From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yb1-xb2c.google.com (mail-yb1-xb2c.google.com [IPv6:2607:f8b0:4864:20::b2c]) by sourceware.org (Postfix) with ESMTPS id D85B63858402 for ; Mon, 18 Oct 2021 16:17:01 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org D85B63858402 Received: by mail-yb1-xb2c.google.com with SMTP id n65so2637002ybb.7 for ; Mon, 18 Oct 2021 09:17:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=G2ho0HqKkOnjLkXbh/8vVNQtQ4Qh8eY/X05SEmiXrt4=; b=1BuVp0fErj/n334wX1cKeFIyFrDWDagAj0Fs9pFYLSlJb77CFBsCp7b4XkrZw2TFNt ePHTJ53MDw2VdUbu7WzGbzaETlgjjioitXQNNhvLzWeAcTIvUBWyqv94Aoj6GVZb8G5h npCGH8V9MdrlrXH3V29Xx9Gl30lYdlYfFvieiBQk79NBRRCBWpNzcKBOTk+zRhBfwFCZ SoPRfOxXeEBlk8r7mHU3JqVBczOXwi3hHf/j9PL7QQWyRu768WMx/UJGHgKHbjNeWHUP ALiLPyymv3L4Yjqm1DNgKdzZx6yj92WzPlAxv7RcWM+Mt1X/PHqRFvYZ8BIZWh7InpBv IVTw== X-Gm-Message-State: AOAM533Oot7jSwxH28amHIg4U1fx2CT4MRRAIBwGS/Mf/V+KTNoTv50W ICsd1OfxSKyy+4F+aruvpPZzcvwUX1v6443bervWxw== X-Google-Smtp-Source: ABdhPJz0psZiflVcRNM8ZGHofClHAUk3C8QHIOrR4bL4s43Aiit/y0uHTDpdzmad/ZV6hFPMAbhaOZQan3PoORxPpns= X-Received: by 2002:a25:bdc5:: with SMTP id g5mr32133835ybk.403.1634573821202; Mon, 18 Oct 2021 09:17:01 -0700 (PDT) MIME-Version: 1.0 References: <20211017005020.2645717-1-maskray@google.com> In-Reply-To: From: =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= Date: Mon, 18 Oct 2021 09:16:49 -0700 Message-ID: Subject: Re: [PATCH v2] elf: Support DT_RELR relative relocation format [BZ #27924] To: "H.J. Lu" Cc: GNU C Library , Binutils Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-24.0 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, 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=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) 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, 18 Oct 2021 16:17:06 -0000 On Mon, Oct 18, 2021 at 7:42 AM H.J. Lu wrote: > > On Sat, Oct 16, 2021 at 5:50 PM Fangrui Song via Binutils > wrote: > > > > PIE and shared objects usually have many relative relocations. In > > 2017/2018, SHT_RELR/DT_RELR was proposed on > > https://groups.google.com/g/generic-abi/c/bX460iggiKg/m/GxjM0L-PBAAJ > > ("Proposal for a new section type SHT_RELR") and is a pre-standard. RELR > > usually takes 3% or smaller space than R_*_RELATIVE relocations. The > > virtual memory size of a mostly statically linked PIE is typically 5~10% > > smaller. > > > > --- > > > > Notes I will not include in the submitted commit: > > > > Available on https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/maskray/relr > > > > "pre-standard": even Solaris folks are happy with the refined generic-abi > > proposal. Cary Coutant will apply the change > > https://sourceware.org/pipermail/libc-alpha/2021-October/131781.html > > > > This patch is simpler than Chrome OS's glibc patch and makes ELF_DYNAMIC_DO_RELR > > available to all ports. I don't think the current glibc implementation > > supports ia64 in an ELFCLASS32 container. That said, the style I used is > > works with an ELFCLASS32 container for 64-bit machine if ElfW(Addr) is > > 64-bit. > > > > * Chrome OS folks have carried a local patch since 2018 (latest version: > > https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-libs/glibc/files/local/glibc-2.32). > > I.e. this feature has been battle tested. > > * Android bionic supports 2018 and switched to DT_RELR==36 in 2020. > > * The Linux kernel has supported CONFIG_RELR since 2019-08 > > (https://git.kernel.org/linus/5cf896fb6be3effd9aea455b22213e27be8bdb1d). > > * A musl patch (by me) exists but is not applied: > > https://www.openwall.com/lists/musl/2019/03/06/3 > > * rtld-elf from FreeBSD 14 will support DT_RELR. > > > > I believe upstream glibc should support DT_RELR to benefit all Linux > > distributions. I filed some feature requests to get their attention: > > > > * Gentoo: https://bugs.gentoo.org/818376 > > * Arch Linux: https://bugs.archlinux.org/task/72433 > > * Debian https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996598 > > * Fedora https://bugzilla.redhat.com/show_bug.cgi?id=2014699 > > > > As of linker support (to the best of my knowledge): > > > > * LLD support DT_RELR. > > * https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-devel/binutils/files/ > > has a gold patch. > > * GNU ld feature request https://sourceware.org/bugzilla/show_bug.cgi?id=27923 > > > > I wish that GNU ld and gold maintainers can implement the feature as well :) > > > > Tested on aarch64 and x86_64. > > > > Changes from v1 (https://sourceware.org/pipermail/libc-alpha/2021-October/131768.html) > > * Fix style, simplify code > > * Improve test > > --- > > configure | 31 +++++++++++++++++++++++++++++++ > > configure.ac | 4 ++++ > > elf/Makefile | 4 ++++ > > elf/dynamic-link.h | 28 ++++++++++++++++++++++++++++ > > elf/elf.h | 13 +++++++++++-- > > elf/get-dynamic-info.h | 3 +++ > > elf/tst-relr.c | 27 +++++++++++++++++++++++++++ > > 7 files changed, 108 insertions(+), 2 deletions(-) > > create mode 100644 elf/tst-relr.c > > > > diff --git a/configure b/configure > > index 3227e434d3..fdab6a97ef 100755 > > --- a/configure > > +++ b/configure > > @@ -6067,6 +6067,37 @@ $as_echo "$libc_linker_feature" >&6; } > > config_vars="$config_vars > > have-depaudit = $libc_cv_depaudit" > > > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --pack-dyn-relocs=relr" >&5 > > +$as_echo_n "checking for linker that supports --pack-dyn-relocs=relr... " >&6; } > > +libc_linker_feature=no > > +if test x"$gnu_ld" = x"yes"; then > > + cat > conftest.c < > +int _start (void) { return 42; } > > +EOF > > + if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp > > + -Wl,--pack-dyn-relocs=relr -nostdlib -nostartfiles > > + -fPIC -shared -o conftest.so conftest.c > > + 1>&5' > > + { { eval echo "\"\$as_me\":${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 > > +if test $libc_linker_feature = yes; then > > + libc_cv_relr=yes > > +else > > + libc_cv_relr=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-relr = $libc_cv_relr" > > + > > { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5 > > $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; } > > libc_linker_feature=no > > diff --git a/configure.ac b/configure.ac > > index 00f49f09f7..96110f9d7d 100644 > > --- a/configure.ac > > +++ b/configure.ac > > @@ -1354,6 +1354,10 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x], > > [libc_cv_depaudit=yes], [libc_cv_depaudit=no]) > > LIBC_CONFIG_VAR([have-depaudit], [$libc_cv_depaudit]) > > > > +LIBC_LINKER_FEATURE([--pack-dyn-relocs=relr], [-Wl,--pack-dyn-relocs=relr], > > + [libc_cv_relr=yes], [libc_cv_relr=no]) > > +LIBC_CONFIG_VAR([have-relr], [$libc_cv_relr]) > > + > > LIBC_LINKER_FEATURE([--no-dynamic-linker], > > [-Wl,--no-dynamic-linker], > > [libc_cv_no_dynamic_linker=yes], > > diff --git a/elf/Makefile b/elf/Makefile > > index bf45d8ee24..2c4cdfac68 100644 > > --- a/elf/Makefile > > +++ b/elf/Makefile > > @@ -245,6 +245,10 @@ tests-special += $(objpfx)tst-audit14-cmp.out $(objpfx)tst-audit15-cmp.out \ > > $(objpfx)tst-audit16-cmp.out > > endif > > endif > > +ifeq ($(have-relr),yes) > > +tests += tst-relr > > +LDFLAGS-tst-relr += -Wl,--pack-dyn-relocs=relr > > +endif > > endif > > Is DT_RELR only generated for PIE? If yes, you need to add it > to tests-pie and compile it as PIE. PIE and shared objects. PDE doesn't need relative relocations. It is useful to ensure that -Wl,--pack-dyn-relocs=relr doesn't cause breakage to PDE. > [hjl@gnu-cfl-2 tmp]$ gcc -pie -fPIE -O2 tst-relr.c > -Wl,--pack-dyn-relocs=relr -fuse-ld=lld > [hjl@gnu-cfl-2 tmp]$ ./a.out > Segmentation fault (core dumped) > [hjl@gnu-cfl-2 tmp]$ > > Given that the current lld implementation generates broken > binaries for existing glibc without any warning at run-time, > we should use a different linker command line option to > implement it properly so that the new binary will fail to > run on glibc without DT_RELR support at run-time. I don't think so. LLD's design is to be machine agnostic and NOT make decisions which would vary on different machines. --pack-dyn-relocs=relr is not the default. The user tells LLD to use DT_RELR and the user is responsible for making sure target ld.so supports DT_RELR. LLD is often used as a cross linker. The host ld.so doesn't support LLD doesn't mean that LLD should disable the format. For example, it is totally fine to link a FreeBSD executable on a Linux machine.