Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752500AbaKYKak (ORCPT ); Tue, 25 Nov 2014 05:30:40 -0500 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:38706 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752472AbaKYKae (ORCPT ); Tue, 25 Nov 2014 05:30:34 -0500 Date: Tue, 25 Nov 2014 10:30:34 +0000 From: Will Deacon To: AKASHI Takahiro Cc: "keescook@chromium.org" , Catalin Marinas , "dsaxena@linaro.org" , "arndb@arndb.de" , "linux-arm-kernel@lists.infradead.org" , "linaro-kernel@lists.linaro.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call Message-ID: <20141125103034.GC30251@arm.com> References: <1416273038-15590-1-git-send-email-takahiro.akashi@linaro.org> <1416273038-15590-3-git-send-email-takahiro.akashi@linaro.org> <20141118140425.GM18842@arm.com> <546C58DB.5080204@linaro.org> <20141119190601.GM15985@arm.com> <546D7860.2010300@linaro.org> <20141120191738.GV19126@arm.com> <547432D2.7000909@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <547432D2.7000909@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 Tue, Nov 25, 2014 at 07:42:10AM +0000, AKASHI Takahiro wrote: > On 11/21/2014 04:17 AM, Will Deacon wrote: > > On Thu, Nov 20, 2014 at 05:13:04AM +0000, AKASHI Takahiro wrote: > >> On 11/20/2014 04:06 AM, Will Deacon wrote: > >>> On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote: > >>>> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly > >>>> replaced with -1 by a tracer, and, in this sense, it is *skipped*. > >>> > >>> Ok, but now userspace sees -ENOSYS for a skipped system call in that case, > >>> whereas it would usually see whatever the trace put in x0, right? > >> > >> If you don't really like this behavior, how about this patch instead of my [2/6] patch? > >> > >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > >> index 726b910..1ef57d0 100644 > >> --- a/arch/arm64/kernel/entry.S > >> +++ b/arch/arm64/kernel/entry.S > >> @@ -668,8 +668,15 @@ ENDPROC(el0_svc) > >> * switches, and waiting for our parent to respond. > >> */ > >> __sys_trace: > >> + cmp w8, #-1 // default errno for invalid > >> + b.ne 1f // system call > >> + mov x0, #-ENOSYS > >> + str x0, [sp, #S_X0] > >> +1: > >> mov x0, sp > >> bl syscall_trace_enter > >> + cmp w0, #-1 // skip the syscall? > >> + b.eq __sys_trace_return_skipped > >> adr lr, __sys_trace_return // return address > >> uxtw scno, w0 // syscall number (possibly new) > >> mov x1, sp // pointer to regs > >> @@ -684,6 +691,7 @@ __sys_trace: > >> > >> __sys_trace_return: > >> str x0, [sp] // save returned x0 > >> +__sys_trace_return_skipped: > >> mov x0, sp > >> bl syscall_trace_exit > >> b ret_to_user > >> > >> With this change, I believe, syscall(-1) returns -ENOSYS by default whether traced > >> or not, and still you can change a return value when tracing. > >> (But a drawback here is that a tracer will see -ENOSYS in x0 even at syscall entry > >> for syscall(-1).) > > > > But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't > > be treated any differently to syscall(42) with respect to restarting, > > exactly like x86. > > Can you elaborate a bit more as to "restarting?" Sorry, I meant skipping. There was another thread about syscall restarting at the same time I wrote that, so my mind was elsewhere! > We can't make any assumption about the number of arguments taken by *invalid* syscall(-1) > and so changing a value in x0 (or any other registers) doesn't make any difference. > () Ok, that's a fair point. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/