Re: [Bochs-developers] Fetch Operands

[César <div...@gm...>]

Hi,

thank you, your comments were very helpful.


César.

2012/1/24 Stanislav <stl...@gm...>:
> Hi,
>
> Of course you cannot use functions like read_RMW_virtual_byte because of two
> reasons:
>        a) you don’t understand what the function does
>             this particular function does checks store access permission
> and call for OS exception if store permission check fail
>                     I guess checking for store permission for load
> instruction is not very good idea :)
>        b) In general you won't like to affect guest code with your
> instrumentation, definition not cause guest code exceptions.
>             If you want to read guest memory on physical address you have
> dbg_ function to do it. One to read memory and another one to translate
> linear address to physical.
>                    Look on the debugger code - it has plenty of examples.
>
> For having smth like - just in case I am working on this :)
> But don't expect any working prototype in next month or two - it will take a
> lot of time.
>
> Stanislav
>
> P.S> BTW in your instrumentation code you have a problem.
> You read ins->b1() which is meaningless. Real opcode could 2-byte or even
> 3-byte but b1() is truncated to 8 bit only.
> So opcode 0x10 and 2-byte opcode 0x0f 0x10 will look the same for you (same
> b1()).
> You should use i->getIaOpcode() instead. So in your case it will be like:
>
> switch (ins->getIaOpcode ()) {
>        case BX_IA_CMP_EbIb:
>            printCmp(ins, 1, AL, ins->Ib());
>            break;
>        case BX_IA_CMP_EwIw:  // cmp ax, imm16
>             printCmp(ins, 2, EAX, ins->Id());
>             break;
>       case BX_IA_CMP_EdId:  // cmp eax,
>             printCmp(ins, 2, EAX, ins->Id());
>             break;
>  }
>
> And etc. For every uniq opcode there is uniq enum in getIaOpcode()
>
> -----Original Message-----
> From: César [mailto:div...@gm...]
> Sent: Tuesday, January 24, 2012 11:55 PM
> To: Stanislav
> Cc: boc...@li...
> Subject: Re: [Bochs-developers] Fetch Operands
>
> Hi, thank you Stanislav.
>
> I mean the actual value used to compute the outcome of the instruction,
>
> I know how the opcode/operands are encoded in the instruction and, indeed, I
> did some work to 'parse' its prefix/opcode/modRm/* fields and check if the
> current instruction is relevant to the experiment.
> However the modR/m part is a bit trickier and I tried to use the same
> function that bochs does (that pointed by i->ResolveModrm), but apparently
> when the following line execute it mess something in the O.S memory and it
> doesn't boot:
>
> BX_CPU_C::read_RMW_virtual_dword(ins->seg(), eaddr);
>
> I think would be great to have is something like:
>
> bxInstruction::howManyOperands();
> bxInstruction::getOperand( Bit8u );
>
> An part of my previous instrumentation code is attached.
>
>
> César.
>
> 2012/1/24 Stanislav <stl...@gm...>:
>> What do you mean the value of first operand ?
>>
>> If you have instruction like:
>>
>>        cmp ecx, [ebx+esp+2]
>>
>> what is going to be a first operand ? BX_32BIT_REG_ECX (equal to 1) ?
>> or actual value of ecx register ?
>> what is going to be a second operand ? Here it is a memory location,
>> so the second operand is not very well defined.
>>
>> Anyway, for each and every instruction operands defined differently,
>> some reg from nnn() and write to rm(), most of the instruction do vise
> versa.
>> Some instructions access memory, some not, some instructions access
>> even multiple memory locations.
>>
>> Stanislav
>>
>> -----Original Message-----
>> From: César [mailto:div...@gm...]
>> Sent: Tuesday, January 24, 2012 7:40 PM
>> To: boc...@li...
>> Subject: [Bochs-developers] Fetch Operands
>>
>> Hi,
>>
>> What is the easiest way to get an instruction operand data? For
>> example, lets say the current instruction is a CMP r/m32, r32, how
>> obtain the value of the first operand (the r/m32 part) ?
>>
>> I tried to use:
>>
>> bx_address eaddr = BX_CPU_CALL_METHODR(ins->ResolveModrm, (ins));
>> op1 = BX_CPU_C::read_RMW_virtual_dword(ins->seg(), eaddr);
>>
>> (with the proper checks for ensuring the first operand is r/m32).
>> However, strange results show up and the O.S doesn't boot.
>>
>>
>> thanks in advance.
>> César.
>>
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