Re: Questionable breakpoint stepping code

Vladimir Prus
2007-11-26T19:16:43Z

On Monday 26 November 2007 21:49:27 Michael Snyder wrote:
> On Fri, 2007-11-23 at 16:56 +0300, Vladimir Prus wrote:
> > The infrun.c:handle_inferiour_event function has
> > this code block:
> >
> > if (thread_hop_needed)
> > {
> > ........
> > remove_status = remove_breakpoints ();
> > /* Did we fail to remove breakpoints? If so, try
> > to set the PC past the bp. (There's at least
> > one situation in which we can fail to remove
> > the bp's: On HP-UX's that use ttrace, we can't
> > change the address space of a vforking child
> > process until the child exits (well, okay, not
> > then either :-) or execs. */
> > if (remove_status != 0)
> > {
> > /* FIXME! This is obviously non-portable! */
> > write_pc_pid (stop_pc + 4, ecs->ptid);
> > /* We need to restart all the threads now,
> > * unles we're running in scheduler-locked mode.
> > * Use currently_stepping to determine whether to
> > * step or continue.
> > */
> > /* FIXME MVS: is there any reason not to call resume()? */
> > if (scheduler_mode == schedlock_on)
> > target_resume (ecs->ptid,
> > currently_stepping (ecs), TARGET_SIGNAL_0);
> > else
> > target_resume (RESUME_ALL,
> > currently_stepping (ecs), TARGET_SIGNAL_0);
> > prepare_to_wait (ecs);
> > return;
> > }
> >
> > The code is a bit scary -- specifically I sure don't want GDB to mess
> > with PC values like this on x86, if removing breakpoints fails in any way.
> > The essential bits of this code are present as of revision 1.1 of infrun.c
> > (added in 1999).
> >
> > So:
> > 1. Anybody knows if this code is still needed for modern HPUX?
> > 2. Can we have it wrapped in #ifdef, and if so, which one?
> >
> > - Volodya
>
> Hi Volodya,
>
> I think it's my code. It's not really related specifically
> to HPUX, that comment was there in the previous iteration and
> I just kept it.
>
> The several state variables with "thread_hop" as part of their
> names are related to single-stepping in the presence of thread-
> specific breakpoints. They are meant to solve the problem of
> what to do if you are doing a step, and you hit a thread-specific
> breakpoint, but with the wrong thread.

Yes, I know that.

>
> You need to do a kind of special single-step to get past that
> particular breakpoint, then return to the single-stepping
> infrun state.

Right.

> As for the scheduler-locking code, that pertains to a
> different but not wholly unrelated functionality (set
> scheduler-locking), which affects which threads can run
> at which times.

I know that too.

>
> As for your last question, no, I don't believe we approve
> of ifdefs...

Well, the question, then is -- how do we make this code work
correctly even if the instruction at PC is not 4 bytes in size?
Calling disassembler seems a plausible approach.

- Volodya