Re: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x)

[Oleg Nesterov <oleg@redhat.com>]

On 01/06, 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".
>
> 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.

Confused again, perhaps I just misunderstood what you mean...

Without utrace, tracehook_signal_handler() doesn't send SIGTRAP, it
merely does ptrace_notify(SIGTRAP), this means that

> 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.

even without utrace we can have unexpected SIGTRAP.

Oleg.