From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 556 invoked by alias); 19 Mar 2014 16:41:34 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 547 invoked by uid 89); 19 Mar 2014 16:41:33 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.8 required=5.0 tests=AWL,BAYES_00,SPF_HELO_PASS,SPF_PASS,T_RP_MATCHES_RCVD autolearn=ham version=3.3.2 X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Wed, 19 Mar 2014 16:41:30 +0000 Received: from int-mx01.intmail.prod.int.phx2.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id s2JGfOSd019714 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK); Wed, 19 Mar 2014 12:41:25 -0400 Received: from [127.0.0.1] (ovpn01.gateway.prod.ext.ams2.redhat.com [10.39.146.11]) by int-mx01.intmail.prod.int.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id s2JGfMVF029548; Wed, 19 Mar 2014 12:41:23 -0400 Message-ID: <5329C8B2.4040409@redhat.com> Date: Wed, 19 Mar 2014 16:41:00 -0000 From: Pedro Alves User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Eli Zaretskii CC: brobecker@adacore.com, gdb-patches@sourceware.org Subject: Re: [PATCH] Fix "PC register is not available" issue References: <83txawa9wk.fsf@gnu.org> <20140318161608.GD4282@adacore.com> <83pplja2h9.fsf@gnu.org> <20140318165413.GE4282@adacore.com> <83k3bra0rx.fsf@gnu.org> <5328835C.4010908@redhat.com> <83ioraam9m.fsf@gnu.org> <53296C3B.4040507@redhat.com> <83a9cm9mwr.fsf@gnu.org> In-Reply-To: <83a9cm9mwr.fsf@gnu.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-SW-Source: 2014-03/txt/msg00462.txt.bz2 On 03/19/2014 04:24 PM, Eli Zaretskii wrote: >> Date: Wed, 19 Mar 2014 10:06:51 +0000 >> From: Pedro Alves >> CC: brobecker@adacore.com, gdb-patches@sourceware.org >> >> On 03/19/2014 03:40 AM, Eli Zaretskii wrote: >>>> Date: Tue, 18 Mar 2014 17:33:16 +0000 >>>> From: Pedro Alves >>>> CC: Joel Brobecker , gdb-patches@sourceware.org >>>> >>>> I see that the GetThreadContext call (do_windows_fetch_inferior_registers) >>>> doesn't check for errors (I think it should (*)). It'd be interesting to know whether gdb can >>>> actually read the registers off of this thread >>> >>> How to see those registers? >> >> Just "info registers" ? > > That's what I thought, but ... > >> If we can't even read registers off of it, and GetThreadContext >> is failing, it means after your patch we'll be showing bogus >> register contents for these threads. > > ...how do you tell bogus register contents from correct contents? > It's not like I know which register should have what value at any > given time, do I? The point is that GDB ignores GetThreadContext errors, and so if indeed GetThreadContext fails, GDB happily proceeds decoding a bogus th->context. I mean, we should do this in do_windows_fetch_inferior_registers: - GetThreadContext (th->h, &th->context); + CHECK (GetThreadContext (th->h, &th->context)); So that GetThreadContext fails, we at least see a warning. I assume that if GetThreadContext does not fail, then the register contents are correct. > >> But I think GetThreadContext will indeed succeed for these threads. > > Well, at least MSDN begs to differ: > > You cannot get a valid context for a running thread. Use the > SuspendThread function to suspend the thread before calling > GetThreadContext. I mean it'll succeed because we only ever read registers when threads are stopped for debug event. I don't mean to imply that those threads are special WRT to GetThreadContext. It's not valid to get a context for a _running_ thread. But after a debug event, no thread is running at all. The OS already stopped threads for us. > >> AFAIK, we don't really need the SuspendThread calls when handling >> a debug event, given that when WaitForDebugEvent returns a >> stop event, all threads have already been stopped by the OS for us. > > Yes, AFAIK that's true. Alright, we were talking past each other then. I did a little websearch, and I found evidence of other debuggers also not using SuspendThread after events: http://www.ollydbg.de/Help/t_run.htm "indebugevent Application is paused on debug event, therefore Suspendallthreads() does not need to call SuspendThread()" > >> We really only need to SuspendThread threads when we might want >> to leave most threads paused on the next resume, for e.g., when >> stepping over a breakpoint. The suspend count handling in >> windows-nat.c is quite messy, and looking at the code, it doesn't >> look like we actually get that right, given we only SuspendThread >> threads if we try to read their registers, and so if nothing reads >> registers off all threads when e.g., handling a breakpoint that >> we decide needs to be stepped over (which we don't), then we end >> up resuming threads we shouldn't. > > That's assuming that stepping resumes threads. I'm not sure, but I > really don't know enough about debugging APIs on Windows. There's no special step request in the debug API. The way to set a thread stepping is to enable the trace flag in eflags: if (step) { /* Single step by setting t bit. */ struct regcache *regcache = get_current_regcache (); struct gdbarch *gdbarch = get_regcache_arch (regcache); windows_fetch_inferior_registers (ops, regcache, gdbarch_ps_regnum (gdbarch)); th->context.EFlags |= FLAG_TRACE_BIT; } >> It'll likely show us the thread is stopped at some ntdll.dll function >> or some such, and from the function name we will likely >> be able to infer what/which thread is this, like, e.g., whether >> it's a thread injected with DebugBreakProcess or some such >> (internally by one of the system dlls or the dlls your app >> links with). > > I'll see what I can find about that, but I doubt you'd see something > telltale in the backtrace. (The thread started by Windows for > debugging is not part of this issue; I never saw the threads that are > to have any debug-related functions on their callstacks.) Thanks! -- Pedro Alves