQ3n/WqEEOIrSk/l2IPHTq31qPZQQ=; b=HT8QNPNlaTWf7Ubr/OZZgIE+awUf8gUK17/eok03OF0filb5qSnUerCFqHRn9WXXu/ pv/99fMltv5Bm/Jj/SfoqhYF+aosaO1R9cnZVBY5RTWPxfRvUbUeIa+Nb0/LStVE68oh /sExMuKPtio4wNqEtgJBjE5eDVNfmwswtVvsNVRV21b+xJQmhpI+d5C4XvCb7k+D5RMB 8JizVji4W6dFDPnEgDvzJfy2IqjMnUNM6f7sRWW+cNhPFA+aDL1g3KI7kLQgldZ3Xtds qsVuqPrYnJPTAn3z11xgAPz4LPod4FJkbro+Jb7U8nDN/a6frP0Oclb3mMfzZmEK5tdS iQlg== X-Gm-Message-State: AOAM531oB4fc/D24v/4pza6Nf+wRrKFD0px+rZxfzPLRCwlLvmj78l8M gQk/znsRKXJWKHyKlA0umZQGUw== X-Google-Smtp-Source: ABdhPJxsG1rRibxCntJNeXvdDR26PdetWj9rGPVDw6K/xbunrd2tYDVzUez1MEwOQGulvxkhuy3n3g== X-Received: by 2002:a17:90a:1463:: with SMTP id j90mr11310120pja.205.1616388035290; Sun, 21 Mar 2021 21:40:35 -0700 (PDT) Received: from google.com ([2620:15c:2ce:0:683f:d5f2:bcd4:6d19]) by smtp.gmail.com with ESMTPSA id f15sm11703385pgr.90.2021.03.21.21.40.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 21 Mar 2021 21:40:34 -0700 (PDT) Date: Sun, 21 Mar 2021 21:40:31 -0700 From: Fangrui Song To: "H.J. Lu" Cc: GNU C Library , Florian Weimer Subject: Re: [PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492] Message-ID: <20210322044031.4goih7otuijx4xwg@google.com> References: <87zgz4ygui.fsf@oldenburg.str.redhat.com> <20210322040044.2559211-1-maskray@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Spam-Status: No, score=-27.1 required=5.0 tests=BAYES_00, DKIMWL_WL_MED, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, ENV_AND_HDR_SPF_MATCH, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, USER_IN_DEF_DKIM_WL, USER_IN_DEF_SPF_WL 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 04:40:40 -0000 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 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 <> +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 <> +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 # 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.