Re: [PATCH, v3] Expect SI_KERNEL or TRAP_BRKPT si_code values for MIPS breakpoint traps

[Pedro Alves <palves@redhat.com>]

On 02/23/2016 02:00 AM, Luis Machado wrote:
> On 02/22/2016 07:41 PM, Pedro Alves wrote:
>> On 02/22/2016 10:20 PM, Luis Machado wrote:
>>
>>> diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
>>> index ba58717..72b32b1 100644
>>> --- a/gdb/nat/linux-ptrace.h
>>> +++ b/gdb/nat/linux-ptrace.h
>>> @@ -140,11 +140,16 @@ struct buffer;
>>>      in SPU code on a Cell/B.E.  However, SI_KERNEL is never seen
>>>      on a SIGTRAP for any other reason.
>>>
>>> +   The MIPS kernel uses the default si_code of SI_KERNEL for software
>>> +   breakpoints, hardware watchpoints and SIGTRAP's in general.
>>
>> If we get this for both software breakpoints and hardware watchpoints,
>> then it seems to me that this change still leaves watchpoints broken,
>> as I can't see how check_stopped_by_watchpoint is reached, in either
>> gdb/linux-nat.c or gdbserver/linux-low.c.
> 
> With or without this specific breakpoint fix, hardware watchpoints are 
> already broken for MIPS AFAICS.

Yep, hence my "still".  ;-)

> 
> MIPS' kernel has never set si_code to anything other than SI_KERNEL. 
> When the change to expect TRAP_HWBKPT was committed, then hardware 
> watchpoints stopped working properly for MIPS.
> 
> Now all hardware watchpoints produce are breakpoint-like traps that get 
> silently ignored by GDB as "delayed software breakpoint trap". Neither 
> GDB nor GDBserver can tell when a hardware watchpoint really happened by 
> using si_code information.
> 
> But this seems to be a different issue and i think it should be handle 
> separately.

I think it's all the same, and that we'll need to change
linux-nat.c:save_sigtrap and the equivalent bits in gdbserver/linux-low.c.

So if we pick the table I added for x86, and fill it in
for MIPS, this is what we get:

   | what                                     | si_code        |
   |------------------------------------------+----------------|
   | software breakpoints                     | SI_KERNEL      |
   | single-steps                             | N/A            |
   | single-stepping a syscall                | N/A            |
   | user sent SIGTRAP                        | 0              |
   | exec SIGTRAP (when no PTRACE_EVENT_EXEC) | doesn't matter |
   | hardware breakpoints                     | N/A            |
   | hardware watchpoints                     | SI_KERNEL      |

So the only distinction we need to make is between software
breakpoints SI_KERNEL, and hardware watchpoints SI_KERNEL.

And we can accurately do that with
mips_linux_stopped_by_watchpoint / mips_stopped_by_watchpoint.

Is my understanding correct?

Thanks,
Pedro Alves