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-Status: No, score=-4.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server2.sourceware.org [IPv6:2620:52:3:1:0:246e:9693:128c]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id 88BCC1F55B for ; Tue, 26 May 2020 14:53:56 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 5A1D63892024; Tue, 26 May 2020 14:53:28 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5A1D63892024 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1590504808; bh=J03/Qz2NTKez9n+J8FkkHyKFt/pjXwvDlXKd3V/IOls=; h=Date:To:In-Reply-To:References:Subject:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=LGCEMGKo3UaaIWtSQQPZP/AvJLCOrEBHlNf380w81gxEGacnNktG5HMvGIpMb5sxz 0Ap1+LiN7mSaeKnSMvJoUSB0ZmDtLIqeeu4BUqo4BjVIzR/kdYQho2m1rPRUf5ay0V 6lQNANtAnn44+j9Kd9aEG+NSg1Rh/ZZA/rynq7Wg= Received: from mail.efficios.com (mail.efficios.com [167.114.26.124]) by sourceware.org (Postfix) with ESMTPS id 83F273892024 for ; Tue, 26 May 2020 14:53:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 83F273892024 Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 4323D2530AB; Tue, 26 May 2020 10:53:25 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id Wd-fD4srGR5b; Tue, 26 May 2020 10:53:24 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id D2F562530AA; Tue, 26 May 2020 10:53:24 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com D2F562530AA X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id TN0OeTI4tPQP; Tue, 26 May 2020 10:53:24 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id BFB152530A9; Tue, 26 May 2020 10:53:24 -0400 (EDT) Date: Tue, 26 May 2020 10:53:24 -0400 (EDT) To: Florian Weimer Message-ID: <1931644690.34207.1590504804638.JavaMail.zimbra@efficios.com> In-Reply-To: <87ftbmpxqi.fsf@oldenburg2.str.redhat.com> References: <20200501021439.2456-1-mathieu.desnoyers@efficios.com> <87v9kqbzse.fsf@oldenburg2.str.redhat.com> <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> <87367ovy6k.fsf@oldenburg2.str.redhat.com> <108939265.33525.1590428184533.JavaMail.zimbra@efficios.com> <87lflerhqt.fsf@oldenburg2.str.redhat.com> <1701081361.34159.1590503556923.JavaMail.zimbra@efficios.com> <87ftbmpxqi.fsf@oldenburg2.str.redhat.com> Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3928 (ZimbraWebClient - FF76 (Linux)/8.8.15_GA_3928) Thread-Topic: glibc: Perform rseq registration at C startup and thread creation (v19) Thread-Index: +WF+p2h1qwweiPpTqvCExj6y4Uu9/w== 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: , From: Mathieu Desnoyers via Libc-alpha Reply-To: Mathieu Desnoyers Cc: Rich Felker , libc-alpha , Peter Zijlstra , linux-api , Boqun Feng , Will Deacon , linux-kernel , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" ----- On May 26, 2020, at 10:38 AM, Florian Weimer fweimer@redhat.com wrote= : > * Mathieu Desnoyers: >=20 >> AFAIU, the only gain here would be to make sure we don't emit useless >> ";" in the "/* nothing */" case. But does it matter ? >=20 > I don't think C allows empty constructs like this at the top level. >=20 >>>>> And something similar for _Alignas/attribute aligned, >>>> >>>> I don't see where _Alignas is needed here ? >>>> >>>> For attribute aligned, what would be the oldest supported C and C++ >>>> standards ? >>>=20 >>> There are no standardized attributes for C, there is only _Alignas. >>> C++11 has an alignas specifier; it's not an attribute either. I think >>> these are syntactically similar. >> >> There appears to be an interesting difference between attribute aligned >> and alignas. It seems like alignas cannot be used on a structure declara= tion, >> only on fields, e.g.: >> >> struct blah { >> int a; >> } _Alignas (16); >> >> o.c:3:1: warning: useless =E2=80=98_Alignas=E2=80=99 in empty declaratio= n >> } _Alignas (16); >> >> But >> >> struct blah { >> int _Alignas (16) a; >> }; >=20 > Like the attribute, it needs to come right after the struct keyword, I > think. (Trailing attributes can be ambiguous, but not in this case.) Nope. _Alignas really _is_ special :-( struct _Alignas (16) blah { int a; }; p.c:1:8: error: expected =E2=80=98{=E2=80=99 before =E2=80=98_Alignas=E2=80= =99 struct _Alignas (16) blah { Also: struct blah _Alignas (16) { int a; }; p.c:1:27: error: expected identifier or =E2=80=98(=E2=80=99 before =E2=80= =98{=E2=80=99 token struct blah _Alignas (16) { >=20 >> is OK. So if I change e.g. struct rseq_cs to align >> the first field: >> >> struct rseq_cs >> { >> /* Version of this structure. */ >> uint32_t rseq_align (32) version; >> /* enum rseq_cs_flags. */ >> uint32_t flags; >> uint64_t start_ip; >> /* Offset from start_ip. */ >> uint64_t post_commit_offset; >> uint64_t abort_ip; >> }; >> >> It should work. >=20 > Indeed. OK, so let's go for that approach. >=20 >> /* Rely on GNU extensions for older standards and tls model. */ >> #ifdef __GNUC__ >> # ifndef rseq_alignof >> # define rseq_alignof(x) __alignof__ (x) >> # endif >> # ifndef rseq_alignas >> # define rseq_alignas(x) __attribute__ ((aligned (x))) >> # endif >> # define rseq_tls_model_ie __attribute__ ((__tls_model__ ("initial-exec"= ))) >> #else >> /* Specifying the TLS model on the declaration is optional. */ >> # define rseq_tls_model_ie /* Nothing. */ >> #endif >> >> /* Fall back to __thread for TLS storage class. */ >> #ifndef rseq_tls_storage_class >> # define rseq_tls_storage_class __thread >> #endif >=20 > If they are only used in the glibc headers, they should have __rseq > prefixes, so that application code doesn't start using them (in case we > have to change/fix them, or move the into later). OK will do. >=20 > Rest looks fine. One last thing I'm planning to add in sys/rseq.h to cover acessing the rseq_cs pointers with both the UAPI headers and the glibc struct rseq declarations: /* The rseq_cs_ptr macro can be used to access the pointer to the current rseq critical section descriptor. */ #ifdef __LP64__ # define rseq_cs_ptr(rseq) \ ((const struct rseq_cs *) (rseq)->rseq_cs.ptr) #else /* __LP64__ */ # define rseq_cs_ptr(rseq) \ ((const struct rseq_cs *) (rseq)->rseq_cs.ptr.ptr32) #endif /* __LP64__ */ Does it make sense ? Thanks, Mathieu >=20 > Thanks, > Florian --=20 Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com