From patchwork Thu Sep 21 12:13:12 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Markus Armbruster X-Patchwork-Id: 13393890 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.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 A5DDAE71060 for ; Thu, 21 Sep 2023 12:13:40 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.606350.944263 (Exim 4.92) (envelope-from ) id 1qjIYl-00080C-Bu; Thu, 21 Sep 2023 12:13:31 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 606350.944263; Thu, 21 Sep 2023 12:13:31 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1qjIYl-0007zw-8Q; Thu, 21 Sep 2023 12:13:31 +0000 Received: by outflank-mailman (input) for mailman id 606350; Thu, 21 Sep 2023 12:13:29 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1qjIYj-0005zm-TO for xen-devel@lists.xenproject.org; Thu, 21 Sep 2023 12:13:29 +0000 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id 44f7ffa1-5878-11ee-9b0d-b553b5be7939; Thu, 21 Sep 2023 14:13:28 +0200 (CEST) Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-641-zZ3TtG8EPVuYsO7sGpTg5g-1; Thu, 21 Sep 2023 08:13:21 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 22AFD101B047; Thu, 21 Sep 2023 12:13:15 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.25]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D468751E3; Thu, 21 Sep 2023 12:13:14 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 824C621E690E; Thu, 21 Sep 2023 14:13:12 +0200 (CEST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 44f7ffa1-5878-11ee-9b0d-b553b5be7939 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1695298407; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NwdQRx9eHHEXA/94dHOQriykfQNAQJKQnyavt6LfIyk=; b=irdxbfXeWbj5tu0vB5p1diBw5wBaYiXzb2BTo+EaP3zKGIBEYxBZvbLKtxyGBdGn/ul8Zy UTKAmO6uTgD0VRNig5ZoqZhIf5OCYhFq/kYo+eXEptK4495st9PXE7rnPX1i3C5prA3197 sFs0z1gfqC734AlfkPNR4ingZuxyoOo= X-MC-Unique: zZ3TtG8EPVuYsO7sGpTg5g-1 From: Markus Armbruster To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, hreitz@redhat.com, eblake@redhat.com, vsementsov@yandex-team.ru, jsnow@redhat.com, idryomov@gmail.com, pl@kamp.de, sw@weilnetz.de, sstabellini@kernel.org, anthony.perard@citrix.com, paul@xen.org, pbonzini@redhat.com, marcandre.lureau@redhat.com, berrange@redhat.com, thuth@redhat.com, philmd@linaro.org, stefanha@redhat.com, fam@euphon.net, quintela@redhat.com, peterx@redhat.com, leobras@redhat.com, kraxel@redhat.com, qemu-block@nongnu.org, xen-devel@lists.xenproject.org, alex.bennee@linaro.org, peter.maydell@linaro.org Subject: [PATCH v3 7/7] qobject atomics osdep: Make a few macros more hygienic Date: Thu, 21 Sep 2023 14:13:12 +0200 Message-ID: <20230921121312.1301864-8-armbru@redhat.com> In-Reply-To: <20230921121312.1301864-1-armbru@redhat.com> References: <20230921121312.1301864-1-armbru@redhat.com> MIME-Version: 1.0 Content-type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Variables declared in macros can shadow other variables. Much of the time, this is harmless, e.g.: #define _FDT(exp) \ do { \ int ret = (exp); \ if (ret < 0) { \ error_report("error creating device tree: %s: %s", \ #exp, fdt_strerror(ret)); \ exit(1); \ } \ } while (0) Harmless shadowing in h_client_architecture_support(): target_ulong ret; [...] ret = do_client_architecture_support(cpu, spapr, vec, fdt_bufsize); if (ret == H_SUCCESS) { _FDT((fdt_pack(spapr->fdt_blob))); [...] } return ret; However, we can get in trouble when the shadowed variable is used in a macro argument: #define QOBJECT(obj) ({ \ typeof(obj) o = (obj); \ o ? container_of(&(o)->base, QObject, base) : NULL; \ }) QOBJECT(o) expands into ({ ---> typeof(o) o = (o); o ? container_of(&(o)->base, QObject, base) : NULL; }) Unintended variable name capture at --->. We'd be saved by -Winit-self. But I could certainly construct more elaborate death traps that don't trigger it. To reduce the risk of trapping ourselves, we use variable names in macros that no sane person would use elsewhere. Here's our actual definition of QOBJECT(): #define QOBJECT(obj) ({ \ typeof(obj) _obj = (obj); \ _obj ? container_of(&(_obj)->base, QObject, base) : NULL; \ }) Works well enough until we nest macro calls. For instance, with #define qobject_ref(obj) ({ \ typeof(obj) _obj = (obj); \ qobject_ref_impl(QOBJECT(_obj)); \ _obj; \ }) the expression qobject_ref(obj) expands into ({ typeof(obj) _obj = (obj); qobject_ref_impl( ({ ---> typeof(_obj) _obj = (_obj); _obj ? container_of(&(_obj)->base, QObject, base) : NULL; })); _obj; }) Unintended variable name capture at --->. The only reliable way to prevent unintended variable name capture is -Wshadow. One blocker