Messages in this thread | | | Date | Mon, 8 Nov 2010 20:18:13 +0100 | From | Oleg Nesterov <> | Subject | Re: Q: perf_event && task->ptrace_bps[] |
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On 11/08, Frederic Weisbecker wrote: > > On Mon, Nov 08, 2010 at 03:56:47PM +0100, Oleg Nesterov wrote: > > Hello. > > > > I am trying to understand the usage of hw-breakpoints in arch_ptrace(). > > ptrace_set_debugreg() and related code looks obviously racy. Nothing > > protects us against flush_ptrace_hw_breakpoint() called by the dying > > tracee. Afaics we can leak perf_event or use the already freed memory > > or both. > > > > Am I missed something? > > > > Looking into the git history, I don't even know which patch should be > > blamed (if I am right), there were too many changes. I noticed that > > 2ebd4ffb6d0cb877787b1e42be8485820158857e "perf events: Split out task > > search into helper" moved the PF_EXITING check from find_get_context(). > > This check coould help if sys_ptrace() races with SIGKILL, but it was > > racy anyway. > > > > It is not clear to me what should be done. Looking more, I do not > > understand the scope of perf_event/ctx at all, sys_perf_event_open() > > looks wrong too, see the next email I am going to send. > > > > Oleg. > > But I don't understand how ptrace_set_debugreg() and flush_old_exec() can > happen at the same time.
This can't happen. But I meant do_exit()->flush_ptrace_hw_breakpoint()
> The parent can only do the ptrace request when > the child is stopped, right?
Yes. But nothing can "pin" TASK_TRACED.
We know that a) the tracee was stopped() when sys_ptrace() was called and b) its task_struct can't go away. That is all. The tracee can be killed at any moment, and sys_ptrace() can race with with flush_ptrace_hw_breakpoint().
> I am certainly missing something obvious.
Perhaps ;) Or, it is quite possible I missed something, I never read this code before and it is certainly not trivial.
Oleg.
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