From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-x62e.google.com (mail-pl1-x62e.google.com [IPv6:2607:f8b0:4864:20::62e]) by sourceware.org (Postfix) with ESMTPS id 8BAF83858C27 for ; Thu, 1 Apr 2021 00:02:52 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 8BAF83858C27 Received: by mail-pl1-x62e.google.com with SMTP id f17so91946plr.0 for ; Wed, 31 Mar 2021 17:02:52 -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=DqVmuyb1CwpfPKRAwFhBaqs5PVp+FrwL5QTft0s2GBU=; b=b3lEogZntro/uZq7hCoHxeb7qpgjCnDk1vqrhzpOruokQM7Ts8x499phAuOPleWXH6 LNV/ZqRmnMgTWzwGbkVIGKvUOYCdEYYc9ZKqrzQjdsxd6g35dNSQK88taNN8FGyKRuNa RukMInPXOiIntcX7LqWgSUMJNGSSLavJtSwDXhS0ACv0FpoRr5U2vccad014wQnpmJse fQZMIm6zEUVWzEdxxp1OxTgHbs/Pu1clR+OXzi9HcASmm/sU1lPknI1Uo6xQIuy+NNlz xvLHjuv0bQDFPjVLnlzDEkcRfOVlV/Bu37oVyWbJO4WwQbRxLo3lrzkF44DmCZpUEYIZ n2pg== X-Gm-Message-State: AOAM531AvuKWZmAODOM7KgMX41Xj1GWAuQX9vCZSw0gCBzVTOZ6royJu bZRYAuK+oDV7DONHxwMxWOV9aw== X-Google-Smtp-Source: ABdhPJwWXGfNIbfCeqSXxnDpJhOCNS5MNMo7M9plJOXjUy8A17hvPCnEBIb+8fq2p12mAnS6B000mw== X-Received: by 2002:a17:902:8482:b029:e6:325b:5542 with SMTP id c2-20020a1709028482b02900e6325b5542mr5330171plo.70.1617235371005; Wed, 31 Mar 2021 17:02:51 -0700 (PDT) Received: from google.com ([2620:15c:2ce:0:9106:8d2e:6ec7:9708]) by smtp.gmail.com with ESMTPSA id k15sm3504777pfi.0.2021.03.31.17.02.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 Mar 2021 17:02:50 -0700 (PDT) Date: Wed, 31 Mar 2021 17:02:47 -0700 From: Fangrui Song To: "H.J. Lu" Cc: GNU C Library , Florian Weimer Subject: Re: [PATCH v4] Set the retain attribute on _elf_set_element if CC supports [BZ #27492] Message-ID: <20210401000247.ksohwngbzh2z37ov@google.com> References: <20210331193923.2602174-1-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: Thu, 01 Apr 2021 00:02:55 -0000 On 2021-03-31, H.J. Lu wrote: >On Wed, Mar 31, 2021 at 12:39 PM Fangrui Song 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). >> --- >> Changes in v1 -> v2: >> * Define attribute_used_retain_section >> Changes in v2 -> v3: >> * Use attribute_used_retain instead attribute_used_retain_section >> Changes in v3 -> v4: >> * Add LIBC_LINKER_FEATURE detection for -z start-stop-gc and add tst-cleanup* tests >> --- >> config.h.in | 3 +++ >> configure | 58 ++++++++++++++++++++++++++++++++++++++++++ >> configure.ac | 20 +++++++++++++++ >> include/libc-symbols.h | 14 +++++++--- >> libio/Makefile | 32 +++++++++++++++++++++++ >> libio/tst-cleanup.c | 33 ++++++++++++++++++++++++ >> libio/tst-cleanup.exp | 1 + >> 7 files changed, 157 insertions(+), 4 deletions(-) >> create mode 100644 libio/tst-cleanup.c >> create mode 100644 libio/tst-cleanup.exp >> >> diff --git a/config.h.in b/config.h.in >> index ca1547ae67..95af1ef229 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 fcf43bf7de..fa86348342 100755 >> --- a/configure >> +++ b/configure >> @@ -626,6 +626,7 @@ fno_unit_at_a_time >> libc_cv_has_glob_dat >> libc_cv_hashstyle >> libc_cv_fpie >> +libc_cv_z_start_stop_gc >> libc_cv_z_execstack >> libc_cv_z_combreloc >> ASFLAGS_config >> @@ -4105,6 +4106,29 @@ fi >> $as_echo "$libc_cv_textrel_ifunc" >&6; } >> >> >> +# Check if gcc supports attribute retain as it is used in attribute_used_retain 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; } >> @@ -5871,6 +5895,40 @@ fi >> $as_echo "$libc_linker_feature" >&6; } >> >> >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports -z start-stop-gc" >&5 >> +$as_echo_n "checking for linker that supports -z start-stop-gc... " >&6; } >> +libc_linker_feature=no >> +if test x"$gnu_ld" = x"yes"; then >> + libc_linker_check=`$LD -v --help 2>/dev/null | grep "\-z start-stop-gc"` >> + if test -n "$libc_linker_check"; then >> + cat > conftest.c <> +int _start (void) { return 42; } >> +EOF >> + if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp >> + -Wl,-z,start-stop-gc -nostdlib -nostartfiles >> + -fPIC -shared -o conftest.so conftest.c >> + 1>&5' >> + { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5 >> + (eval $ac_try) 2>&5 >> + ac_status=$? >> + $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5 >> + test $ac_status = 0; }; } >> + then >> + libc_linker_feature=yes >> + fi >> + rm -f conftest* >> + fi >> +fi >> +if test $libc_linker_feature = yes; then >> + libc_cv_z_start_stop_gc=yes >> +else >> + libc_cv_z_start_stop_gc=no >> +fi >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5 >> +$as_echo "$libc_linker_feature" >&6; } >> +config_vars="$config_vars >> +have-z-start-stop-gc = $libc_cv_z_start_stop_gc" >> + >> { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5 >> $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; } >> libc_linker_feature=no >> diff --git a/configure.ac b/configure.ac >> index fce967f2c2..4e2907da92 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 retain as it is used in attribute_used_retain 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 $libc_cv_gcc_retain = yes; then >> + AC_DEFINE(HAVE_GCC_RETAIN) >> +fi >> + >> # Check if gcc warns about alias for function with incompatible types. >> AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types], >> libc_cv_gcc_incompatible_alias, [dnl >> @@ -1317,6 +1333,10 @@ LIBC_LINKER_FEATURE([-z execstack], [-Wl,-z,execstack], >> [libc_cv_z_execstack=yes], [libc_cv_z_execstack=no]) >> AC_SUBST(libc_cv_z_execstack) >> >> +LIBC_LINKER_FEATURE([-z start-stop-gc], [-Wl,-z,start-stop-gc], >> + [libc_cv_z_start_stop_gc=yes], [libc_cv_z_start_stop_gc=no]) >> +LIBC_CONFIG_VAR([have-z-start-stop-gc], [$libc_cv_z_start_stop_gc]) >> + >> LIBC_LINKER_FEATURE([--no-dynamic-linker], >> [-Wl,--no-dynamic-linker], >> [libc_cv_no_dynamic_linker=yes], >> diff --git a/include/libc-symbols.h b/include/libc-symbols.h >> index 546fc26a7b..b311c90f0c 100644 >> --- a/include/libc-symbols.h >> +++ b/include/libc-symbols.h >> @@ -352,6 +352,12 @@ for linking") >> >> */ >> >> +#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. */ >> @@ -367,12 +373,12 @@ 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 >> >> /* Define SET as a symbol set. This may be required (it is in a.out) to >> diff --git a/libio/Makefile b/libio/Makefile >> index 12ce41038f..439d22e057 100644 >> --- a/libio/Makefile >> +++ b/libio/Makefile >> @@ -195,6 +195,20 @@ ifeq (yes,$(build-shared)) >> tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \ >> $(objpfx)tst-bz24228-mem.out >> endif >> + >> +tests += tst-cleanup-default >> +tests-static += tst-cleanup-default >> +tests-special += $(objpfx)tst-cleanup-default-cmp.out >> +LDFLAGS-tst-cleanup-default = -Wl,--gc-sections >> + >> +ifeq ($(have-z-start-stop-gc),yes) >> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc >> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc >> +tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \ >> + $(objpfx)tst-cleanup-nostart-stop-gc-cmp.out >> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc > >Won't this test fail if linker supports -z start-stop-gc and GCC doesn't support >retain attribute? Yes, but that doesn't happen in practice. If ld supports -z start-stop-gc, the GNU as should be new and the GCC should support 'retain'. But perhaps it is good to prevent the issue, I can change +ifeq ($(have-z-start-stop-gc),yes) to +ifeq ($(have-gnu-retain)$(have-z-start-stop-gc),yesyes) and add a LIBC_CONFIG_VAR in configure.ac. >> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc >> +endif >> endif >> >> include ../Rules >> @@ -224,6 +238,24 @@ $(objpfx)tst_wprintf2.out: $(gen-locales) >> $(objpfx)tst-wfile-sync.out: $(gen-locales) >> endif >> >> +$(objpfx)tst-cleanup-default-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-default.out >> + cmp $^ > $@; \ >> + $(evaluate-test) >> +$(objpfx)tst-cleanup-default.o: tst-cleanup.c >> + $(compile.c) -o $@ >> + >> +$(objpfx)tst-cleanup-start-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-start-stop-gc.out >> + cmp $^ > $@; \ >> + $(evaluate-test) >> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c >> + $(compile.c) -o $@ >> + >> +$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-nostart-stop-gc.out >> + cmp $^ > $@; \ >> + $(evaluate-test) >> +$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c >> + $(compile.c) -o $@ >> + >> $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen >> $(SHELL) $< $(common-objpfx) '$(test-program-prefix)' \ >> $(common-objpfx)libio/; \ >> diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c >> new file mode 100644 >> index 0000000000..7f0a34a91e >> --- /dev/null >> +++ b/libio/tst-cleanup.c >> @@ -0,0 +1,33 @@ >> +/* Copyright (C) 2021 Free Software Foundation, Inc. >> + This file is part of the GNU C Library. >> + >> + The GNU C Library is free software; you can redistribute it and/or >> + modify it under the terms of the GNU Lesser General Public >> + License as published by the Free Software Foundation; either >> + version 2.1 of the License, or (at your option) any later version. >> + >> + The GNU C Library is distributed in the hope that it will be useful, >> + but WITHOUT ANY WARRANTY; without even the implied warranty of >> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU >> + Lesser General Public License for more details. >> + >> + You should have received a copy of the GNU Lesser General Public >> + License along with the GNU C Library; if not, see >> + . */ >> + >> +/* Test that stdout is flushed after atexit callbacks were run. */ >> + >> +#include >> +#include >> + >> +void >> +hook (void) >> +{ >> + puts ("hello"); >> +} >> + >> +int >> +main (void) >> +{ >> + atexit (hook); >> +} >> diff --git a/libio/tst-cleanup.exp b/libio/tst-cleanup.exp >> new file mode 100644 >> index 0000000000..ce01362503 >> --- /dev/null >> +++ b/libio/tst-cleanup.exp >> @@ -0,0 +1 @@ >> +hello >> -- >> 2.31.0.291.g576ba9dcdaf-goog >> > > >-- >H.J.