[libvirt] creating stream

[Michal Privoznik]

On 04/21/2017 01:36 PM, Vasiliy Tolstov wrote:
> 2017-04-21 14:14 GMT+03:00 Michal Privoznik <mprivozn at redhat.com>:
>> That's how strings are encoded in RPC. the first four bytes say how long the
>> string is followed by N bytes (padded to 4 bytes boundary) of the actual
>> string.
>
>
> I know, I'm use https://godoc.org/github.com/davecgh/go-xdr/xdr2 and
> as i see EncodeString already to that. And all other functions what
> operates on strings works fine. Fow example LookupDomainByName. Or
> StoragePool functions
>
> Bad payload from own prog:
>         0x0000:  6001 2a87 006c 0640 2a04 ac00 0006 0001  `.*..l.@*.......
>         0x0010:  0000 0000 0000 0001 2a04 ac00 0004 0010  ........*.......
>         0x0020:  0ec4 7aff fe54 cb0a e8b2 407d e44c 1249  ..z..T....@}.L.I
>         0x0030:  eafc 4a72 8018 00e1 ffba 0000 0101 080a  ..Jr............
>         0x0040:  2ef0 9ea7 02ad afcb 0000 004c 2000 8086  ...........L....
>         0x0050:  0000 0001 0000 00c9 0000 0000 0000 0009  ................
>         0x0060:  0000 0000 0000 0006 3134 3331 3737 0000  ........143177..
>         0x0070:  49bf 97e1 4bda 5749 ca89 0000 7db6 2f02  I...K.WI....}./.
>         0x0080:  0000 0001 0000 0007 7365 7269 616c 3000  ........serial0.
>         0x0090:  0000 0001                                ....
>
> Good payload from virsh:
>         0x0000:  6009 09de 0070 0640 2a04 ac00 0006 0001  `....p.@*.......
>         0x0010:  0000 0000 0000 0001 2a04 ac00 0004 0010  ........*.......
>         0x0020:  0ec4 7aff fe54 cb0a e8a8 407d e159 1a3e  ..z..T....@}.Y.>
>         0x0030:  2e25 4a72 8018 00ea ffbe 0000 0101 080a  .%Jr............
>         0x0040:  2eed 6e7c 02ac e3be 0000 0050 2000 8086  ..n|.......P....
>         0x0050:  0000 0001 0000 00c9 0000 0000 0000 0009  ................
>         0x0060:  0000 0000 0000 0006 3134 3331 3737 0000  ........143177..
>         0x0070:  49bf 97e1 4bda 5749 ca89 0000 7db6 2f02  I...K.WI....}./.
>         0x0080:  0000 0001 0000 0001 0000 0007 7365 7269  ............seri

So the extra "byte" (in fact 4 bytes) are at 0x0084-0x0087. So this is 
how strings are encoded. There are basically two types of strings when 
it comes to XDR. The first one is nonnull_string, which are encoded 
[length][string]. With just this it would be impossible to differentiate 
between NULL and an empty string "" as both would be encoded in the same 
way. Therefore, there is remote_string (libvirt terminology) or 
xdr_pointer (XDR terminology) which encodes a pointer as [bool][pointer] 
where bool is either 00 00 00 00 meaning the pointer is NULL or 
something else (e.g. 00 00 00 01) meaning the pointer has a non-NULL 
value. So long story short, what you can see here is:

00 00 00 01 00 00 00 07 73 65 72 69 61 6c 30

which is the same as (regroup)

[00 00 00 01] [00 00 00 07] [73 65 72 69 61 6c 30]

which means: the following pointer is a non-NULL pointer, and it is a 
string of the length 7 and the string is "serial0".

>         0x0090:  616c 3000 0000 0000                      al0.....

Michal