[erlang-questions] hash_state() and Segmentation Fault

[Hans Nilsson R]

The hash_state() is just an Erlang reference.  It is created in the C-function hash_init_nif. It references an area with a C-struct evp_md_ctx containing the EVP_MD_CTX* returned from OpenSSL cryptolib.

The area is allocated by enif_alloc_resource  (see http://erlang.org/doc/man/erl_nif.html#enif_alloc_resource ) and the Erlang reference is created by enif_make_resource (same refman page)

Read about the nif resource mechanism in http://erlang.org/doc/man/erl_nif.html  section "Resource objects".

/Hans

On 4/15/19 12:03 PM, Valentin Micic wrote:
> Hmmm… I may need to restate the question:
> 
> Does anyone know where can one find a description of the hash_state() structure, as used by crypto:hash_xxx functions?
> 
> Thanks in advance.
> 
> V/
> 
> On 10 Apr 2019, at 1:08 PM, Valentin Micic wrote:
> 
>> Hi, 
>>
>> I've been investigating a feasibility of saving of hash_state() -- used as a part of erlang:md5_init/md5_update/md5_final and/or their functional equivalents in crypto library; so it could be used later to implicitly reinitialize hash calculations.
>> Well, (duh!) of course it is feasible; however, my concern was that the structure of this opaque value may change between different versions of Erlang run-time, and I was interested to see how these functions would behave when fake values for hash_state()  are given.
>>
>> The results are interesting.
>>
>> A call to,  say, erlang:md5_final( <<0:88/unsigned_integer-unit:8>> ) (*), or 
>>  erlang:md5_final( <<1212312312:88/unsigned_integer-unit:8>> ), or, indeed
>> crypto:hash_final( {md5, <<0:92/unsigned-integer-unit:8>>} )
>>
>> will all produce:
>>
>> <<176,230,65,201,152,204,62,174,111,162,248,114,109,152,205,221>>
>>
>> Presumably this may be due to some default value that has been used for all invalid values for hash_sate().
>>
>> However, a call using a fake (yet "non-zero") value in  crypto:hash_final( {md5, <<1:92/unsigned-integer-unit:8>>} )  results in run-time crashing and reporting segmentation fault (and this cannot be a good thing, right?).
>>
>> As it appears that some internal tests are performed in order to verify the hash_state() value, would  it possible to extend these test to cover other values without imposing unnecessary performance penalty?
>>
>> Or, alternatively, is there any way that this test could be performed externally (e.g. when in doubt and before calling a function that may crash the run-time)… in other words, is it possible to publish descriptions (e.g. structure) of various hash_state() values?
>>
>> Kind regards
>>
>> V/
>>
>> (*) 88 corresponds to a size (in octets) of the erlang:md5_xxx hash_state() value, and conversely, 92 is a number of octets in md5 hash_state() equivalent used by crypto library.
>>
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