for enabling it is shadowing hiding in function-like macros like qdict_put(dict, "name", qobject_ref(...)) qdict_put() wraps its last argument in QOBJECT(), and the last argument here contains another QOBJECT(). Use dark preprocessor sorcery to make the macros that give us this problem use different variable names on every call. Signed-off-by: Markus Armbruster Reviewed-by: Eric Blake Reviewed-by: Philippe Mathieu-Daudé --- include/qapi/qmp/qobject.h | 10 ++++++++-- include/qemu/atomic.h | 17 ++++++++++++----- include/qemu/compiler.h | 3 +++ include/qemu/osdep.h | 27 ++++++++++++++++++++------- 4 files changed, 43 insertions(+), 14 deletions(-) diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h index 9003b71fd3..89b97d88bc 100644 --- a/include/qapi/qmp/qobject.h +++ b/include/qapi/qmp/qobject.h @@ -45,10 +45,16 @@ struct QObject { struct QObjectBase_ base; }; -#define QOBJECT(obj) ({ \ +/* + * Preprocessor sorcery ahead: use a different identifier for the + * local variable in each expansion, so we can nest macro calls + * without shadowing variables. + */ +#define QOBJECT_INTERNAL(obj, _obj) ({ \ typeof(obj) _obj = (obj); \ - _obj ? container_of(&(_obj)->base, QObject, base) : NULL; \ + _obj ? container_of(&_obj->base, QObject, base) : NULL; \ }) +#define QOBJECT(obj) QOBJECT_INTERNAL((obj), MAKE_IDENTFIER(_obj)) /* Required for qobject_to() */ #define QTYPE_CAST_TO_QNull QTYPE_QNULL diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index d95612f7a0..f1d3d1702a 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -157,13 +157,20 @@ smp_read_barrier_depends(); #endif -#define qatomic_rcu_read(ptr) \ - ({ \ +/* + * Preprocessor sorcery ahead: use a different identifier for the + * local variable in each expansion, so we can nest macro calls + * without shadowing variables. + */ +#define qatomic_rcu_read_internal(ptr, _val) \ + ({ \ qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \ - typeof_strip_qual(*ptr) _val; \ - qatomic_rcu_read__nocheck(ptr, &_val); \ - _val; \ + typeof_strip_qual(*ptr) _val; \ + qatomic_rcu_read__nocheck(ptr, &_val); \ + _val; \ }) +#define qatomic_rcu_read(ptr) \ + qatomic_rcu_read_internal((ptr), MAKE_IDENTFIER(_val)) #define qatomic_rcu_set(ptr, i) do { \ qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \ diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h index a309f90c76..03236d830c 100644 --- a/include/qemu/compiler.h +++ b/include/qemu/compiler.h @@ -37,6 +37,9 @@ #define tostring(s) #s #endif +/* Expands into an identifier stemN, where N is another number each time */ +#define MAKE_IDENTFIER(stem) glue(stem, __COUNTER__) + #ifndef likely #define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0) diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h index 2897720fac..a80f49d1ab 100644 --- a/include/qemu/osdep.h +++ b/include/qemu/osdep.h @@ -383,19 +383,28 @@ void QEMU_ERROR("code path is reachable") * determined by the pre-processor instead of the compiler, you'll * have to open-code it. Sadly, Coverity is severely confused by the * constant variants, so we have to dumb things down there. + * + * Preprocessor sorcery ahead: use different identifiers for the local + * variables in each expansion, so we can nest macro calls without + * shadowing variables. */ -#undef MIN -#define MIN(a, b) \ +#define MIN_INTERNAL(a, b, _a, _b) \ ({ \ typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ _a < _b ? _a : _b; \ }) -#undef MAX -#define MAX(a, b) \ +#undef MIN +#define MIN(a, b) \ + MIN_INTERNAL((a), (b), MAKE_IDENTFIER(_a), MAKE_IDENTFIER(_b)) + +#define MAX_INTERNAL(a, b, _a, _b) \ ({ \ typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ _a > _b ? _a : _b; \ }) +#undef MAX +#define MAX(a, b) \ + MAX_INTERNAL((a), (b), MAKE_IDENTFIER(_a), MAKE_IDENTFIER(_b)) #ifdef __COVERITY__ # define MIN_CONST(a, b) ((a) < (b) ? (a) : (b)) @@ -416,14 +425,18 @@ void QEMU_ERROR("code path is reachable") /* * Minimum function that returns zero only if both values are zero. * Intended for use with unsigned values only. + * + * Preprocessor sorcery ahead: use different identifiers for the local + * variables in each expansion, so we can nest macro calls without + * shadowing variables. */ -#ifndef MIN_NON_ZERO -#define MIN_NON_ZERO(a, b) \ +#define MIN_NON_ZERO_INTERNAL(a, b, _a, _b) \ ({ \ typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ _a == 0 ? _b : (_b == 0 || _b > _a) ? _a : _b; \ }) -#endif +#define MIN_NON_ZERO(a, b) \ + MIN_NON_ZERO_INTERNAL((a), (b), MAKE_IDENTFIER(_a), MAKE_IDENTFIER(_b)) /* * Round number down to multiple. Safe when m is not a power of 2 (see