From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757502AbaGWI0z (ORCPT ); Wed, 23 Jul 2014 04:26:55 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:36301 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757367AbaGWI0w (ORCPT ); Wed, 23 Jul 2014 04:26:52 -0400 Date: Wed, 23 Jul 2014 09:25:05 +0100 From: Will Deacon To: AKASHI Takahiro Cc: Kees Cook , Will Drewry , Catalin Marinas , "dsaxena@linaro.org" , "linux-arm-kernel@lists.infradead.org" , "linaro-kernel@lists.linaro.org" , LKML Subject: Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations Message-ID: <20140723082505.GB27260@arm.com> References: <1406020499-5537-1-git-send-email-takahiro.akashi@linaro.org> <1406020499-5537-2-git-send-email-takahiro.akashi@linaro.org> <53CF5E53.3060409@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53CF5E53.3060409@linaro.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote: > On 07/23/2014 05:15 AM, Kees Cook wrote: > > On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro > > wrote: > >> asmlinkage int syscall_trace_enter(struct pt_regs *regs) > >> { > >> + unsigned long saved_x0, saved_x8; > >> + > >> + saved_x0 = regs->regs[0]; > >> + saved_x8 = regs->regs[8]; > >> + > >> if (test_thread_flag(TIF_SYSCALL_TRACE)) > >> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); > >> > >> + regs->syscallno = regs->regs[8]; > >> + if ((long)regs->syscallno == ~0UL) { /* skip this syscall */ > >> + regs->regs[8] = saved_x8; > >> + if (regs->regs[0] == saved_x0) /* not changed by user */ > >> + regs->regs[0] = -ENOSYS; > > > > I'm not sure this is right compared to other architectures. Generally > > when a tracer performs a syscall skip, it's up to them to also adjust > > the return value. They may want to be faking a syscall, and what if > > the value they want to return happens to be what x0 was going into the > > tracer? It would have no way to avoid this -ENOSYS case. I think > > things are fine without this test. > > Yeah, I know this issue, but was not sure that setting a return value > is mandatory. (x86 seems to return -ENOSYS by default if not explicitly > specified.) > Is "fake a system call" a more appropriate word than "skip"? > > I will defer to Will. I agree with Kees -- iirc, I only suggested restoring x8. Will