Re: BCD Decoding

[Evan Huus <eapache () gmail com>]

On Tue, Dec 17, 2013 at 6:13 PM, Pascal Quantin
<pascal.quantin () gmail com> wrote:
> Hi Even,
>
> in 3GPP world BCD encoding starts with the least significant nibble. That's
> why tvb_bcd_dig_to_wmwm_packet_str() behaves like this. Changing it to
> decode the most significant nibble first would break all the dissectors
> currently using this function.

OK, just wondering.

> The "stop condition" for the most significant nibble set to 0xf is just to
> detect the filler digit in case you have an odd number of digits. In case of
> even number, the length itself is sufficient and you do not need a filler,
> so no "stop condition" is required.

In that case, what should the decoder do if it encounters a 0xf nibble
embedded in a value (ie due to a malformed packet instead of
indicating a stop condition)? Currently our behaviour is rather
undefined:
- if 0xf is in the high nibble, decoding stops even though the whole
length has not yet been decoded (ie if we pass a len of 12 but the
very first nibble is 0xf then we don't decode anything at all)
- if 0xf is in the low nibble, we read past the end of the digit array
(dgt_set_t) and decode it as a garbage value

Throwing an exception seems a little extreme, but I'm not sure what else to do.

> 2013/12/17 Evan Huus <eapache () gmail com>
> > Alexis's ASAN build recently caught an error in
> > tvb_bcd_dig_to_wmem_packet_str in which it appears that if the least
> > significant nibble of the decoded byte is 0xf then we read one element
> > past the end of the 14-element digit array.
> >
> > If the most significant nibble is 0xf we treat that as a stop
> > condition. Is the correct approach to treat a least significant nibble
> > of 0xf as a stop condition also?
> >
> > While in the neighbourhood - shouldn't we be decoding the more
> > significant nibble first, not second? Wiki states that most BCD
> > implementations are big-endian...
> >
> > Cheers,
> > Evan
> >
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