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