Messages in this thread | | | Date | Wed, 22 Aug 2012 15:48:37 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC 5/5 v2] uprobes: add global breakpoints |
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On 08/21, Sebastian Andrzej Siewior wrote: > > This patch adds the ability to hold the program once this point has been > passed and the user may attach to the program via ptrace.
Sorry Sebastian, I didn't even try to read the patch ;) Fortunately I am not maintainer, I can only reapeat that you do not need to convince me.
> Oleg: The change in ptrace_attach() is still as it was. I tried to > address Peter concern here. > Now what options do I have here: > - not putting the task in TASK_TRACED but simply halt. This would work > without a change to ptrace_attach() but the task continues on any > signal. So a signal friendly task would continue and not notice a > thing.
TASK_KILLABLE
> - putting the TASK_TRACED
This is simply wrong, in many ways.
For example, what if the probed task is already ptraced? Or debugger attaches via PTRACE_SEIZE? How can debugger know it is stopped? uprobe_wait_traced() goes to sleep in TASK_TRACED without notification. And it does not set ->exit_code, this means do_wait() won't work. And note ptrace_stop()->recalc_sigpending_tsk().
> @@ -76,6 +79,7 @@ struct uprobe_task { > > unsigned long xol_vaddr; > unsigned long vaddr; > + int skip_handler;
I am trying to guess what this skip_handler does...
> --- a/kernel/events/uprobes.c > +++ b/kernel/events/uprobes.c > @@ -1513,7 +1513,16 @@ static void handle_swbp(struct pt_regs *regs) > goto cleanup_ret; > } > utask->active_uprobe = uprobe; > - handler_chain(uprobe, regs); > + if (utask->skip_handler) > + utask->skip_handler = 0; > + else > + handler_chain(uprobe, regs); > + > + if (utask->state == UTASK_TRACE_WOKEUP_TRACED) { > + send_sig(SIGTRAP, current, 0); > + utask->skip_handler = 1; > + goto cleanup_ret; > + } > if (uprobe->flags & UPROBE_SKIP_SSTEP && can_skip_sstep(uprobe, regs)) > goto cleanup_ret; > > @@ -1528,7 +1537,7 @@ cleanup_ret: > utask->active_uprobe = NULL; > utask->state = UTASK_RUNNING; > } > - if (!(uprobe->flags & UPROBE_SKIP_SSTEP)) > + if (!(uprobe->flags & UPROBE_SKIP_SSTEP) || utask->skip_handler)
Am I understand correctly?
If it was woken by PTRACE_ATTACH we set utask->skip_handler = 1 and re-execute the instruction (yes, SIGTRAP, but this doesn't matter). When the task hits this bp again we skip handler_chain() because it was already reported.
Yes? If yes, I don't think this can work. Suppose that the task dequeues a signal before it returns to the usermode to re-execute and enters the signal handler which can hit another uprobe.
And this can race with uprobe_register() afaics.
Oleg.
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