Re: [RFC] initial support for watchpoints in frysk

[Phil Muldoon <pmuldoon@redhat.com>]

Thiago Jung Bauermann wrote:
> Hi,
>   

Hi Thiago,


> Since fhpd is complaining rather loudly about not supporting watchpoints
> in Power, I decided to add such support. :-)
>   
> I'd like to ask your opinion, especially regarding two
> methods I had to add to frysk.sys.Ptrace and
> frysk.proc.live.LinuxPtraceTask: setDebugReg and getDebugReg. They are
> necessary because on the PowerPC architecture, debug registers are a
> priviledged resource, and userland need to access them via
> PTRACE_SET_DEBUGREG and PTRACE_GET_DEBUGREG. What do you think, are
> these changes ok?
>   


The watchpoint specific code looks ok. I just added an alignment check 
yesterday to x86 and x8664, so if this is important to PPC then should 
add the check too.

I'm a bit worried about the live.LinuxPtraceTask setDebugReg and 
getDebugReg. This is a PPC specific function but it is in a non-ISA 
class. So I think it would lead to confusion. If you were to call this 
on x86 and x8664 what would happen?

Access to the x86 and x8664 debug status registers are performed as follows:

    return task.getRegister(X8664Registers.DEBUG_STATUS);

I think this call eventually resolves to 
RegisterBanks.getBankArrayRegister() which again resolves into ISA 
specific methods to get the register. Andrew who did the register code 
would know better here.

> Now regarding why the patch doesn't work: in order to verify if the
> process stopped because of the hardware watchpoint, I need to examine
> the siginfo structure which comes with the SIGTRAP. This information is
> not available to Frysk yet, so I'd like your opinion on the following
> approach:
>
>   

On x86/x8664 we can just look at the bits in the debug status register 
when a stop event/sigtrap has arrived to see if  it is a watchpoint 
event. If you look at the handleTrappedEvent in the Running and the 
Stepping states you can see there is a very specific sequence of checks 
here. I'm guessing this won't work on the PPC then as you need to 
examine the siginfo structure. What data are you looking for in there?


> If my understanding is correct, this would make the siginfo available to
> LinuxPtraceTaskState.Running.handleStoppedEvent, 

Should that be LinuxPtraceTaskState.Running.handleTrappedEvent and 
LinuxPtraceTaskState.Stepping.handleTrappedEvent? These two states right 
now are the only states that handle watchpoints. (ie sigtrap occurs, 
check if it is a watchpoint, if not, pass the sigtrap along).


Regards

Phil