Libguestfs - Aug 2014 - About the return value of value_value

Hi,

I have been working on a Python application that uses hivex. Meanwhile I have 
encountered some Python bindings issues which could be fixed.

The next issue I see now is about the value_value function. This is briefly 
documented as: "return data length, data type and data of a value".

For Perl, Python and OCaml, this is not true. A tuple is returned for both 
without the length (as this can be calculated from the data value). Ruby is 
the outlier here that uses a dictionary with three keys. I am not familar with 
Ruby and neither do I know Ruby users of hivex.

The documentation should likely be fixed to exclude the length, but what about 
the Ruby API? Is it correct or should a documentation note be added that Ruby 
differs
 
Kind regards,
Peter
https://lekensteyn.nl

On Sun, Aug 10, 2014 at 05:04:01PM +0200, Peter Wu
wrote:
> Hi,
> 
> I have been working on a Python application that uses hivex. Meanwhile I
have
> encountered some Python bindings issues which could be fixed.
> 
> The next issue I see now is about the value_value function. This is briefly
> documented as: "return data length, data type and data of a
value".
>
> For Perl, Python and OCaml, this is not true. A tuple is returned
> for both without the length (as this can be calculated from the data
> value). Ruby is the outlier here that uses a dictionary with three
> keys. I am not familar with Ruby and neither do I know Ruby users of
> hivex.
>
> The documentation should likely be fixed to exclude the length, but
> what about the Ruby API? Is it correct or should a documentation
> note be added that Ruby differs
Note that the documentation applies to the C API (where length is
returned).  Since the same generated documentation is used for the
other languages too, that can make it a bit patchy.

In Ruby it seems as if the length could be calculated from the string.
On the other hand, I'm not sure there is any point in intentionally
removing the length from the return value, as that might break callers
for no particular reason.

The best plan here is probably to add a note to the Ruby documentation
for RLenTypeVal saying what the hash contains on Ruby.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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(renaming subject as I am partially getting off-topic)

On Sunday 10 August 2014 16:26:07 Richard W.M. Jones
wrote:
> > The next issue I see now is about the value_value function. This is
> > briefly  documented as: "return data length, data type and data
of a
> > value".
> > 
> > For Perl, Python and OCaml, this is not true. A tuple is returned
> > for both without the length (as this can be calculated from the data
> > value). Ruby is the outlier here that uses a dictionary with three
> > keys. I am not familar with Ruby and neither do I know Ruby users of
> > hivex.
> > 
> > The documentation should likely be fixed to exclude the length, but
> > what about the Ruby API? Is it correct or should a documentation
> > note be added that Ruby differs
> 
> Note that the documentation applies to the C API (where length is
> returned).  Since the same generated documentation is used for the
> other languages too, that can make it a bit patchy.
The Python documentation is scare on the type of the various parameters and 
return values. Moreover, it states

    "Read the hivex(3) man page to find out how to use the API."

Perhaps a second API should be created that is more pythonic (read: easier to 
use)? I mean, right now you have to use this (with some patches[0][1], also 
available at git[2]):

import hivex
from hivex.hive_types import *

h = hivex.Hivex("system", write=True)
ccs_name    = "ControlSet001"
ccs         = h.node_get_child(h.root(), ccs_name)
services    = h.node_get_child(ccs, "Services")
svc_viostor = h.node_get_child(services, "viostor")
start_id    = h.node_get_value(svc_viostor, "Start")
#node_type, node_value = h.value_value(start_id)
dword_value = h.value_dword(start_id)
if node_value != 4:
    new_value = {
        "key": "Start",
        # constant from hivex.hive_types
        "t": REG_DWORD,
        # alternative of b'\4\0\0\0'
        "value": 4
    }
    h.node_set_value(svc_viostor, new_value)
h.commit()

It would be great if something like this could be done instead:

import hivex

hive = hivex.Hivex2("system", write=True)
ccs_name = "ControlSet001"
svc_viostor = hive.root()[ccs_name].Services.viostor
if svc_viostor.Start != 4:
    # Automatically detect that int '4' is an DWORD
    svc_viostor.Start = 4
svc.commit()

I (ab)use the __getattr__ methods if an object to allow this kind of 
modifications. See also the RegistryHandle helper class at
https://github.com/Lekensteyn/qemu-tools/blob/master/vbox-to-qemu.py 
(_import_callback at line 216 may also be interesting)

This is a quick implementation with not much thought put into it, but what do 
you think of the idea to make an easier API next to the current one?

In the current implementation, Python 3 bytes (Python 2 strings) are treated 
as plain bytes(*). That is fine, but Unicode is not handled correctly. This 
might also be an opportunity to treat Unicode strings as UTF-16 (LE) strings 
which must be nul-terminated. So u'Bar' should become
b'B\0a\0r\0\0\0'.

 (*) Actually, Hivex 1.3.10 is broken in Python 3 and tries to convert all 
strings from UTF-8 to bytes and segfaults on other input which does not work 
for UTF-16 strings[0].
> In Ruby it seems as if the length could be calculated from the string.
> On the other hand, I'm not sure there is any point in intentionally
> removing the length from the return value, as that might break callers
> for no particular reason.
> 
> The best plan here is probably to add a note to the Ruby documentation
> for RLenTypeVal saying what the hash contains on Ruby.
... and mention that all other language bindings return a tuple / list / array 
with just two elements as the length can be found from the value?

Kind regards,
Peter
https://lekensteyn.nl

 [0]: https://www.redhat.com/archives/libguestfs/2014-August/msg00050.html
 [1]: https://www.redhat.com/archives/libguestfs/2014-August/msg00053.html
 [2]: https://github.com/Lekensteyn/hivex/compare/master...develop