Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758020AbaKTTRm (ORCPT ); Thu, 20 Nov 2014 14:17:42 -0500 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:35659 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756755AbaKTTRl (ORCPT ); Thu, 20 Nov 2014 14:17:41 -0500 Date: Thu, 20 Nov 2014 19:17:38 +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: <20141120191738.GV19126@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <546D7860.2010300@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 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. 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/