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    SubjectRe: Is PTRACE_ATTACH lazy?
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    Victor Zandy <zandy@cs.wisc.edu> writes:
    > For example, I find that if I make two calls like this
    >
    > ret = ptrace(PTRACE_ATTACH, pid, 0, 0);
    > ret = ptrace(PTRACE_DETACH, pid, 0, 0);
    >
    > the second call will fail, with ESRCH.
    ...
    > My guess is that the traced process has not had its state set to
    > TASK_STOPPED before the second call to ptrace is executed. (Does the
    > traced process need to be scheduled before it will change to
    > TASK_STOPPED?)
    >
    > Does anyone know for sure why this happens?

    Your guess is about right. A SIGSTOP is sent during the PTRACE_ATTACH

    send_sig(SIGSTOP, child, 1);

    and will complete (shortly) thereafter.


    > Can anyone suggest a workaround that to allow me make one or
    > more ptrace calls immediately after a PTRACE_ATTACH?

    The parent has to do a wait4 to wait for the child to stop. (You could also
    observe the stop by watching /proc/n/status, etc, but this isn't really
    practical.)

    --Mike

    --
    Any sufficiently adverse technology is indistinguishable from Microsoft.

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