From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-x433.google.com (mail-pf1-x433.google.com [IPv6:2607:f8b0:4864:20::433]) by sourceware.org (Postfix) with ESMTPS id A53913858004 for ; Tue, 23 Mar 2021 04:15:02 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org A53913858004 Received: by mail-pf1-x433.google.com with SMTP id h3so12941329pfr.12 for ; Mon, 22 Mar 2021 21:15:02 -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=CdC2g1ZUd4vZmkp4+fUMaxI6JQ4iiX2jcneqxRaV9rQ=; b=MXSoWM5eqJOdUegqQGiEPnkzkDg0bwEMm1LxvcTTPOKTQaPi4UPp2QeymNV4VjJjYX R1ed9yYg2tOnS3+/ksToXCO0o62K5BC8xwLfvvqQOQNQbRTDjtYOmq7a1vkzpi5xYI+c vu8WHNgWczoq3RT2e3971uWLnkBoG4XKJE/UF9cpBMS21jQPD6HPwRQe1Le/aK0v0sk0 mgYuDuluAmhe2ooRYQOBpfxu/RL2ewiVtf43oEV4UDC0mhGZfZdzKSaq5BbzrYxNbuFG qxPOi8HbFqnyDTbxMiDLP61V5fhM5vFd7uDB4nZhAEv8UJcwGTjPE6Ay90E6DGpvWt1W 6w7g== X-Gm-Message-State: AOAM5321yTaLPeOgzK/3yub9tv3RZ6TJPEKT2SefRuzAuIDI2tbGM9gA 4FmdENHIoMKClN00YLY8SubBWA== X-Google-Smtp-Source: ABdhPJz8fy7jYSHBgXjv7DedtOdwpBhZK0SS47gJhgSsB63xdIqk5+0wShfbJiPLTqrFpSxsksyOig== X-Received: by 2002:a17:902:6a8c:b029:e6:ef44:6a1e with SMTP id n12-20020a1709026a8cb02900e6ef446a1emr2301476plk.64.1616472901509; Mon, 22 Mar 2021 21:15:01 -0700 (PDT) Received: from google.com ([2620:15c:2ce:0:a99a:4680:c176:4aad]) by smtp.gmail.com with ESMTPSA id fv9sm933791pjb.23.2021.03.22.21.15.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Mar 2021 21:15:00 -0700 (PDT) Date: Mon, 22 Mar 2021 21:14:57 -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: <20210323041457.p7ds4qp7qchxobk5@google.com> References: <87zgz4ygui.fsf@oldenburg.str.redhat.com> <20210322040044.2559211-1-maskray@google.com> <20210322044031.4goih7otuijx4xwg@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=-27.0 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: Tue, 23 Mar 2021 04:15:06 -0000 On 2021-03-22, H.J. Lu wrote: >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 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 > >__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. +#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. */ @@ -377,12 +383,12 @@ 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