Messages in this thread | | | From | Roland McGrath <> | Subject | Re: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x) | Date | Wed, 6 Jan 2010 13:08:12 -0800 (PST) |
| |
> 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".
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.
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).
Thanks, Roland
| |