Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751254AbaKTFNO (ORCPT ); Thu, 20 Nov 2014 00:13:14 -0500 Received: from mail-pd0-f173.google.com ([209.85.192.173]:63649 "EHLO mail-pd0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750752AbaKTFNN (ORCPT ); Thu, 20 Nov 2014 00:13:13 -0500 Message-ID: <546D7860.2010300@linaro.org> Date: Thu, 20 Nov 2014 14:13:04 +0900 From: AKASHI Takahiro User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Will Deacon 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 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> In-Reply-To: <20141119190601.GM15985@arm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/20/2014 04:06 AM, Will Deacon wrote: > On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote: >> On 11/18/2014 11:04 PM, Will Deacon wrote: >>> On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote: >>>> >>>> + if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) { >>>> + /* >>>> + * user-issued syscall(-1): >>>> + * RESTRICTION: We always return ENOSYS whatever value is >>>> + * stored in x0 (a return value) at this point. >>>> + * Normally, with ptrace off, syscall(-1) returns -ENOSYS. >>>> + * With ptrace on, however, if a tracer didn't pay any >>>> + * attention to user-issued syscall(-1) and just let it go >>>> + * without a hack here, it would return a value in x0 as in >>>> + * other system call cases. This means that this system call >>>> + * might succeed and see any bogus return value. >>>> + * This should be definitely avoided. >>>> + */ >>>> + regs->regs[0] = -ENOSYS; >>>> + } >>> >>> I'm still really uncomfortable with this, and it doesn't seem to match what >>> arch/arm/ does either. >> >> Yeah, I know but >> as I mentioned before, syscall(-1) will be signaled on arm, and so we don't >> have to care about a return value :) > > What does x86 do? On x86, syscall(-1) returns -ENOSYS if not traced, and we can change a return value if traced. >>> Doesn't it also prevent a tracer from skipping syscall(-1)? >> >> 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? Yes. 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).) -Takahiro AKASHI > 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/