Wireshark · Wireshark-dev: Re: [Wireshark-dev] dissect_per_constrained_integer() with no_bound (MAX in ASN.1)

[Pascal Quantin <pascal.quantin@xxxxxxxxx>]

Hi Pavel,

2016-12-21 15:37 GMT+01:00 Pavel Strnad <strnadp@xxxxxxxxxx>:

Hello,

I am trying to understand the difference in usage of NO_BOUND or UINT_MAX in the place of max parameter in dissect_per_constrained_integer() function. In my case aligned PER variant.

 

From packet-per.h:

#define NO_BOUND -1

guint32 dissect_per_constrained_integer(tvbuff_t *tvb, guint32 offset, asn1_ctx_t *actx, proto_tree *tree, int hf_index, guint32 min, guint32 max, guint32 *value, gboolean has_extension);

 

Based on that it looks like that there is no different dissection of following two asn.1 definitions?

Correct, semi-constrained integer does not seem to be managed properly. This matches the comment found in line 1283.

1)      seconds     INTEGER (0..4294967295)

2)      seconds     INTEGER (0..MAX) where MAX translates to NO_BOUND=4294967295 using asn2wrs

 

Reading X.691 (aligned PER) wireshark seems to dissect well the 1^st case that is using size constraint but not the 2^nd case

where semi-constraint size is used and the length determinant should include padding bits in the case of aligned PER.

 

I would like to try to fix it myself but will need some hint how to differentiate these two cases and keep API unchanged?

IMHO you can't without changing the API. Maybe we would need to extra booleans indicating whether the min and / or max values are bounded or not, and adapt asn2wrs generator accordingly. This way we could drop the NO_BOUND define

Note that this sounds like a non trivial project because:
- as indicated in the comments, dissect_per_constrained_integer only handle 32 bits integers and dissect_per_constrained_integer_64b only handles 64 bits integers, so both needs to be adapted

- MIN or MAX parameters do not seem properly handled in other types also, and any change done in dissect_per_constrained_integer(_64b) needs to be reflected in the caller functions and any other making use of NO_BOUND

Regards,

Pascal.