From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 70389C433F5 for ; Mon, 4 Apr 2022 06:28:41 +0000 (UTC) Received: from localhost ([::1]:38186 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nbGCe-0001SM-8H for qemu-devel@archiver.kernel.org; Mon, 04 Apr 2022 02:28:40 -0400 Received: from eggs.gnu.org ([209.51.188.92]:51636) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nbG8J-0000e8-T8 for qemu-devel@nongnu.org; Mon, 04 Apr 2022 02:24:12 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:46938) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nbG8E-0005PJ-SU for qemu-devel@nongnu.org; Mon, 04 Apr 2022 02:24:09 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1649053445; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hVm4IPSMjuL70Vi4DriMR/QPpw+xesOfdIqo+xFByhA=; b=MsugfGhnGWAEV07LWG01enEo4uL23zgTLF26rHylW0/k3dmg5A9znWz+vwpzw2fVVszwTN slXoHMHfJFArMKytGu+MAhwWhXShaOjgboYonxRSCqvLP+SbdlY6vxUHTPGtdAOVuqMoVw Ty+MxPVzvME8SRTDWeifiPEZpKJh3pE= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-600-TEdNgcM9OSiLK-DEhxmMjg-1; Mon, 04 Apr 2022 02:24:04 -0400 X-MC-Unique: TEdNgcM9OSiLK-DEhxmMjg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 162333806721; Mon, 4 Apr 2022 06:24:04 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.36.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E75D140F1C2E; Mon, 4 Apr 2022 06:24:03 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id BC25021E6906; Mon, 4 Apr 2022 08:24:02 +0200 (CEST) From: Markus Armbruster To: Paolo Bonzini Subject: Re: use of uninitialized variable involving visit_type_uint32() and friends References: <87y20p88qq.fsf@pond.sub.org> <875ynt54pk.fsf@pond.sub.org> <33548764-9f91-b4df-c2b6-b897713d56fd@redhat.com> Date: Mon, 04 Apr 2022 08:24:02 +0200 In-Reply-To: <33548764-9f91-b4df-c2b6-b897713d56fd@redhat.com> (Paolo Bonzini's message of "Fri, 1 Apr 2022 17:46:39 +0200") Message-ID: <87ee2d1i4d.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Paolo Bonzini writes: > On 4/1/22 15:11, Markus Armbruster wrote: >>> If it can do really serious interprocedural analysis, it _might_ be able >>> to see through the visitor constructor and know that the "value = *obj" >>> is not initialized (e.g. "all callers of object_property_set use an >>> input visitor"). I doubt that honestly, but a man can dream. >> >> I'm wary of arguments based on "a sufficiently smart compiler can"... > > Absolutely. > >>> Because it communicates what the caller expects: "I have left this >>> uninitialized because I expect my "v" argument to be the kind of visitor >>> that fills it in". It's this argument that gives me the confidence >>> needed to shut up Coverity's false positives. >>> >>> Embedding the visitor type in the signature makes it impossible not to >>> pass it, unlike e.g. an assertion in every getter or setter. >> >> I think we got two kinds of code calling visitor methods: >> >> 1. Code for use with one kind of visitor only >> >> We get to pass a literal argument to the additional parameter you >> propose. >> >> 2. Code for use with arbitrary visitors (such as qapi-visit*.c) >> >> We need to pass v->type, where @v is the existing visitor argument. >> Except we can't: struct Visitor and VisitorType are private, defined >> in . Easy enough to work around, but has a distinct >> "this design is falling apart" smell, at least to me. > > Hmm, maybe that's a feature though. If we only need v->type in .c files > for the generated visit_type_* functions, then it's not a huge deal that > they will have to include . All callers outside > generated type visitors (which includes for example QMP command > marshaling), instead, would _have_ to pass visitor type constants and > make it clear what direction the visit is going. I quoted the generated qapi-visit*.c as an example. There may handwritten instances, too. >> Note that "intent explicit in every method call" is sufficient, but not >> necessary for "intent is locally explicit, which lets us dismiss false >> positives with confidence". We could do "every function that calls >> methods". Like checking a precondition. We already have >> visit_is_input(). We could have visit_is_output(). >> >> The sane way to make output intent explicit is of course passing the >> thing by value rather than by reference. To get that, we could generate >> even more code. So, if the amount of code we currently generate isn't >> disgusting enough, ... > > Yeah, that would be ugly. Or, we could generate the same code plus some > static inline wrappers that take a > > struct InputVisitor { > Visitor dont_use_me_it_hurts; > } > struct OutputVisitor { > Visitor dont_use_me_it_hurts; > } > > That would be zero-cost abstraction at runtime. Looks worth exploring!