Re: [PATCH] Fix read and access watchpoint can't stop

[Hev <r@hev.cc>]

Hello,

On Thu, Jun 18, 2020 at 9:35 PM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Hev <r@hev.cc>
> > Date: Thu, 18 Jun 2020 10:05:14 +0800
> > Cc: gdb-patches@sourceware.org
> >
> > > >       /* We were stopped by a hardware watchpoint, but the target could
> > > >          not report the data address.  We must check the watchpoint's
> > > >          value.  Access and read watchpoints are out of luck; without
> > >
> > > The comment below the line you suggest to modify explains why we don't
> > > handle access and read watchpoints here.  So I don't understand the
> > > rationale for this change (it might be a good idea to explain it in
> > > more detail in the log message).  What am I missing?
> > >
> > > Thanks.
> >
> > This problem only occurs on the target that target_stopped_data_address() returns false, and
> > b->watchpoint_triggered is set to  watch_triggered_unknown when watchpoint is triggered. so, In
> > bpstat_check_watchpoint(), the last branch needs to be hit to enable check value:
> >
> >  4961       int must_check_value = 0;
> >  4962
> >  4963       if (b->type == bp_watchpoint)
> >  4964     /* For a software watchpoint, we must always check the
> >  4965        watched value.  */
> >  4966     must_check_value = 1;
> >  4967       else if (b->watchpoint_triggered == watch_triggered_yes)
> >  4968     /* We have a hardware watchpoint (read, write, or access)
> >  4969        and the target earlier reported an address watched by
> >  4970        this watchpoint.  */
> >  4971     must_check_value = 1;
> >  4972       else if (b->watchpoint_triggered == watch_triggered_unknown
> >  4973            && b->type == bp_hardware_watchpoint)
> >  4974     /* We were stopped by a hardware watchpoint, but the target could
> >  4975        not report the data address.  We must check the watchpoint's
> >  4976        value.  Access and read watchpoints are out of luck; without
> >  4977        a data address, we can't figure it out.  */
> >  4978     must_check_value = 1;
> >
> > However, the b->type is the actual watchpoint type, bp_hardware_watchpoint for write-only,
> > bp_read_watchpoint for read-only, and bp_access_watchpoint for read-write, so it will not hit when read-only
> > and read-write watching.
> >
> > On mips we don't know the low order 3 bits of the data address, so we must return false in
> > stopped_data_address(). This is why we have not found the problem on other platforms (x86/Aarch64/...).
>
> I understand, but how will this work on MIPS, given that
> is_hardware_watchpoint allows any type of watchpoints?  How will GDB
> know which watchpoint triggered if there's no address?
>
> And what is the value of bs->breakpoint_at, and how is it different
> from b->type?
>
> Thanks.

I looked at the code again. Although this changes can keep bs->stop
still equal 0 to make works, But it is not a right way.

Thanks for review.

-- 
Best regards!
Hev
https://hev.cc