From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_EF,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server1.sourceware.org [209.132.180.131]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id B0DA61F462 for ; Tue, 30 Jul 2019 14:04:47 +0000 (UTC) DomainKey-Signature: a=rsa-sha1; c=nofws; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:date:from:to:cc:subject:message-id:in-reply-to :references:mime-version:content-type; q=dns; s=default; b=oeImJ ebkuo/X18duPStLGd+aohMuBuAG2rEzLMgcG6Ezd6x5ud5Vlxw9hFyW2hDCP9EnV bZUNzJ44q2x4lApCTmz3LZByhZq7Pow82tZj0Y41TR6n2kYukCoMIFKCTpLWRdJ3 UspawhrExY7rkpL7LbEhKP/0SLpjnP2hBap9BQ= DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:date:from:to:cc:subject:message-id:in-reply-to :references:mime-version:content-type; s=default; bh=lCI9hXMFDOb zPwY8nXhtVjlw/uM=; b=n2A05z3GdyNQ/YSy4BSUYbPOCMsWbUsvMY9atM9MJ4E w33dYEnS6J+kjv+pWOzHV1EiotBLN83+pRgyYZMl3sX6V6IWzQamvUKJVQFJ8r3H nhNajBzMs4KbmI5hJYbeaTbh33Kd/TCXBr7XmyJ97ee0LVliaVr8ep5On+cEigEg = Received: (qmail 70092 invoked by alias); 30 Jul 2019 14:04:45 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Received: (qmail 70079 invoked by uid 89); 30 Jul 2019 14:04:44 -0000 Authentication-Results: sourceware.org; auth=none X-HELO: mail-out.m-online.net Date: Tue, 30 Jul 2019 16:04:30 +0200 From: Lukasz Majewski To: Zack Weinberg Cc: Joseph Myers , Wolfgang Denk , GNU C Library , Alistair Francis , Alistair Francis , Adhemerval Zanella , Arnd Bergmann Subject: Re: Accelerating Y2038 glibc fixes Message-ID: <20190730160430.6b6f302e@jawa> In-Reply-To: References: <20190712072103.D3DBC24003A@gemini.denx.de> <20190726123902.6f8813da@jawa> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; boundary="Sig_/nuRExRUY1USkXb8KqIHmAEn"; protocol="application/pgp-signature" --Sig_/nuRExRUY1USkXb8KqIHmAEn Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi Zack, > On Fri, Jul 26, 2019 at 6:39 AM Lukasz Majewski wrote: > ... > > > > See for example [1] - there are just 7 lines of "code". But > > > > Joseph does not accept our patches. The arguments he gives are > > > > not on a technical level; =20 > ... > > Our goal is to add a solid foundation for the Y2038 work, so we > > would know the direction where we are heading. =20 > ... > > If you think that it would be better and most of all faster if you > > rewrite the description, then I don't mind. > > > > It would be great if you could do it sooner than latter as this > > slows down our development. =20 > ... > > The most recent version of this patch set (v8): > > https://patchwork.ozlabs.org/patch/1117100/ =20 >=20 > I haven=E2=80=99t been following the details of these patches super caref= ully, > and I=E2=80=99m not sure I understand what _Joseph=E2=80=99s_ concerns wi= th your > writing is. However, I=E2=80=99m a native English speaker, I=E2=80=99ve = read over the > text in the patch at , I > do think I understand the issues at a high level, and I do think the > meaning of __ASSUME_TIME64_SYSCALLS could be explained more clearly. > I=E2=80=99m prepared to work with you to come up with better wording=20 Thanks for offering your help. > but I > need to ask you a bunch of questions. Could you please reply to each > of the queries marked Qn below? >=20 > As I understand it, we have five distinct cases to consider: >=20 > 1. All future LP64 ABIs and all but one existing LP64 ABI, identified > by __WORDSIZE =3D=3D 64: time_t is already a 64-bit integer type and > all of the relevant system calls already accept it in that form. > glibc=E2=80=99s implementation of, for instance, clock_gettime may con= tinue > to invoke the system call numbered __NR_clock_gettime. This is exactly how we shall proceed with machines having __WORDSIZE=3D=3D64 (e.g. x86_64, armv8, etc). They now