Wireshark-dev: Re: [Wireshark-dev] Does proto_tree_add_bits_item treat the bits in a little end
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?
--
Kind regards,
Peter Wu
https://lekensteyn.nl
