From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-x436.google.com (mail-pf1-x436.google.com [IPv6:2607:f8b0:4864:20::436]) by sourceware.org (Postfix) with ESMTPS id BE6033858400; Mon, 18 Oct 2021 14:42:46 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org BE6033858400 Received: by mail-pf1-x436.google.com with SMTP id m26so14872909pff.3; Mon, 18 Oct 2021 07:42:46 -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=NXlPSQcDTJgGXoIZD/FjmM5IXOcJJZYNJB0tbaiRdN0=; b=Dh7OCmn5sCqlAPpRSm0rQC3wI0Bo7IjCmwRaUFcnDQlfBLETvhrLrNXvWEnViI2Cle 10fPxvvd7aIkhPFwckHJsecwgxq43H4f9ciW/RCVZUiTfIZqk9O2UaIdl6O8XSgPpqSv JAJfjyTuwHZ+Z3cdiN7yG0apmQf4E5Jp5aesz3v84QEUOuaItA7OKozbm4AQUYyx9Pgs wH5wvZhQ6QxVHNVWNwCETcpFkUK5QhHEZynCzlLnezq1xCtT+wpXRWGRVKNw0UcBgj1x kEh2p6YSC2c4te96didvd3kY/Qxpa+bo70J9cub7pQTihk5K+fnDgiX8YAN7Bt2LMJgt NoCQ== X-Gm-Message-State: AOAM5339V9gyvpgJ8ZOqzO8/JaNzWO2OPYZm891MNI7/VadCjpUIBI5J n4aKiyTXb1xfCvbpA7Ujt4Yu0ZQpIueIQyWKyyGcYIJ+sN8= X-Google-Smtp-Source: ABdhPJw4bC7TUnt6d5P+KURvY2DtXDOybjOg7Pib7r0Q1hYl5sUO0RIBzUTQaiFqMyZiQJOgvr+wu9jmkz7EqkOXaUE= X-Received: by 2002:a62:7752:0:b0:44c:eb65:8561 with SMTP id s79-20020a627752000000b0044ceb658561mr29337393pfc.43.1634568165660; Mon, 18 Oct 2021 07:42:45 -0700 (PDT) MIME-Version: 1.0 References: <20211017005020.2645717-1-maskray@google.com> In-Reply-To: <20211017005020.2645717-1-maskray@google.com> From: "H.J. Lu" Date: Mon, 18 Oct 2021 07:42:09 -0700 Message-ID: Subject: Re: [PATCH v2] elf: Support DT_RELR relative relocation format [BZ #27924] To: Fangrui Song Cc: GNU C Library , Binutils Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-3030.2 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP 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 14:42:50 -0000 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. [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. -- H.J.