From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oa1-x35.google.com (mail-oa1-x35.google.com [IPv6:2001:4860:4864:20::35]) by sourceware.org (Postfix) with ESMTPS id 3670E3858D33 for ; Sat, 7 Jan 2023 00:02:16 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 3670E3858D33 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-oa1-x35.google.com with SMTP id 586e51a60fabf-150b06cb1aeso3286842fac.11 for ; Fri, 06 Jan 2023 16:02:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.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=qIJ643GXUmL2tKbySi+jbtggQrIqed0smm7c2jIvFSA=; b=fTcicF1h30yVhF8zRUOW+y88EV9yJp41kYGJq9htYQDQYXst9ecZndjddwEppwV9rF e7Y7lckvyD0S6yoYVGDqAaCMB04k1L0XkGk21ublQHScyH/okra99AU6+rAbNF/+94Bk h/mr9jG1sImVcl7legxzcd5MfX8Gy27nA/I6MRY+lWnMd94NqP7G8YX472zX3ZLFWwTi YrYX87JcYjqIhDbsDJD1UxtlsSDbghwzn5zuCo8XVyv8Ck7EZw6t3S7faN2K52DNJabr d093hVtfLwG1IzE0kZAa73NUypMxmYs3QdLUbBbxhZlTtR02FLzzHQQoPdz1zrgDLL9O yPoA== 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=qIJ643GXUmL2tKbySi+jbtggQrIqed0smm7c2jIvFSA=; b=VwJCh2Gy4rLTxV/B8tL2CEFB1wZ3HwtLVhTMHDo/lYQuVbGZdP4Geh7QF4HFBz1PqL RBL5zT75SVoH024P4120gduSSH3gc563NVK7uvBLaXA1GffAv7bWT3fQrFWGPEgzTCm0 mLOWGP8wok7cfzmaHJoiAwwRM6rt4UFyQZG3edWuuX7OUDyXmpgDzSUoj+VOXIIDf25B iq6GdUBvjzqb3xDtGfHMtPk6RbofzY1Ff2Cmbmwpba8BTqX3qUOFUq3Sp6Jvss1CoF03 742G61GkPYnjTGYDXvy9sZa+a5mAFo78foc1QrXJLLwSWC0pM0yU39HMP10SuNTzfyPd ubxg== X-Gm-Message-State: AFqh2kpMg6ZtQX1mqaEBhsdbGt6moaZquqqLbrCxjiuowR5vJeRSs38C XPJ9G+rAZax/yQz33V/7xR6QGRGUxABNgUfeYE6OEZ32stQ= X-Google-Smtp-Source: AMrXdXtm2bMsVNMoD05+vndZHO1o4PS3JZ1u5k9nvqGiFPVrZAh9jgQ5NCI6JzEbGthW7JQfTU8TtfVb4ig0h0oxNPw= X-Received: by 2002:a05:6870:c07:b0:150:ccf2:326b with SMTP id le7-20020a0568700c0700b00150ccf2326bmr1412774oab.266.1673049735581; Fri, 06 Jan 2023 16:02:15 -0800 (PST) MIME-Version: 1.0 References: <20230105210542.3573076-1-maskray@google.com> In-Reply-To: From: "H.J. Lu" Date: Fri, 6 Jan 2023 16:01:39 -0800 Message-ID: Subject: Re: [PATCH] ld: Allow R_X86_64_GOTPCREL for call *__tls_get_addr@GOTPCREL(%rip) To: Fangrui Song Cc: binutils@sourceware.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-3021.1 required=5.0 tests=BAYES_00,BODY_8BITS,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=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Fri, Jan 6, 2023 at 3:52 PM Fangrui Song wrote: > > On Fri, Jan 6, 2023 at 3:20 PM H.J. Lu wrote: > > > > On Fri, Jan 6, 2023 at 3:02 PM Fangrui Song wrote: > > > > > > On Fri, Jan 6, 2023 at 2:42 PM H.J. Lu wrote: > > > > > > > > On Fri, Jan 6, 2023 at 1:44 PM Fangrui Song wr= ote: > > > > > > > > > > On Fri, Jan 6, 2023 at 1:27 PM H.J. Lu wrot= e: > > > > > > > > > > > > On Fri, Jan 6, 2023 at 1:25 PM Fangrui Song wrote: > > > > > > > > > > > > > > On Fri, Jan 6, 2023 at 1:14 PM H.J. Lu = wrote: > > > > > > > > > > > > > > > > On Fri, Jan 6, 2023 at 10:48 AM Fangrui Song wrote: > > > > > > > > > > > > > > > > > > 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 Binutil= s > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > _Thread_local int a; > > > > > > > > > > > int main() { return a; } > > > > > > > > > > > > > > > > > > > > > > % gcc -fno-plt -fpic a.c -fuse-ld=3Dbfd -Wa,-mrelax-r= elocations=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_transi= tion): 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_GOT= PCRELX was > > > > > > > > > > required for call, R_X86_64_GOTPCREL should be invalid = in this TLS