support 64 bit time with non suffixed syscalls. In the patch [1] the __WORDSIZE =3D=3D 64 check covers this. >=20 > 2. The exception to (1) is Alpha. That is an exclusively LP64 > architecture, but in glibc 2.0 it used 32-bit time_t, and we still > have compatibility code for that case. The compatibility symbols > invoke a set of compatibility syscalls with =E2=80=98osf=E2=80=99 in t= heir names: > for instance, gettimeofday@GLIBC_2.0 invokes __NR_osf_gettimeofday. > Not all of the time-related functions in glibc have compatibility > symbols, only those that existed in version 2.0. >=20 > Your patches do not touch this compatibility code at all, as far as > I can see.=20 Yes, you are correct. I was not even aware of such a case (as I found Alpha as 64 bit arch when I did my checking). > Alpha being out of production, and binaries compiled > against glibc 2.0 being rare anyway, it would only make sense to > involve this code in your patches if it reduced the overall volume > of compatibility code somehow, but regardless we need to make sure > it doesn't break. As you mentioned - we shall not break existing binaries. However, I'm not sure if we shall spent more time/effort on the arch being near EOL (or at least being out of production now). >=20 > 3. x32 (recently new 32-bit ABI for x86): like case 1, time_t is > already 64-bit and we use unsuffixed system calls. The text says > this case is identified by __WORDSIZE =3D=3D 32 && __TIMESIZE =3D=3D 6= 4, > but the code actually checks __SYSCALL_WORDSIZE. >=20 > Q1: Which condition correctly identifies this case, __TIMESIZE =3D=3D > 64 or __SYSCALL_WORDSIZE =3D=3D 64? It is: (__WORDSIZE =3D=3D 32) && ((defined __SYSCALL_WORDSIZE &&__SYSCALL_WORDSIZE =3D=3D 64)) Only x32 defines the __SYSCALL_WORDSIZE =3D=3D 64 (as it has __WORDSIZE =3D= =3D 32, but supports 64 bit syscalls ABI). >=20 > Q2: Could we perhaps ensure that __TIMESIZE and/or > __SYSCALL_WORDSIZE is defined to 64 whenever __WORDSIZE =3D=3D 64? Then > we could collapse this into case 1. __TIMESIZE =3D=3D 64 for x32.=20 The x32 uses the same set of syscalls (e.g. clock_gettime) as in point 1 (as for example x86_64). >=20 > 4. Brand-new (added in kernel 5.1 or later) 32-bit ABIs: time_t will > always be 64-bit, This would be true after we make the "switch" to support Y2038 aware systems. Please find example implementation [2] from this patch series [3] (it adds example code for converting __clock_settime to support 64 bit time when __ASSUME_TIME64_SYSCALLS is defined). > _but_ glibc=E2=80=99s implementation of time-related APIs > may need to invoke system calls with suffixed names: clock_gettime > invoking __NR_clock_gettime64, for instance. Also, the kernel will > not provide all of the time-related system calls that have > historically existed; glibc must, for instance, implement > gettimeofday in terms of clock_gettime. Yes, correct. Some syscalls would be emulated (as they are not or will not be converted to 64 bit version). >=20 > Q3: What macros are defined for this case? There are no macros yet available. >=20 > Q4: Does glibc need to call system calls with suffixed names in > this case? I think yes - for example the gettimeofday would internally use clock_gettime64 (vDSO if available). >=20 > Q4a: If the answer to Q4 is =E2=80=98yes=E2=80=99, why is that, and ca= n we change > the kernel so that it=E2=80=99s the same as x32 and the LP64 architect= ures? We need new set of syscalls for 64 bit time support on 32 bit archs (WORDSIZE=3D=3D32); for example x32/LP64 would still use clock_settime syscall (number X). To have the same functionality (64 bit time support) 32 bit archs would need to use clock_settime64 (number 404 on armv7) And here the __ASSUME_TIME64_SYSCALLS comes into play. If the arch is