Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 24 Mar 2003 09:54:54 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 24 Mar 2003 09:54:54 -0500 Received: from crack.them.org ([65.125.64.184]:11501 "EHLO crack.them.org") by vger.kernel.org with ESMTP id ; Mon, 24 Mar 2003 09:54:50 -0500 Date: Mon, 24 Mar 2003 10:05:53 -0500 From: Daniel Jacobowitz To: raj Cc: linux-kernel@vger.kernel.org, zandy@cs.wisc.edu Subject: Re: [PATCH] ptrace on stopped processes (2.4) Message-ID: <20030324150552.GA26287@nevyn.them.org> Mail-Followup-To: raj , linux-kernel@vger.kernel.org, zandy@cs.wisc.edu References: <1047936295.3e763d273307c@www-auth.cs.wisc.edu> <20030324040908.GA19754@nevyn.them.org> <3E7EA4B2.5010306@cs.wisc.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3E7EA4B2.5010306@cs.wisc.edu> User-Agent: Mutt/1.5.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4116 Lines: 95 On Sun, Mar 23, 2003 at 10:24:50PM -0800, raj wrote: > Daniel Jacobowitz wrote: > > >On Mon, Mar 17, 2003 at 03:24:55PM -0600, Rajesh Rajamani wrote: > > > > > >>Hi All, > >>I'm working on adding a function > >> > >>void debugbreak(void); > >> > >>to glibc. I got the inspiration for this from Windows. Windows has a > >>DebugBreak() function, which when invoked from a process, spawns a > >>debugger, which then attaches to the process. I think this would be > >>invaluable to linux developers in situations where they currently have to > >>put an infinite loop and attach to a running process (say, to stop in a > >>library that has been LD_PRELOADed). > >> > >>To this end, I've been experimenting and found out that gdb can't attach > >>to a process that has been stopped. I'd like to send a SIGSTOP as soon as > >>debugbreak() is invoked, so that all threads are stopped in a state close > >>to the one they were in, when the debugbreak() was invoked. > >> > >>I spoke to Vic Zandy about this and he informed me that he had submitted > >>a patch > >>that would allow ptrace to attach to stopped processes also (the thread of > >>discussion is pasted below). I believe the patch was not accepted at > >>that time. > >> I was wondering what the official line on this is? If there are no > >> serious > >>objections, will the community consider accepting the patch? It would go > >>a long > >>way in helping me accomplish my goal. > >> > >> > > > >The question is, what _should_ happen when yu attach to a stopped > >process? If the tracer receives the same one SIGSTOP that it normally > >would, then it will just resume the program as if it weren't stopped. > >Does that make sense or not? > > > Ideally, a stopped process should not be resumed, until it receives a > SIGCONT. I ran two tests to understand the effect of this patch. > > 1. Attaching to a stopped process using gdb. This did not cause the > tracee to continue. No, that's not what I meant. When you attach using GDB, there is no way for GDB to determine if the process was previously stopped or running. > 2. stracing a stopped process. This caused the stopped process to > continue. On further investigation, I found out that strace calls > ptrace(PTRACE_SYSCALL), which restarts the child, but arranges for the > child to be stopped at the next entry to or exit from a system call. > > IMHO, the new ptrace semantics adds a feature to strace. If someone > tries to run strace a stopped process without the patch, it will just > hang, whereas with the patch, it'll let the process continue. Even > without the patch, if a process that sends itself a SIGCONT is straced, > the process will not stop. I do not think this is an issue, considering > that ppl will not get any info by running strace on a stopped process, > until they send a SIGCONT. > > In conclusion, the action taken by the tracer depends on the mechanism > it uses for monitoring the system calls. That said, it'll definitely be > useful if the tracer could find out the state of the tracee at the time > of attachment and take appropriate action. Given below, is a port of > Vic's patch to 2.5.xx series - > -------------------------------------------------------------------------------- > -- kernel/ptrace.c 2003-03-23 22:15:33.000000000 -0800 > +++ kernel/ptrace.c 2003-03-23 22:19:27.000000000 -0800 > @@ -115,7 +115,10 @@ > __ptrace_link(task, current); > write_unlock_irq(&tasklist_lock); > > - force_sig_specific(SIGSTOP, task); > + if (task->state != TASK_STOPPED) > + force_sig_specific(SIGSTOP, task); > + else > + task->exit_code = SIGSTOP; > return 0; > > bad: I'll test this here, give me a day or two. -- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer - 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/