From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x1032.google.com (mail-pj1-x1032.google.com [IPv6:2607:f8b0:4864:20::1032]) by sourceware.org (Postfix) with ESMTPS id 56040385843D for ; Fri, 6 Jan 2023 18:48:50 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 56040385843D Authentication-Results: sourceware.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=google.com Received: by mail-pj1-x1032.google.com with SMTP id m7-20020a17090a730700b00225ebb9cd01so6029913pjk.3 for ; Fri, 06 Jan 2023 10:48:50 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=YHq3ook/jMkRYdOaKeTVcEbxgoKH++0eLgubdmT7jwY=; b=eVbI97ojGbUmUSruzLY+6N9sDfBtcsljyknrY30/h+OEr5FCDegZKezdf/fghY+yOA t/v7UXbODKy9OMoY66iOBrBfqN7R563aT+7Wrln7gwd32tt+JXKSoQ2PAWllrClWYDkq y/gGbHaCc5RCk+66YYjc0UiwrT6GnjJ54FHqZ2quU48aP7G1WkOWJ9NNQ9vtqxwHOawD KePdgstsNN8gZc64PSsSsKSlU6ndF4SVaexGXUhdJCh2iQBOIllJj3MPeiTYOaSSVp5+ lX2PrGiEEOO8Uz87Nm7y+zRa5rDY/bmX7mgwcIwOh7Qz/dRtX2sNwK/iKF/yDBZ9M/0r niSA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=YHq3ook/jMkRYdOaKeTVcEbxgoKH++0eLgubdmT7jwY=; b=BjRg1BTX2kcMeBSGTNzzJ+vnMtfvZDBKqPfP7KaN/7isR+rPc+OAZtarpBdfoXFZbw xxnPoCW4i1LFhHDpcgElaQwxJ57geMe0l0lMMJd72ASKzNPVi/AuLJjnoWYelMuCARas ASbc3W5fA1qVatcSdhFAw+HX6k4ZPJs/VJljub/1Yija+cMAnuJQw6WZT+MN/0tgw6v2 uFpuoaoe6sUj3CZYJUL0rhNy1eUUjGaNgsB0tzjMfY2gAJTaQSypDgNP6GNkZil4ZWjg k8aWQ820h3B36SfpN129QyMOkW7Ox/ri5cTfSrRxrxVBH7UrQN4ciEnEej/9EddW9sZV /CRA== X-Gm-Message-State: AFqh2kpVeWalkV6bEfhPG03gR7C9oimdYca+LVmftSt3bNQeFpW2adaH tWmPYYAf0U54CWEa5DRrSfxxQeJO24XH5j4bcKMbOlSi/nF+Y8/Y X-Google-Smtp-Source: AMrXdXsIX9Xv7G70B5xRUBEArpUWdB6geVM4rC8w9GMG0jp9MUaA0Ylqk+LfeVyJNdfwa3ARL6TD4LhNEQ6G8/rvGR8= X-Received: by 2002:a17:90a:fe85:b0:226:ad9c:bad with SMTP id co5-20020a17090afe8500b00226ad9c0badmr1021093pjb.199.1673030929248; Fri, 06 Jan 2023 10:48:49 -0800 (PST) MIME-Version: 1.0 References: <20230105210542.3573076-1-maskray@google.com> In-Reply-To: From: Fangrui Song Date: Fri, 6 Jan 2023 10:48:36 -0800 Message-ID: Subject: Re: [PATCH] ld: Allow R_X86_64_GOTPCREL for call *__tls_get_addr@GOTPCREL(%rip) To: "H.J. Lu" Cc: binutils@sourceware.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-23.4 required=5.0 tests=BAYES_00,BODY_8BITS,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=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Fri, Jan 6, 2023 at 9:04 AM H.J. Lu wrote: > > On Thu, Jan 5, 2023 at 1:06 PM Fangrui Song via Binutils > wrote: > > > > _Thread_local int a; > > int main() { return a; } > > > > % gcc -fno-plt -fpic a.c -fuse-ld=3Dbfd -Wa,-mrelax-relocations=3Dno > > /usr/bin/ld.bfd: /tmp/ccSSBgrg.o: TLS transition from R_X86_64_TLSGD to= R_X86_64_GOTTPOFF against `a' at 0xd in section `.text' failed > > /usr/bin/ld.bfd: failed to set dynamic section sizes: bad value > > collect2: error: ld returned 1 exit status > > > > This commit fixes the issue. > > > > PR ld/24784 > > * bfd/elf64-x86-64.c (elf_x86_64_check_tls_transition): Allow > > R_X86_64_GOTPCREL. > > --- > > bfd/elf64-x86-64.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c > > index 914f82d0151..095fe2e0fe6 100644 > > --- a/bfd/elf64-x86-64.c > > +++ b/bfd/elf64-x86-64.c > > @@ -1241,7 +1241,7 @@ elf_x86_64_check_tls_transition (bfd *abfd, > > if (largepic) > > return r_type =3D=3D R_X86_64_PLTOFF64; > > else if (indirect_call) > > - return r_type =3D=3D R_X86_64_GOTPCRELX; > > + return (r_type =3D=3D R_X86_64_GOTPCRELX || r_type =3D=3D R= _X86_64_GOTPCREL); > > else > > return (r_type =3D=3D R_X86_64_PC32 || r_type =3D=3D R_X86_= 64_PLT32); > > } > > -- > > 2.39.0.314.g84b9a713c41-goog > > > > Since the new TLS sequence was added after R_X86_64_GOTPCRELX was > required for call, R_X86_64_GOTPCREL should be invalid in this TLS sequen= ce. > > -- > H.J. I have multiple arguments (albeit no single one is very strong) that this 1-deletion-1-addition change provides benefits for users (IMHO with no burden to binutils at all). Some projects may add -Wa,-mrelax-relocations=3Dno to work around older GNU ld. Then the project's toolchain requirement may increase and no longer need to work around older GNU ld. But a distribution may for some reason use a global -fno-plt (e.g. Arch Linux) and then run into this TLS GD/LD->IE/LE optimization issue. rust src/ci/docker/host-x86_64/*musl/Dockerfile openjdk/jdk19u make/autoconf/flags-cflags.m4 (this file appears to be copied into quite a few projects) Linux kernel arch/x86/boot/compressed/Makefile (not a good example as it doesn't use TLS AFAICT) R_X86_64_GOTPCREL isn't purely usefull. It may help linker design: for R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX, the linker can make a decision upfront whether a GOT entry is needed (this affects the size of .got, which may affect section layout and whether other relocations may overflow). This may increase risk of 32-bit relocation overflow. R_X86_64_GOTPCREL can mitigate the risk while being aware to the user. rustc somehow disables x86 relaxed relocations and defaults to `-Z plt=3Dno` and now relies on llvm-project to work around the GNU ld compatibility issue. --=20 =E5=AE=8B=E6=96=B9=E7=9D=BF