Messages in this thread | | | Date | Thu, 7 Jan 2010 10:16:19 +0100 | From | Martin Schwidefsky <> | Subject | Re: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x) |
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On Wed, 6 Jan 2010 13:08:12 -0800 (PST) Roland McGrath <roland@redhat.com> 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 use
> 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, Martin.
"Reality continues to ruin my life." - Calvin.
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