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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5661FC433E0 for ; Wed, 29 Jul 2020 08:51:29 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 1DB3F2075D for ; Wed, 29 Jul 2020 08:51:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ATe7TYZ8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1DB3F2075D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55934 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1k0ho8-0001Dm-CK for qemu-devel@archiver.kernel.org; Wed, 29 Jul 2020 04:51:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59926) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k0hlR-0006im-QR for qemu-devel@nongnu.org; Wed, 29 Jul 2020 04:48:41 -0400 Received: from us-smtp-delivery-74.mimecast.com ([63.128.21.74]:48348) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1k0hlN-0004ad-Uf for qemu-devel@nongnu.org; Wed, 29 Jul 2020 04:48:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1596012516; 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=N5JujI55DsHbmtMCcqPwJoGcU8MI0DZ+oEEsuGJhGho=; b=ATe7TYZ8ljNfFPFRKgmBk50XXlGTPkhPfMmufY+NGHgTQxyXdCIVsgj8Iw8rjksR2Fxrvq biJnp05Bi7JN1v9duh1/Nugfmv8fzHyCprmnsnvv9Te48lKzJiam0JWQbRWEe1f+y+kzdJ x7J4gSOcor3qg8pwqMUBumwCuyCbf64= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-78-i24op6P5NiKh3vTBh9Az-g-1; Wed, 29 Jul 2020 04:48:34 -0400 X-MC-Unique: i24op6P5NiKh3vTBh9Az-g-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E36A21DE1 for ; Wed, 29 Jul 2020 08:48:33 +0000 (UTC) Received: from titinator (ovpn-114-132.ams2.redhat.com [10.36.114.132]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6786610098AB; Wed, 29 Jul 2020 08:48:29 +0000 (UTC) References: <87eeouafe0.fsf@dusky.pond.sub.org> User-agent: mu4e 1.5.2; emacs 26.3 From: Christophe de Dinechin To: Markus Armbruster Subject: Re: Missing qapi_free_Type in error case for qapi generated code? In-reply-to: <87eeouafe0.fsf@dusky.pond.sub.org> Date: Wed, 29 Jul 2020 10:48:24 +0200 Message-ID: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dinechin@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.74; envelope-from=dinechin@redhat.com; helo=us-smtp-delivery-74.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/29 01:09:48 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020-07-29 at 10:34 CEST, Markus Armbruster wrote... > Eric Blake writes: > >> On 7/28/20 10:26 AM, Christophe de Dinechin wrote: >>> The qapi generated code for qmp_marshal_query_spice seems to be missing a >>> resource deallocation for "retval". For example, for SpiceInfo: >>> >> >>> retval = qmp_query_spice(&err); >>> error_propagate(errp, err); >>> if (err) { >>> /* retval not freed here */ >> >> Because it should be NULL here. Returning an error AND an object is >> frowned on. > > It's forbidden, actually. The QMP handler must either succeed and > return a value, or fail cleanly. OK. Then I guess Eric's suggestion to add an assert is the correct approach, with the caveat you identified. > > Since it has to return a value even when it fails, it returns an error > value then. "Cleanly" means the error value does not require cleanup. > > The generated marshalling function relies on this: it *ignores* the > error value. > >>> /* Missing: qapi_free_SpiceInfo(retval); */ >>> goto out; >>> } >>> >>> qmp_marshal_output_SpiceInfo(retval, ret, errp); >> >> And here, retval was non-NULL, but is cleaned as a side-effect of >> qmp_marshal_output_SpiceInfo. >> >>> >>> out: >> >> So no matter how you get to the label, retval is no longer valid >> memory that can be leaked. >> >>> visit_free(v); >>> v = qapi_dealloc_visitor_new(); >>> visit_start_struct(v, NULL, NULL, 0, NULL); >>> visit_end_struct(v, NULL); >>> visit_free(v); >>> } >>> #endif /* defined(CONFIG_SPICE) */ >>> >>> Questions: >>> >>> - Is the query code supposed to always return NULL in case of error? >> >> Yes. If not, that is a bug in qmp_query_spice. > > Correct. > >>> In the >>> case of hmp_info_spice, there is no check for info==NULL, so on the > > I'm blind. Where? In hmp_info_spice, there is this code: info = qmp_query_spice(NULL); if (!info->enabled) { monitor_printf(mon, "Server: disabled\n"); goto out; } I guess this code relies on qmp_query_spice never returning an error. Why is that a safe assumption? This came to my attention because I wanted to return an error and a NULL value for modular spice if the module is not available. > >>> contrary, it seems to indicate that a non-null result is always expected, >>> and that function does call qapi_free_SpiceInfo >> >> Calling qapi_free_SpiceInfo(NULL) is a safe no-op. Or if you expect >> the function to always succeed, you could pass &error_abort as the >> errp parameter. >> >>> >>> - If not, is there an existing shortcut to generate the correct deallocation >>> code for return types that need it? You can't just use >>> qapi_free_%(c_type)s because that would generate an extra * character, >>> i.e. I get "SpiceInfo *" and not "SpiceInfo". >> >> Ah, you're debating about editing scripts/qapi/commands.py. If >> anything, an edit to add 'assert(!retval)' if qmp_COMMAND failed might >> be smarter than trying to add code to free retval. > > This is more complicated than it may seem. > > The "natural" error value for a pointer-valued function is NULL. I'm > confident the handlers use it. assert(!retval) should work. > > For functions returning something else, people may have different ideas > on what to return on error. To make assert(!retval) work, they need to > return something "falsish". I'm not ready to bet my own money on all of > them doing that. That's what I was thinking too. Glad to confirm I was not reading that code too wrong. > > Aside: only functions in pragma returns-whitelist can return > non-pointer. > >>> - If not, is there any good way to know if the type is a pointer type? >>> (A quick look in cripts/qapi/types.py does not show anything obvious) > > No clean way exists, simply because there has been no need. So far, > we've always found a reasonable way to generate code that works whether > types are pointers in C or not. > >> Look at scripts/qapi/schema.py; each QAPI metatype has implementations >> of .c_name and .c_type that determine how to represent that QAPI >> object in C. You probably want c_name instead of c_type when >> constructing the name of a qapi_free_FOO function, but that goes back >> to my question of whether such a call is even needed. > > Method c_name() returns a string you can interpolate into C identifiers. > > Method c_type() returns a string you can use as C type in generated > code. That part I understood. I was looking for something like method c_basetype() or something like that for the non-pointer type of a pointer. > > The QAPI scripts could use more comments. +1. -- Cheers, Christophe de Dinechin (IRC c3d)