[sr-dev] ID generation in rls/notify.c was Re: Random number generation

Hugh Waite hugh.waite at crocodile-rcs.com
Fri Apr 20 16:03:32 CEST 2012


Yes, Peter's commit fixed the issue I saw. It removed the srand() from 
RLS so that it no longer affected the Via branch param generator.

Do you think that the Via branch should be protected against this? For 
example using the srutils unique ID function instead of rand()?

Hugh

On 20/04/2012 11:39, Daniel-Constantin Mierla wrote:
> Hello,
>
> had not time yet to get and look properly at this thread, is the issue 
> fixed by Peter's commit related to some srand() usage?
>
> Cheers,
> Daniel
>
> On 4/16/12 1:15 PM, Hugh Waite wrote:
>> I now realise that rls was deliberately using the same sequence of 
>> random numbers to satisfy RFC4662 §5.5
>>
>>     5.5. Instance Attributes
>>
>>     Each resource element contains zero or more instance elements. These
>>     instance elements are used to represent a single notifier for the
>>     resource. For event packages that allow forking, multiple virtual
>>     subscriptions may exist for a given resource. Multiple virtual
>>     subscriptions are represented as multiple instance elements in the
>>     corresponding resource element. For subscriptions in which forking
>>     does not occur, at most one instance will be present for a given
>>     resource.
>>
>>     The "id" attribute contains an opaque string used to uniquely
>>     identify the instance of the resource. The "id" attribute is unique
>>     only within the context of a resource. Construction of this string
>>     is an implementation decision. Any mechanism for generating this
>>     string is valid, as long as uniqueness within the resource is
>>     assured.
>>
>> Seeding srand() with the values 0, 1, 2... always gives the same 
>> sequence of ID strings. Maybe someone has an opinion on whether this 
>> should be implemented differently, or whether a value should be 
>> stored for situations where multiple resource instances are 
>> removed/reordered.
>>
>> Maybe kamailio should use a unique ID (from srutils) in the Via 
>> header for uac requests (in modules/tm/h_table.h) anyway, to prevent 
>> a module from affecting Via branch params by using srand().
>>
>> Regards,
>> Hugh
>>
>> On 13/04/2012 15:26, Hugh Waite wrote:
>>> Hi,
>>> One of the places that uses rand() is 
>>> modules/tm/h_table.c:init_synonym_id . Requests generated by 
>>> kamailio (as a uac) use this to generate the Via branch tag. If any 
>>> module calls srand(), it affects the Via branch for subsequent 
>>> requests.
>>>
>>> The actual bug we found is in rls/notify.c:generate_string(). The 
>>> add_resource_instance() function will re-seed srand() with 0 (zero), 
>>> leading to nearly every NOTIFY sent by rls having the same "random" 
>>> number in the Via branch. I am sure this was the cause of lost 
>>> replies, timeouts and dropped subscriptions that we were seeing (and 
>>> appears to have gone away after removing it).
>>>
>>> Although, I could just remove the srand from rls notify.c, I 
>>> wondered if it should be using a different random function, and also 
>>> whether init_synonym_id should use something more unique for the Via 
>>> branch parameter.
>>>
>>> A quick check has shown a few places that call srand() within the 
>>> code, although they probably have less drastic consequences.
>>>
>>> Regards,
>>> Hugh
>>>
>>> On 13/04/2012 14:01, Daniel-Constantin Mierla wrote:
>>>> Hello,
>>>>
>>>> I haven't looked at the issue reported by Hugh, so by now just 
>>>> comments on srutils/sruid.
>>>>
>>>> The idea was to have an unique id generator without linking to an 
>>>> external library -- my first purpose was to use it for temporary 
>>>> GRUU ids (RFC5627), I am just about to push to master the gruu 
>>>> support in registrar/usrloc.
>>>>
>>>> Then I thought it might be useful in other places, such as dialog 
>>>> unique id.
>>>>
>>>> I added it as part of lib, since its target usage was for modules 
>>>> so far, but if needed for some core processing, the two files 
>>>> (rather small by now) can be moved in the core.
>>>>
>>>> Cheers,
>>>> Daniel
>>>>
>>>> On 4/13/12 2:50 PM, Henning Westerholt wrote:
>>>>> On Friday 13 April 2012, Hugh Waite wrote:
>>>>>> I have a question about random number generation within kamailio.
>>>>>>
>>>>>> A number of modules use rand() to get a random value and in some 
>>>>>> places
>>>>>> is re-seeding with srand(). I believe this is dangerous because 
>>>>>> rand()
>>>>>> is used in the Via branch tag generator.
>>>>>> We have detected some real bugs (where srand is reseeding with 0 for
>>>>>> every message, causing transaction mis-matching) but I'm not sure 
>>>>>> of the
>>>>>> correct way to fix this (other than remove srand()).
>>>>>>
>>>>>> Should all modules be using a 'core' random function (e.g. in 
>>>>>> srutils?)
>>>>>> ? And if so, is this library documented?
>>>>>>
>>>>>> Regards,
>>>>>> Hugh
>>>>> Hi Hugh,
>>>>>
>>>>> for the purpose getting a pseudo-random number (i.e. not for 
>>>>> cryptographic
>>>>> functionality) we should consolidate on a single random function. 
>>>>> There is the
>>>>> recent introduced srutils/sruid code, then there exists a (IMHO 
>>>>> stronger)
>>>>> pseudo-random number generator in rand/fastrand and then there is 
>>>>> of course
>>>>> rand().
>>>>>
>>>>> Maybe Daniel can comment about the purpose of the srutils 
>>>>> function, IMHO
>>>>> consolidating on fastrand or one of the stronger function (d_rand 
>>>>> etc..) from
>>>>> stdlib.h would be fine.
>>>>>
>>>>> The re-seeding the internal state of rand() with srand during 
>>>>> runtime sounds
>>>>> wrong toe me and should be removed/ fixed.
>>>>>
>>>>> Viele Grüße/ best regards,
>>>>>
>>>>> Henning Westerholt
>>>>>
>>>>> _______________________________________________
>>>>> sr-dev mailing list
>>>>> sr-dev at lists.sip-router.org
>>>>> http://lists.sip-router.org/cgi-bin/mailman/listinfo/sr-dev
>>>>
>>>
>>>
>>
>>
>> -- 
>> Hugh Waite
>> Senior Design Engineer
>> Crocodile RCS Ltd.
>>
>>
>> _______________________________________________
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>
> -- 
> Daniel-Constantin Mierla
> Kamailio Advanced Training, April 23-26, 2012, Berlin, Germany
> http://www.asipto.com/index.php/kamailio-advanced-training/


-- 
Hugh Waite
Senior Design Engineer
Crocodile RCS Ltd.

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