sequence. > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > H.J. > > > > > > > > > > > > > > > > > > I have multiple arguments (albeit no single one is very s= trong) that > > > > > > > > > this 1-deletion-1-addition change provides benefits for u= sers (IMHO > > > > > > > > > with no burden to binutils at all). > > > > > > > > > > > > > > > > > > Some projects may add -Wa,-mrelax-relocations=3Dno to wor= k around older > > > > > > > > > GNU ld. Then the project's toolchain requirement may incr= ease 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 optim= ization > > > > > > > > > issue. > > > > > > > > > > > > > > > > > > rust src/ci/docker/host-x86_64/*musl/Dockerfile > > > > > > > > > openjdk/jdk19u make/autoconf/flags-cflags.m4 (this file a= ppears to be > > > > > > > > > copied into quite a few projects) > > > > > > > > > Linux kernel arch/x86/boot/compressed/Makefile (not a goo= d example as > > > > > > > > > it doesn't use TLS AFAICT) > > > > > > > > > > > > > > > > > > R_X86_64_GOTPCREL isn't purely usefull. It may help linke= r 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 defaul= ts to `-Z > > > > > > > > > > > > > > > > Why is that? > > > > > > > > > > > > > > It's assuredly a rust's problem and I am trying to fix that i= n > > > > > > > https://github.com/rust-lang/rust/pull/106511 > > > > > > > > > > > > > > The -Wa,-mrelax-relocations=3Dno problem may affect more pac= kages. > > > > > > > > > > > > -mrelax-relocations=3Dno should be a workaround for the older l= inker. It > > > > > > shouldn't be used with the current linker. > > > > > > > > > > A project may choose to work with many linker versions. > > > > > For simplicity, before it decides to drop compatibility with GNU > > > > > ld<2.26 (AIUI GOTPCRELX was supported in 2.26), > > > > > it may unconditionally add -Wa,-mrelax-relocations=3Dno, instead = of > > > > > > > > -mrelax-relocations=3Dno is only supported with the newer binutils. > > > > > > The relocatable object file producer and the consumer may be on > > > different machines and use different binutils versions. > > > https://github.com/rust-lang/rust/commit/305aca86f9d8d132650b495f610f= 9abe5239fec6 > > > added -Wa,-mrelax-relocations=3Dno so that the relocatable object fil= es > > > can be used on a user machine with an old ld. > > > > But -fno-plt may not work with the old linker. For this matter, > > -Wa,-mrelax-relocations=3Dno doesn't work with ld today. When > > -Wa,-mrelax-relocations=3Dno is used, no features from newer linkers > > can be used. > > A project may add -Wa,-mrelax-relocations=3Dno so that its prebuilt > relocatable object files can be linked on a machine with old ld. > > A distribution (like Arch Linux and rust) using new ld may decide to > use -fno-plt globally. If they don't strip or override the project > -Wa,-mrelax-relocations=3Dno, > they may run into the -fpic -fno-plt -Wa,-mrelax-relocations=3Dno incompa= tibility. -Wa,-mrelax-relocations=3Dno should be removed when -fno-plt is added. > In the GCC/gas model, GCC doesn't know whether its emitted assembly > will be used with -mrelax-relocations=3Dno. > GCC just emits the -fno-plt form of TLS GD/LD code sequence. If the > assembly is fed into gas with -mrelax-relocations=3Dno, the output will > be broken with current ld. > > I have tested some TLS examples and this simple patch work. One can > argue that GNU ld either suppresses GD/LD=3D>IE/LE optimization or > support this old relocation type. > It appears that supporting the old relocation