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Date: Fri, 15 Jul 2011 20:41:59 +0300 From: Vladimir Zapolskiy <vzapolskiy@...il.com> To: Oleg Nesterov <oleg@...hat.com> Cc: Evgeniy Polyakov <zbr@...emap.net>, "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org Subject: Re: [PATCH] connector: add an event for monitoring process tracers Oleg, first of all thank you for a good review, please see my comments below. On Wed, Jul 13, 2011 at 6:09 PM, Oleg Nesterov <oleg@...hat.com> wrote: > On 07/12, Vladimir Zapolskiy wrote: >> >> Note, a detach signal is not emitted, if a tracer process terminates >> without explicit PTRACE_DETACH request. Such cases can be covered >> listening to PROC_EVENT_EXIT connector events. > > Hmm. More and more reasons to make the implicit detach sleepable... > > But. There is another case. The (dead) tracee can be detached via > do_wait(). Perhaps this falls into "covered listening to EXIT" too, > but imho makes sense to document in the changelog. Oh, and probably > we will add the ability to detach a zombie... > > I don't really understand why do you need this, but I won't argue. > I found that implicit ptrace detach codepath is quite mutable and vast, and I don't want to interfere in that changes without knowing even basic pitfalls. Somehow the sending a connector signal on explicit detach is quite sufficient at least for the most of the proc connector usecases I can imagine, because hopefully almost(?) all implicit detach cases are related to tracer or tracee thread completion, and that is supposed to be reported to userspace via do_exit()/proc_exit_connector() path. > As for the patch, > >> +void proc_ptrace_connector(struct task_struct *task) >> +{ >> + struct cn_msg *msg; >> + struct proc_event *ev; >> + struct timespec ts; >> + __u8 buffer[CN_PROC_MSG_SIZE]; >> + struct task_struct *tracer; >> + >> + if (atomic_read(&proc_event_num_listeners) < 1) >> + return; >> + >> + msg = (struct cn_msg *)buffer; >> + ev = (struct proc_event *)msg->data; >> + get_seq(&msg->seq, &ev->cpu); >> + ktime_get_ts(&ts); /* get high res monotonic timestamp */ >> + put_unaligned(timespec_to_ns(&ts), (__u64 *)&ev->timestamp_ns); >> + ev->what = PROC_EVENT_PTRACE; >> + ev->event_data.ptrace.process_pid = task->pid; >> + ev->event_data.ptrace.process_tgid = task->tgid; >> + >> + rcu_read_lock(); >> + tracer = tracehook_tracer_task(task); >> + if (tracer) { >> + ev->event_data.ptrace.tracer_pid = tracer->pid; >> + ev->event_data.ptrace.tracer_tgid = tracer->tgid; >> + } else { >> + ev->event_data.ptrace.tracer_pid = 0; >> + ev->event_data.ptrace.tracer_tgid = 0; >> + } > > This doesn't look right. The code uses tracehook_tracer_task() to > figure out whether this task traced or not. But this is racy. > > ptrace_attach: > > ...attach... > > /* WINDOW */ > > proc_ptrace_connector(task); > > The task can exit in between, and the caller's subthread can do > wait4() and release it. In this case proc_ptrace_connector() will > see tracehook_tracer_task() == NULL and report "detach". > > The similar race in ptrace_detach() path. Another tracer can attach > to this task before we proc_ptrace_connector(). > > I think proc_ptrace_connector() needs the explicit "task_struct *tracer" > argument, NULL if ptrace_detach(). Or a simple boolean, the tracer is > current. > > If you think this is fine - I won't argue. > Fixed in the second version of the change, thanks. > > > But in any case, please rediff against > > git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc.git ptrace > > tracehook_tracer_task() was removed, and > >> @@ -260,6 +261,9 @@ out: >> if (wait_trap) >> wait_event(current->signal->wait_chldexit, >> !(task->group_stop & GROUP_STOP_TRAPPING)); >> + if (!retval) >> + proc_ptrace_connector(task); >> + >> return retval; >> } > > this chunk probably should be updated. Rebased, thanks a lot. Vladimir -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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