capable of providing 64 bit time, (no matter if it uses clock_settime or clock_settime64), then __ASSUME_TIME64_SYSCALLS is defined. It is also assumed that both clock_settime64 and clock_settime provide the same ABI, so no code needs to be adjusted in glibc. If code needs to be adjusted (as the calls are not compatible) - a new flag will be introduced (like with semtimedop) > (Either by removing the suffixes, or by _adding_ suffixed aliases > to asm/unistd.h for x32 and LP64 architectures.) Wouldn't this caused the ABI break? >=20 > 5. Historical 32-bit ABIs, where the existing set of system calls > takes 32-bit time_t, and Linux 5.1 added a matching set that takes > 64-bit time_t. For compatibility with old programs that make > direct system calls, the kernel will not rename the __NR_ constants > for the old (32-bit) system calls; instead new constants with =E2=80= =9864=E2=80=99 > or =E2=80=98time64=E2=80=99 suffixes will be added. As in case 4, the= new set of > system calls does not cover all of the historic time-related system > calls. >=20 > In this case, and only this case, glibc=E2=80=99s code needs to accoun= t for > the possibility that the new __NR_ constants are not defined > (because glibc is being compiled against kernel headers from a > version older than 5.1), or that the new system calls are not > available at runtime (glibc was compiled against new kernel headers > but is running with an old kernel). >=20 > The #if is complicated enough that I=E2=80=99m not sure, but I _think_= your > code only defines __ASSUME_TIME64_SYSCALLS when the new constants > are _guaranteed_ to be defined. >=20 > Q5: Is it correct that __ASSUME_TIME64_SYSCALLS is only defined > when the new constants are guaranteed to be defined? No. The __ASSUME_TIME64_SYSCALLS is defined only when the architecture supports 64 bit time related ABI. (either via clock_settime on e.g. x86_64/arm64 or clock_settime64 on arm). Please consult the code for clock_settime [4]. It shows how the __ASSUME_TIME64_SYSCALLS flag is used in practice. >=20 > Q6: All of the other __ASSUME_ constants mean both that we assume > the kernel headers are new enough to provide all the necessary > declarations, and that we assume the feature is available at > runtime: no fallback code will be included in the library. Is this > also the intended meaning of __ASSUME_TIME64_SYSCALLS? The patch [1] defines the __ASSUME_TIME64_SYSCALLS as the ability of the architecture (via kernel syscalls) to provide 64 bit time support. As shown in [4] - the fallback code is called when __ASSUME_TIME64_SYSCALLS is NOT defined and if architecture doesn't support clock_settime64. >=20 > zw Note: [1] - https://patchwork.ozlabs.org/patch/1117100/=20 [2] - https://github.com/lmajewski/y2038_glibc/commit/3d5f3512438de7426acba58c1ed= f53f756f8570b#diff-c051022b496f12bd9028edb46b8ec04d [3] - https://github.com/lmajewski/y2038_glibc/commits/Y2038-2.29-glibc-__clock-i= nternal-struct-timespec-v6 [4] - https://github.com/lmajewski/y2038_glibc/commit/69f842a8519ca13ed11fab0ff1b= cc6fa1a524192 Best regards, Lukasz Majewski -- DENX Software Engineering GmbH, Managing Director: Wolfgang Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de --Sig_/nuRExRUY1USkXb8KqIHmAEn Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEgAyFJ+N6uu6+XupJAR8vZIA0zr0FAl1ATm4ACgkQAR8vZIA0 zr2oAQgAqabwuogK51J4mZSakCklauQtWuAJLLfXxzbqgFiqLfiqP6oIuppS0Rfu CNRNi+RDO18/056ROPUr71TFxIx8z/+fJ02wdXdegP3v1Rwrt7UdEeheBb4Ev9VB 8cRSFM1QAtzlPqzByrZK4VfKDDRJJ32mSYnt2TohG7HjSkoHT8Ka8hnlqp3H/rTn 4ae2v0aQo3laEi90cMiwmRS4eGJqocT9jTfeCaAh0IcsFdis4jgpWFoC/uyoQ0G6 HZwqoW/BjONTyxtWXjIq865WdJVtabFyDpR3M3e4oKw+U9iL1H0n736FGQ16jF3H oDVTlvHzWQTZnHjHWeI/J7WDeDsTHA== =qVdb -----END PGP SIGNATURE----- --Sig_/nuRExRUY1USkXb8KqIHmAEn--