type is the simplest approa= ch. > > > > https://github.com/IHaskell/IHaskell/issues/636 and > > > https://github.com/dcos/dcos/commit/facda25019e07051f501b39720b4e7104= 9bd0030 > > > likely use the same argument. > > > > > > In other cases, the project may use -Wa,-mrelax-relocations=3Dno with > > > Clang (where they assume a not-too-old version), but need to work wit= h > > > system ld (which may be old). > > > > This shouldn't happen with as and ld from the same binutils. > > I agree. > > > > > > doing configure work to check linker support. > > > > > > > > The TLS sequence from -fno-plt doesn't work for the older linker. > > > > The older linker support should be dropped for -fno-plt. > > > > > > > > > Now a user may use -fno-plt (Arch Linux, rustc, maybe Alpine) and= run > > > > > into the aforementioned TLS problem. > > > > > > > > > > This 1-deletion-1-addition change can address this issue with no > > > > > maintenance burden on binutils side in my opinion, > > > > > so I made this patch. > > > > > > > > > > The linker design I described is true as well. Whether GOTPCRELX = leads > > > > > to a GOT entry can be decided at relocation scanning time, before= the > > > > > section layout is decided. > > > > > Users may make a conscious decision to use GOTPCREL to avoid pote= ntial > > > > > relocation overflow risk. > > > > > > > > > > GOTPCREL isn't really dead. It can be used with Intel LAM and tag= ged > > > > > global variables (with non-zero high address bits) > > > > > https://reviews.llvm.org/D111343 > > > > > GOTPCREL instead of GOTPCRELX makes it clear an instruction > > > > > referencing the variable isn't supposed to be relaxed. > > > > > > > > The address of the local symbol, foo, in > > > > > > > > movq foo@GOTPCREL(%rip), %rax > > > > > > > > is assigned by the linker. I am not sure how the tag is involved h= ere. > > > > Besides, it is the call instruction here. > > > > > > This is an auxiliary argument. I wanted to emphasize that GOTPCREL > > > isn't dead and did not intend to use it with this call instruction. > > > If GOTPCRELX is used and the distance between the current location an= d > > > the symbol is larger than 2**31, this will trigger a relocation > > > overflow. > > > > > > This happens with tagged globals with non-zero high address bits. > > > > This sounds needing some linker changes to add tags to data variables. > > I am not sure if GOTPCREL alone is sufficient. > > > > > A linker can fix the problem by avoiding relaxation, increasing the > > > size of .got . This requires that it scans relocations more than once= . > > > If GOTPCRELX is decided upfront whether it needs relaxation or not, o= n > > > an arch which doesn't use range extension thunks like x86, technicall= y > > > relocations can just be scanned once. > > > > > > > > > > > > plt=3Dno` and now relies on llvm-project to work around t= he GNU ld > > > > > > > > > compatibility issue. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > =E5=AE=8B=E6=96=B9=E7=9D=BF > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > H.J. > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > =E5=AE=8B=E6=96=B9=E7=9D=BF > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > H.J. > > > > > > > > > > > > > > > > > > > > -- > > > > > =E5=AE=8B=E6=96=B9=E7=9D=BF > > > > > > > > > > > > > > > > -- > > > > H.J. > > > > > > > > > > > > -- > > > =E5=AE=8B=E6=96=B9=E7=9D=BF > > > > > > > > -- > > H.J. > > > > -- > =E5=AE=8B=E6=96=B9=E7=9D=BF --=20 H.J.