From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752211AbaGaKr0 (ORCPT ); Thu, 31 Jul 2014 06:47:26 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:56849 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751911AbaGaKrY (ORCPT ); Thu, 31 Jul 2014 06:47:24 -0400 Date: Thu, 31 Jul 2014 11:46:53 +0100 From: Will Deacon To: Omar Sandoval Cc: "linux-arm-kernel@lists.infradead.org" , "linux@arm.linux.org.uk" , Catalin Marinas , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] ARM/ARM64: don't enter kgdb when userspace executes a kgdb break instruction. Message-ID: <20140731104653.GM26853@arm.com> References: <20140730071245.GA8954@mew.web-pass.com> <20140730112414.GJ12239@arm.com> <20140731053323.GA15523@mew.web-pass.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140731053323.GA15523@mew.web-pass.com> 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, Jul 31, 2014 at 06:33:23AM +0100, Omar Sandoval wrote: > Hi, Hi Omar, > On Wed, Jul 30, 2014 at 12:24:14PM +0100, Will Deacon wrote: > > Whilst this sounds like a worrying problem, I've failed to reproduce it > > on arm64. Executing a brk instruction with either KGDB_DYN_DGB_BRK_IMM or > > KDBG_COMPILED_DBG_BRK_IMM immediates from userspace results in a SIGTRAP being > > delivered, assumedly because kgdb_handle_exception simply returns when kgdb > > isn't active. > > From what I can tell, the break hooks are registered so long as kgdb is enabled > at all - i.e., the kernel was compiled with CONFIG_KGDB=y and, for example, > CONFIG_KGDB_SERIAL_CONSOLE=y and kgdboc was passed on the kernel command line. > kgdb_handle_exception doesn't seem to check whether the debugger is active. > > > The following (totally untested) diff is simpler for arm64, but again, I'm > > not sure we even have a problem here. > > This diff also fixes the problem. I don't have a strong preference for either > approach, so I can revise the patch with this approach instead if you'd prefer > that. I'll merge the arm64 diff I proposed. Could you repost the ARM part please? > > On which systems have you managed to reproduce this and how? > > I first reproduced this on a Raspberry Pi. The recommended distro, Raspbian, > distributes a kernel compiled with CONFIG_KGDB=y, CONFIG_KGDB_KDB=y, and > CONFIG_KDB_KEYBOARD=y, so it was sufficient to have a keyboard plugged in. > However, I also reproduced it by booting with kgdboc on the command line, as > CONFIG_KGDB_SERIAL_CONSOLE was also enabled. Additionally, I reproduced it and > then verified that my patch fixed it on self-compiled kernels. I think enabling and activating kgdb by default is a pretty crazy thing to do, but I agree that we shouldn't allow userspace to trap into it either. Once you repost the ARM patches, we can look at getting them merged via rmk. Cheers, Will