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csn1: fix M_UINT_OFFSET: show value after applying the offset 

Some integer fields in CSN.1 structures can be encoded with an offset.
A good example is GPRS Mobile Allocation IE defined in 3GPP TS 44.060,
section 12.10a, table 12.10a.1:

  < GPRS Mobile Allocation IE > ::=
    < HSN : bit (6) >
    { 0 | 1  < RFL number list : < RFL number list struct > > }
    {     0  < MA_LENGTH : bit (6) >
             < MA_BITMAP : bit (val(MA_LENGTH) + 1) >
        | 1  { 0 | 1  < ARFCN index list : < ARFCN index list struct > > }
    } ;

so in this case the variable-length MA_BITMAP is defined as follows:

  < MA_BITMAP : bit (val(MA_LENGTH) + 1) >

what basically means that its bit length shall be encoded with
a negative offset 1, therefore the following statements apply:

  MA_LENGTH=0 defines MA_BITMAP of bit length 1
  MA_LENGTH=1 defines MA_BITMAP of bit length 2
  ...
  MA_LENGTH=63 defines MA_BITMAP of bit length 64

== What's wrong? ==

For some reason, Wireshark shows the raw values without applying
the offset.  Here is an example of GPRS Mobile Allocation IE:

  GPRS_Mobile_Allocation
      .... .101  010. .... = HSN: 42
      ...0 .... = RFL_NUMBER Exist: 0
      .... 0... = Mobile Allocation:  (Union)
      u.MA
          .... .001  111. .... = Bit length: 15
          ...0 .... = Bitmap: 0 // 1st
          .... 1... = Bitmap: 1
          .... .0.. = Bitmap: 0
          .... ..1. = Bitmap: 1
          .... ...0 = Bitmap: 0
          1... .... = Bitmap: 1
          .0.. .... = Bitmap: 0
          ..1. .... = Bitmap: 1  // 8th
          ...0 .... = Bitmap: 0
          .... 1... = Bitmap: 1
          .... .0.. = Bitmap: 0
          .... ..1. = Bitmap: 1
          .... ...0 = Bitmap: 0
          1... .... = Bitmap: 1
          .0.. .... = Bitmap: 0
          ..1. .... = Bitmap: 1 // 16th

== Solution ==

Let's use proto_tree_add_uint_bits_format_value(), so we can print
the final value with the offset applied, as well as the original
one and the offset itself:

  GPRS_Mobile_Allocation
      .... .101  010. .... = HSN: 42
      ...0 .... = RFL_NUMBER Exist: 0
      .... 0... = Mobile Allocation:  (Union)
      u.MA
          .... .001  111. .... = Bit length: 16 (Raw 15 + Offset 1)

Change-Id: Ic4eaf2d8a3c2fedca855726e4175ddf47d16c5af
Reviewed-on: https://code.wireshark.org/review/37931
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>
This commit is contained in:
Vadim Yanitskiy 2020-07-23 02:54:20 +07:00 committed by Anders Broman
parent 26c4120204
commit 2046666b97
1 changed files with 9 additions and 3 deletions
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12
epan/dissectors/packet-csn1.c
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@ -278,7 +278,9 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
pui8 = pui8DATA(data, pDescr->offset);
*pui8 = ui8 + (guint8)pDescr->descr.value;
proto_tree_add_bits_item(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits, ENC_BIG_ENDIAN);
proto_tree_add_uint_bits_format_value(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits,
*pui8, "%u (Raw %u + Offset %u)", *pui8, ui8,
(guint8) pDescr->descr.value);
}
else if (no_of_bits <= 16)
{
@ -286,7 +288,9 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
pui16 = pui16DATA(data, pDescr->offset);
*pui16 = ui16 + (guint16)pDescr->descr.value;
proto_tree_add_bits_item(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits, ENC_BIG_ENDIAN);
proto_tree_add_uint_bits_format_value(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits,
*pui16, "%u (Raw %u + Offset %u)", *pui16, ui16,
(guint16) pDescr->descr.value);
}
else if (no_of_bits <= 32)
{
@ -294,7 +298,9 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
pui32 = pui32DATA(data, pDescr->offset);
*pui32 = ui32 + (guint16)pDescr->descr.value;
proto_tree_add_bits_item(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits, ENC_BIG_ENDIAN);
proto_tree_add_uint_bits_format_value(tree, *(pDescr->hf_ptr), tvb, bit_offset, no_of_bits,
*pui32, "%u (Raw %u + Offset %u)", *pui32, ui32,
(guint16) pDescr->descr.value);
}
else
{