Wireshark · Wireshark-dev: Re: [Wireshark-dev] Does proto_tree_add_bits_item treat the bits in a little endian manner as well o

[Richard Sharpe <realrichardsharpe@xxxxxxxxx>]

On Thu, May 17, 2018 at 3:41 AM, Peter Wu <peter@xxxxxxxxxxxxx> wrote:
> On Wed, May 16, 2018 at 08:32:12AM -0700, Richard Sharpe wrote:
>> On Wed, May 16, 2018 at 8:01 AM, Richard Sharpe
>> <realrichardsharpe@xxxxxxxxx> wrote:
>> > Hi folks,
>> >
>> > I am seeing something weird with proto_tree_add_bits_item, although it
>> > could be my misunderstanding as well.
>> >
>> > Attached are two screen shots showing the dissected field and the byte view.
>> >
>> > The code:
>> > --------
>> > static void
>> > dissect_a_control_bsr(proto_tree *tree, tvbuff_t *tvb, int offset,
>> >   guint32 bits _U_, guint32 start_bit)
>> > {
>> >   start_bit += offset * 8;
>> >   proto_tree_add_bits_item(tree, hf_ieee80211_he_bsr_aci_bitmap, tvb,
>> >                         start_bit, 4, ENC_LITTLE_ENDIAN);
>> >   proto_tree_add_bits_item(tree, hf_ieee80211_he_bsr_delta_tid, tvb,
>> >                         start_bit + 4, 2, ENC_LITTLE_ENDIAN);
>> >   proto_tree_add_bits_item(tree, hf_ieee80211_he_bsr_aci_high, tvb,
>> >                         start_bit + 6, 2, ENC_LITTLE_ENDIAN);
>> >   proto_tree_add_bits_item(tree, hf_ieee80211_he_bsr_scaling_factor, tvb,
>> >                         start_bit + 8, 2, ENC_LITTLE_ENDIAN);
>> > --------------
>> >
>> > is dealing with a field that contains 0x8f 0xb0 0x00 0x08.
>> >
>> > The first (always counting from the RHE) six bits (0b001111 from 0x8f)
>> > are the HTC+ control bits (2 bits) and the Aggregate control field
>> > bits (4 bits). These are being dissected correctly as HTC+ = 0x3 and
>> > A-Control as 0x3 as well.
>> >
>> > The next four bits are the ACI Bitmap, and they should be 0b0010 but
>> > they are displaying as 0b1110 as if the bottom two bits of the 0xb
>> > from 0xb0 were being grabbed rather than the bottom two bits of 0x0,
>> >
>> > I hope this is making sense. I guess I need to look more closely at
>> > the code in decode_bits_in_field.
>>
>> I came across this in _tvb_get_bits64:
>>
>>                 /* get bits from any partial octet at the tail */
>>                 if(remaining_bit_length)
>>                 {
>>                         value <<= remaining_bit_length;
>>                         value += (tvb_get_guint8(tvb, octet_offset) >>
>> (8 - remaining_bit_length));
>>                 }
>>         }
>>
>> That would seem to take the top bits from the next byte but that seems
>> incorrect from the point of view of many uses.
>
> It looks valid for me, but only for ENC_BIG_ENDIAN types. Your field is
> ENC_LITTLE_ENDIAN. We have this comment here in tvb_get_bits:
>
>     /* note that encoding has no meaning here, as the tvb is considered to contain an octet array */
>
> Perhaps you could start with a change that modifies tvb_get_bits to
> respect the encoding parameter?

I found another way to do what I wanted ... however, I don't think
there is any architecture that arranges its bits in a big endian
fashion, but maybe that is because I have never looked.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)(传说杜康是酒的发明者)