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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE067C433F5 for ; Mon, 25 Oct 2021 14:10:49 +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 6291D60F70 for ; Mon, 25 Oct 2021 14:10:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 6291D60F70 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:36548 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mf0ga-0001YD-G7 for qemu-devel@archiver.kernel.org; Mon, 25 Oct 2021 10:10:48 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:53932) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mf0du-0005Qn-F8 for qemu-devel@nongnu.org; Mon, 25 Oct 2021 10:08:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:33747) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mf0dr-0006qC-V9 for qemu-devel@nongnu.org; Mon, 25 Oct 2021 10:08:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635170879; 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=ZxL5vHfR45EwUHIvdSwccHh74kKBhMZzDkyhDc4Si5w=; b=AmtUNVE3fCliP/w5nOJ6BcG+uzYNlKaXqLmuftGi+BN+VxYc32OZkYC1RrHeea3n7KDCgl uGIOhhDOBhY+G4/r/wtfeoPBoq2kO+wtvesfuVTsPEUzjqDFhtJNGL/zif5rK5BpDjZxOA IgcPdSOoFDbrBpQ0qMy0NabqJBDrMJw= 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-18-yX0e1oTfO62WPf1jp7RD_A-1; Mon, 25 Oct 2021 10:07:57 -0400 X-MC-Unique: yX0e1oTfO62WPf1jp7RD_A-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E675A10A8E09; Mon, 25 Oct 2021 14:07:56 +0000 (UTC) Received: from localhost (unknown [10.39.192.215]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2310B61064; Mon, 25 Oct 2021 14:07:40 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [RFC 1/2] tls: add macros for coroutine-safe TLS variables Date: Mon, 25 Oct 2021 15:07:15 +0100 Message-Id: <20211025140716.166971-2-stefanha@redhat.com> In-Reply-To: <20211025140716.166971-1-stefanha@redhat.com> References: <20211025140716.166971-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="US-ASCII" Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-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: fweimer@redhat.com, Kevin Wolf , thuth@redhat.com, qemu-block@nongnu.org, Stefan Hajnoczi , Fam Zheng , sguelton@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Compiler optimizations can cache TLS values across coroutine yield points, resulting in stale values from the previous thread when a coroutine is re-entered by a new thread. Serge Guelton developed an __attribute__((noinline)) wrapper and tested it with clang and gcc. I formatted his idea according to QEMU's coding style and wrote documentation. These macros must be used instead of __thread from now on to prevent coroutine TLS bugs. Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1952483 Suggested-by: Serge Guelton Signed-off-by: Stefan Hajnoczi --- MAINTAINERS | 1 + include/qemu/tls.h | 142 +++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 143 insertions(+) create mode 100644 include/qemu/tls.h diff --git a/MAINTAINERS b/MAINTAINERS index 894dc43105..cf225b7029 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -2869,6 +2869,7 @@ M: Kevin Wolf S: Maintained F: util/*coroutine* F: include/qemu/coroutine* +F: include/qemu/tls.h F: tests/unit/test-coroutine.c Buffers diff --git a/include/qemu/tls.h b/include/qemu/tls.h new file mode 100644 index 0000000000..9a5ccc6c52 --- /dev/null +++ b/include/qemu/tls.h @@ -0,0 +1,142 @@ +/* + * QEMU Thread Local Storage + * + * Copyright Red Hat + * + * SPDX-License-Identifier: LGPL-2.1-or-later + * + * This work is licensed under the terms of the GNU LGPL, version 2.1 or later. + * See the COPYING.LIB file in the top-level directory. + * + * It is forbidden to use __thread in QEMU because compiler optimizations may + * cause Thread Local Storage variables to be cached across coroutine re-entry. + * Coroutines can run in more than one thread through the course of their life, + * leading bugs when stale TLS values from the wrong thread are used as a + * result of compiler optimization. + * + * Although avoiding __thread is strictly only necessary when coroutines access + * the variable this is hard to audit or enforce in practice. Therefore + * __thread is forbidden everywhere. This prevents subtle bugs from creeping in + * if a variable that was previously not accessed from coroutines is now + * accessed from coroutines. + * + * An example is: + * + * ..code-block:: c + * :caption: A coroutine that may see the wrong TLS value + * + * static __thread AioContext *current_aio_context; + * ... + * static void coroutine_fn foo(void) + * { + * aio_notify(current_aio_context); + * qemu_coroutine_yield(); + * aio_notify(current_aio_context); // <-- may be stale after yielding! + * } + * + * This header provides macros for safely defining variables in Thread Local + * Storage: + * + * ..code-block:: c + * :caption: A coroutine that safely uses TLS + * + * QEMU_DECLARE_STATIC_TLS(AioContext *, current_aio_context) + * ... + * static void coroutine_fn foo(void) + * { + * aio_notify(get_current_aio_context()); + * qemu_coroutine_yield(); + * aio_notify(get_current_aio_context()); // <-- safe + * } + */ + +#ifndef QEMU_TLS_H +#define QEMU_TLS_H + +/** + * QEMU_DECLARE_TLS: + * @type: the variable's C type + * @var: the variable name + * + * Declare an extern variable in Thread Local Storage from a header file: + * + * .. code-block:: c + * :caption: Declaring an extern variable in Thread Local Storage + * + * QEMU_DECLARE_TLS(int, my_count) + * ... + * int c = get_my_count(); + * set_my_count(c + 1); + * + * Use this instead of: + * + * .. code-block:: c + * :caption: Declaring a TLS variable using __thread + * + * extern __thread int my_count; + * ... + * int c = my_count; + * my_count = c + 1; + */ +#define QEMU_DECLARE_TLS(type, var) \ + __attribute__((noinline)) type get_##var(void); \ + __attribute__((noinline)) void set_##var(type v); \ + +/** + * QEMU_DEFINE_TLS: + * @type: the variable's C type + * @var: the variable name + * + * Define an variable in Thread Local Storage that was previously declared from + * a header file with QEMU_DECLARE_TLS(): + * + * .. code-block:: c + * :caption: Defining a variable in Thread Local Storage + * + * QEMU_DEFINE_TLS(int, my_count) + * + * Use this instead of: + * + * .. code-block:: c + * :caption: Defining a TLS variable using __thread + * + * __thread int my_count; + */ +#define QEMU_DEFINE_TLS(type, var) \ + __thread type qemu_tls_##var; \ + type get_##var(void) { return qemu_tls_##var; } \ + void set_##var(type v) { qemu_tls_##var = v; } + +/** + * QEMU_DEFINE_STATIC_TLS: + * @type: the variable's C type + * @var: the variable name + * + * Define a static variable in Thread Local Storage: + * + * .. code-block:: c + * :caption: Defining a static variable in Thread Local Storage + * + * QEMU_DEFINE_STATIC_TLS(int, my_count) + * ... + * int c = get_my_count(); + * set_my_count(c + 1); + * + * Use this instead of: + * + * .. code-block:: c + * :caption: Defining a static TLS variable using __thread + * + * static __thread int my_count; + * ... + * int c = my_count; + * my_count = c + 1; + */ +#define QEMU_DEFINE_STATIC_TLS(type, var) \ + static __thread type qemu_tls_##var; \ + static __attribute__((noinline)) type get_##var(void); \ + static type get_##var(void) { return qemu_tls_##var; } \ + static __attribute__((noinline)) void set_##var(type v); \ + static void set_##var(type v) { qemu_tls_##var = v; } + +#endif /* QEMU_TLS_H */ -- 2.31.1