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SubjectRe: Is PTRACE_ATTACH lazy?
Victor Zandy <> 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
> 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


Any sufficiently adverse technology is indistinguishable from Microsoft.

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