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    SubjectRe: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x)
    On Wed,  6 Jan 2010 13:08:12 -0800 (PST)
    Roland McGrath <> wrote:

    > > Oh, I am not sure. But I don't understand TIF_SINGLE_STEP on s390,
    > > absolutely.
    > >
    > > For example, why do_signal() sets TIF_SINGLE_STEP? Why can't we do
    > I think we could. That would be more consistent with other machines. On
    > s390, once we set TIF_SINGLE_STEP, we are going to post a SIGTRAP
    > eventually before going to user mode. But then tracehook_signal_handler()
    > also gets stepping=1 and the expected meaning of this is that the arch code
    > is not itself simulating a single-step for the handler setup. So the
    > tracehook (i.e. ptrace/utrace) code does what it does for "need a fake
    > single-step".

    Hmm, command for tracehook_signal_handler say this for stepping:
    @stepping: nonzero if debugger single-step or block-step in

    > In ptrace (including utrace-based ptrace), this winds up with sending a
    > SIGTRAP. So when we finally do get out of do_signal and TIF_SINGLE_STEP
    > causes a second SIGTRAP, it's already pending and the second one makes no
    > difference.

    So we have been lucky so far.

    > But for the general case of utrace, we'll have the UTRACE_SIGNAL_HANDLER
    > report, followed by a SIGTRAP that appears to be an authentic single-step
    > trap, but takes place on the same instruction. If the resumption after the
    > UTRACE_SIGNAL_HANDLER report didn't use stepping, then this is an entirely
    > unexpected extra SIGTRAP. If we do continue stepping, then we are
    > expecting the SIGTRAP, but this gets us a spurious and errnoeous report
    > that looks like the instruction right before the handler's entry point in
    > memory was just executed.
    > [Martin:]
    > > The reason why we set the TIF_SINGLE_STEP bit in do_signal is that we
    > > want to be able to stop the debugged program before the first
    > > instruction of the signal handler has been executed. The PER single
    > > step causes a trap after an instruction has been executed. That first
    > > instruction can do bad things to the arguments of the signal handler..
    > That's what tracehook_signal_handler is for. You're both doing it yourself
    > in the arch code (by setting TIF_SINGLE_STEP), and then telling the generic
    > code to do it (by passing stepping=1 to tracehook_signal_handler).

    Ok, so with the full utrace the semantics of tracehook_signal_handler
    is more than just causing a SIGTRAP. It is an indication for a signal
    AND a SIGTRAP if single-stepping is active. To make both cases work we
    should stop setting TIF_SINGLE_STEP in do_signal and pass
    current->thread.per_info.single_step to tracehook_signal_handler
    instead of test_thread_flag(TIF_SINGLE_STEP).

    blue skies,

    "Reality continues to ruin my life." - Calvin.

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