Re: [RFA/commit] arm-tdep.c: Do not single-step after hitting a watchpoint.

Luis Machado
2014-09-16T13:09:34Z

On 09/16/2014 09:48 AM, Joel Brobecker wrote:
>>> I think the experiments that were run showed that QEMU is in fact
>>> correct and should NOT be changed.
>>
>> Do we know what the Linux kernel's behavior on this one is? I wonder
>> what the stopped data address shows.
>>
>> Someone with access to a board with a relatively new kernel could
>> try that and rule it out, otherwise we risk fixing something for
>> QEMU/bare metal and breaking things for Linux.
>
> When I tested on GNU/Linux, watchpoints simply did not work
> (silently ignored, no signal). I was using an old kernel (2012),
> though; but that's all I had access to. But, all in all, baremetal
> should be our most reliable source of info, though,no? - no software
> layer to murky the waters.
>

It is hard to tell. ARM's documentation is not clear. For example, this
is probably where the stopped data address should be coming from:

--

WFAR - Watchpoint Fault Address Register

The WFAR is updated to indicate the address of the instruction that
accessed the watchpointed address:

- the address of the instruction + 8 in ARM state
- the address of the instruction + 4 in ThumbÂ® state

--

So it seems in line with what we are seeing? The program being trapped
two instructions after the data fault?

If it stops just a single instruction after the data fault, then someone
(probe, emulator or kernel) may be trying to help GDB by decrementing
the data fault address.

Luis