From patchwork Fri Jul 10 13:07:00 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 11656563 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 31AC113B4 for ; Fri, 10 Jul 2020 13:09:01 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (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 098EA2077D for ; Fri, 10 Jul 2020 13:09:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="XvVNtxug"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="F8mgM5FV" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 098EA2077D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To:Message-Id:Date: Subject:To:From:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=XGOO2/UJpRkZ6uQVrrWJY4Wqyf9ub2t+IkInqYsNKDc=; b=XvVNtxugWTNzp1Ji1tQTozdjF KtNobRnGWgBxDQrtCp9MhroJRq4zu96JjlbzxCq8IslaFnMz2UqkY7aa3GNT/Ao+Ez68EV4h6HZRL Cv8UUuoxTjYr/ssnkR7l4nBpkPqNJwY26sQ75v6Ny0ZaVE5tAPR5RSj3VxjwqnwC06d+TLAUHu2I/ 3O2qDmHuOwo0g3uFBrIkiErmdy9EypehSqEBQ8biRkTt/nU5JTXvQHEUSVtZEvhaRCNCmxcQw/egg rgdA71ax9ka0Azb2kANJPucVaC1mDys5AAfPmScQr0qd5PYPesqCEKpn0QGfpwSdMQMK537DyJruM 7qXd/URAQ==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jtskZ-00011N-Er; Fri, 10 Jul 2020 13:07:35 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jtskJ-0000wf-BT for linux-arm-kernel@lists.infradead.org; Fri, 10 Jul 2020 13:07:20 +0000 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 475AF2078B; Fri, 10 Jul 2020 13:07:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594386438; bh=3J6qeziSFmygF8x/K63RJq6MiNghwwS1jTkEAA1aYSs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=F8mgM5FV/udaXsn9SWVEXPLc5sIjBjGe8my9oj6575soE74vum3LbRxes53Vohm9y F1vrnS8ClcPl1tgGregeVD0NbvJ++eE4L0i2cwPs81MsS2W0mKkNwNhu0wxLBoRz55 HocuCw1zmQRTfKmZcWeWBfiXcuzq0p8DMO7J8P4Q= From: Will Deacon To: linux-arm-kernel@lists.infradead.org Subject: [PATCH v3 5/7] arm64: syscall: Expand the comment about ptrace and syscall(-1) Date: Fri, 10 Jul 2020 14:07:00 +0100 Message-Id: <20200710130702.30658-6-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200710130702.30658-1-will@kernel.org> References: <20200710130702.30658-1-will@kernel.org> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200710_090719_624942_33803D13 X-CRM114-Status: GOOD ( 19.62 ) X-Spam-Score: -5.2 (-----) X-Spam-Report: SpamAssassin version 3.4.4 on merlin.infradead.org summary: Content analysis details: (-5.2 points) pts rule name description ---- ---------------------- -------------------------------------------------- -5.0 RCVD_IN_DNSWL_HI RBL: Sender listed at https://www.dnswl.org/, high trust [198.145.29.99 listed in list.dnswl.org] 0.0 SPF_HELO_NONE SPF: HELO does not publish an SPF Record -0.0 SPF_PASS SPF: sender matches SPF record -0.1 DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain -0.0 DKIMWL_WL_HIGH DKIMwl.org - Whitelisted High sender X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Luis Machado , Kees Cook , Will Deacon , catalin.marinas@arm.com, Keno Fischer , kernel-team@android.com Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org If a task executes syscall(-1), we intercept this early and force x0 to be -ENOSYS so that we don't need to distinguish this scenario from one where the scno is -1 because a tracer wants to skip the system call using ptrace. With the return value set, the return path is the same as the skip case. Although there is a one-line comment noting this in el0_svc_common(), it misses out most of the detail. Expand the comment to describe a bit more about what is going on. Cc: Mark Rutland Cc: Keno Fischer Cc: Luis Machado Signed-off-by: Will Deacon --- arch/arm64/kernel/syscall.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c index 98a26d4e7b0c..5f0c04863d2c 100644 --- a/arch/arm64/kernel/syscall.c +++ b/arch/arm64/kernel/syscall.c @@ -124,7 +124,21 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr, user_exit(); if (has_syscall_work(flags)) { - /* set default errno for user-issued syscall(-1) */ + /* + * The de-facto standard way to skip a system call using ptrace + * is to set the system call to -1 (NO_SYSCALL) and set x0 to a + * suitable error code for consumption by userspace. However, + * this cannot be distinguished from a user-issued syscall(-1) + * and so we must set x0 to -ENOSYS here in case the tracer doesn't + * issue the skip and we fall into trace_exit with x0 preserved. + * + * This is slightly odd because it also means that if a tracer + * sets the system call number to -1 but does not initialise x0, + * then x0 will be preserved for all system calls apart from a + * user-issued syscall(-1). However, requesting a skip and not + * setting the return value is unlikely to do anything sensible + * anyway. + */ if (scno == NO_SYSCALL) regs->regs[0] = -ENOSYS; scno = syscall_trace_enter(regs);