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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=Vm16QKtKv0j4RjXEKwkfTX0lVze5FL0Tj1MGscpmVdc=; b=xmjRK/TD/tJVjq885PAWkVRmH oxAfGIc/Ug/HHv5dZsdi1MCmyC3LhduxHIu6efVYjrreWPz89jOkSAl5JCK6RA/PMwlihtJNyzD7N BsMH0ZfnKsVXtsQ8FHtvVtrbXrTyfT84W+0+oYzTDMkBeX/lgJioXWX9PuxysgtSK5MZJ8+TKzql5 WMirW6XlwSc5Wn2pQH59KnA77BqY8YbpFQCxjfHfUU8j0vbglsUqoW3A9VQL6HXMubslXx1BPsMwB zxvEzK/vmpeQpAXIiv8gMvjAXfOdJ1bXpwL0GmQhEFwGW+xQUyqnMqmmARCjb9or/+dthnwby0y8u sErQkpuCw==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jtskT-0000yv-0j; Fri, 10 Jul 2020 13:07:29 +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 1jtskH-0000vt-Ck for linux-arm-kernel@lists.infradead.org; Fri, 10 Jul 2020 13:07:18 +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 7152D2077D; Fri, 10 Jul 2020 13:07:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594386436; bh=+GmSpCJblP3QLDpANhasBeu8bvqhsL9/+Uoaep/XeGM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=XngBm/+iJGGQz+Cm4EblCNyPga0MrA0y06VfnWOYmPsMqAZ1N17Eg7G6UupTYdLyV y4V0RlQSElraR6rg89/MLir4tiWoRmDQGTpnl7kmS3luW3+NF9DpwnmIJ4v5mjWHhU 850BZsph1QfoLdMhd6vS5oU6/mDADnwe7IOha+O8= From: Will Deacon To: linux-arm-kernel@lists.infradead.org Subject: [PATCH v3 4/7] arm64: ptrace: Add a comment describing our syscall entry/exit trap ABI Date: Fri, 10 Jul 2020 14:06:59 +0100 Message-Id: <20200710130702.30658-5-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_090717_546235_2BD2C56D X-CRM114-Status: GOOD ( 15.59 ) 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 Our tracehook logic for syscall entry/exit raises a SIGTRAP back to the tracer following a ptrace request such as PTRACE_SYSCALL. As part of this procedure, we clobber the reported value of one of the tracee's general purpose registers (x7 for native tasks, r12 for compat) to indicate whether the stop occurred on syscall entry or exit. This is a slightly unfortunate ABI, as it prevents the tracer from accessing the real register value and is at odds with other similar stops such as seccomp traps. Since we're stuck with this ABI, expand the comment in our tracehook logic to acknowledge the issue and descibe the behaviour in more detail. Cc: Mark Rutland Cc: Luis Machado Reported-by: Keno Fischer Signed-off-by: Will Deacon --- arch/arm64/kernel/ptrace.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 22f9053b55b6..89fbee3991a2 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -1811,8 +1811,20 @@ static void tracehook_report_syscall(struct pt_regs *regs, unsigned long saved_reg; /* - * A scratch register (ip(r12) on AArch32, x7 on AArch64) is - * used to denote syscall entry/exit: + * We have some ABI weirdness here in the way that we handle syscall + * exit stops because we indicate whether or not the stop has been + * signalled from syscall entry or syscall exit by clobbering a general + * purpose register (ip/r12 for AArch32, x7 for AArch64) in the tracee + * and restoring its old value after the stop. This means that: + * + * - Any writes by the tracer to this register during the stop are + * ignored/discarded. + * + * - The actual value of the register is not available during the stop, + * so the tracer cannot save it and restore it later. + * + * - Syscall stops behave differently to seccomp and pseudo-step traps + * (the latter do not nobble any registers). */ regno = (is_compat_task() ? 12 : 7); saved_reg = regs->regs[regno];