Re: [rfc] Strip Thumb bit from PC returned by arm_get_longjmp_target

[Matthew Gretton-Dann <matthew.gretton-dann@arm.com>]

On Thu, 2010-08-19 at 10:49 +0200, Ulrich Weigand wrote:
> > Matthew Gretton-Dann wrote:
> > 
> > > The problem with the patch is it removes what may be the only way we
> > > have of telling the instruction state of the longjmp target.  If you
> > > have debugging information (mapping symbols at the very least)
> > > everything is fine, but if you don't then how do you tell what the state
> > > is?  arm_pc_is_thumb does use this bit to detect the instruction state
> > > (and arm_breakpoint_from_pc then uses this result to determine the
> > > breakpoint type).
> > 
> > Ah, I see.  I was confused about just where the Thumb bit was supposed
> > to be present and where not, sorry ...
> > 
> > > In the case above I think the correct fix is to make
> > > arm_adjust_breakpoint_address not strip out the address bits (which it
> > > does when trying to work out whether we are single stepping through an
> > > IT block).
> > 
> > Does the patch below seem reasonable to you?
> 
> Actually, it turns out this new patch doesn't work.  It leads to:
> 
> Breakpoint 4 at 0x84ec: file ../../../gdb-head/gdb/testsuite/gdb.threads/threxit-hop-specific.c, line 47.^M
> (gdb) next^M
> ^M
> Program received signal SIGTRAP, Trace/breakpoint trap.^M
> 
> This happens because GDB now no longer recognizes the PC address
> when the breakpoint trap arrives.  The PC is compared against
> the breakpoint location's loc->address value -- which now has the
> Thumb bit set, but the PC doesn't.
> 
> Note that while the Thumb bit gets removes in arm_breakpoint_from_pc,
> this affects only loc->placed_address, not loc->address.
> 
> This seems to indicate that in fact, breakpoint addresses must
> *not* have the Thumb bit set ...
> 
> Do you have a case where this works for you?

I don't have a case.  I think this means that the original patch is the
correct one for the moment - but I'm not a maintainer so this isn't an
approval.  

Also, looking at the code in arm_adjust_breakpoint_address I think there
are other cases that may cause the failure you are seeing above (for
instance when using Thumb-2 in the absence of mapping symbols and
setting the breakpoint on 0x8001).

We still need to come up with a way to be able to better pass the
instruction set state around with the PC (I'll respond to your earlier
email with some of my thoughts).

Thanks,

Matt

-- 
Matthew Gretton-Dann
Principal Engineer - PDSW Tools
ARM Ltd