Re: Q: perf_event && task->ptrace_bps[]

[Frederic Weisbecker]

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. The parent can only do the ptrace request when
the child is stopped, right? But it can't be stopped in flush_old_exec()...?

Not sure how any race can happen here. I am certainly missing something obvious.

Thanks.

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