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Tue, 26 Jan 2021 09:21:26 -0500 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-377-I80_6ty8MLKYB0ht6oPLkg-1; Tue, 26 Jan 2021 09:21:16 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 92E2B108442D; Tue, 26 Jan 2021 14:21:15 +0000 (UTC) Received: from localhost (ovpn-114-175.ams2.redhat.com [10.36.114.175]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 33DB75C1A3; Tue, 26 Jan 2021 14:21:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611670880; 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=pJXqiMlr4xBoN9Ka8VmfDgLQv1vJ7Whu9MEVSwWZ/gg=; b=cZRaHAar02Im3UC9O6d2ONBjhc86Vx4/NW+fLt0pKOY9fiaFxZr0E+ZrD91XyEHBqJObP+ Etscrx6fE/cLj2GO3cBTTRrB9diNo2gurpkhQ2mPyJUbVq8e9n7cEpDaNblHcx4gAQZemM jIvctFSAHUDwJ68SmIlWKiulZixp28s= X-MC-Unique: I80_6ty8MLKYB0ht6oPLkg-1 From: Max Reitz To: qemu-block@nongnu.org Subject: [PULL 25/53] coroutine-sigaltstack: Add SIGUSR2 mutex Date: Tue, 26 Jan 2021 15:19:48 +0100 Message-Id: <20210126142016.806073-26-mreitz@redhat.com> In-Reply-To: <20210126142016.806073-1-mreitz@redhat.com> References: <20210126142016.806073-1-mreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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=-0.255, 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_H3=0.001, RCVD_IN_MSPIKE_WL=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: Kevin Wolf , Peter Maydell , qemu-devel@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) Content-Type: text/plain; charset="utf-8" Disposition (action) for any given signal is global for the process. When two threads run coroutine-sigaltstack's qemu_coroutine_new() concurrently, they may interfere with each other: One of them may revert the SIGUSR2 handler to SIG_DFL, between the other thread (a) setting up coroutine_trampoline() as the handler and (b) raising SIGUSR2. That SIGUSR2 will then terminate the QEMU process abnormally. We have to ensure that only one thread at a time can modify the process-global SIGUSR2 handler. To do so, wrap the whole section where that is done in a mutex. Alternatively, we could for example have the SIGUSR2 handler always be coroutine_trampoline(), so there would be no need to invoke sigaction() in qemu_coroutine_new(). Laszlo has posted a patch to do so here: https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg05962.html However, given that coroutine-sigaltstack is more of a fallback implementation for platforms that do not support ucontext, that change may be a bit too invasive to be comfortable with it. The mutex proposed here may negatively impact performance, but the change is much simpler. Signed-off-by: Max Reitz Message-Id: <20210125120305.19520-1-mreitz@redhat.com> Reviewed-by: Laszlo Ersek Reviewed-by: Vladimir Sementsov-Ogievskiy --- util/coroutine-sigaltstack.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/util/coroutine-sigaltstack.c b/util/coroutine-sigaltstack.c index aade82afb8..e99b8a4f9c 100644 --- a/util/coroutine-sigaltstack.c +++ b/util/coroutine-sigaltstack.c @@ -157,6 +157,7 @@ Coroutine *qemu_coroutine_new(void) sigset_t sigs; sigset_t osigs; sigjmp_buf old_env; + static pthread_mutex_t sigusr2_mutex =3D PTHREAD_MUTEX_INITIALIZER; =20 /* The way to manipulate stack is with the sigaltstack function. We * prepare a stack, with it delivering a signal to ourselves and then @@ -186,6 +187,12 @@ Coroutine *qemu_coroutine_new(void) sa.sa_handler =3D coroutine_trampoline; sigfillset(&sa.sa_mask); sa.sa_flags =3D SA_ONSTACK; + + /* + * sigaction() is a process-global operation. We must not run + * this code in multiple threads at once. + */ + pthread_mutex_lock(&sigusr2_mutex); if (sigaction(SIGUSR2, &sa, &osa) !=3D 0) { abort(); } @@ -234,6 +241,8 @@ Coroutine *qemu_coroutine_new(void) * Restore the old SIGUSR2 signal handler and mask */ sigaction(SIGUSR2, &osa, NULL); + pthread_mutex_unlock(&sigusr2_mutex); + pthread_sigmask(SIG_SETMASK, &osigs, NULL); =20 /* --=20 2.29.2