10:04 a.m.
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
10:26 a.m.
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.
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3:19 p.m.
New subject: New Python API? (was: Re: About the return value of value_value)
(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
4:18 p.m.
New subject: New Python API? (was: Re: About the return value of value_value)
On Sun, Aug 10, 2014 at 10:19:39PM +0200, Peter Wu wrote:
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)?
One (negative) thing we learned doing libvirt is that unless you
generate the language bindings and C API together, the language
bindings inevitably get out of date, or (worse) contain non-systematic
errors which are difficult to discover and correct.
Therefore you're welcome to create a more Pythonic hivex API either on
top of the existing Python API or talking directly to C, but we
couldn't accept it upstream (well, unless it was fully generated and
included in generator.ml, but that seems unlikely to be possible).
Having said that ...
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()
... a possible exception would be if it just involves adding some
extra code to the existing hivex.py file, eg. adding a just the extra
classes with __setattr__ and __getattr__ functions.
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)
Noted. If this could be added to the existing hivex.py ...
[...]
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'.
It's worth saying that encoding in the registry itself is not always
UTF-16LE. It's sometimes UTF-8, ASCII or (in a case I found last
week) an NLS like ISO-8859-1 or Big5. Essentially the consuming app
always has to know what encoding to use. Doing "clever" stuff in the
bindings is therefore almost always going to be wrong in some case.
(This is also why the C functions like hivex_value_string are
deprecated).
(*) 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?
Yup.
Rich.
--
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Read my programming and virtualization blog: http://rwmj.wordpress.com
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6:11 p.m.
New subject: New Python API?
On Sunday 10 August 2014 22:18:04 Richard W.M. Jones wrote:
On Sun, Aug 10, 2014 at 10:19:39PM +0200, Peter Wu wrote:
> 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)?
One (negative) thing we learned doing libvirt is that unless you
generate the language bindings and C API together, the language
bindings inevitably get out of date, or (worse) contain non-systematic
errors which are difficult to discover and correct.
Therefore you're welcome to create a more Pythonic hivex API either on
top of the existing Python API or talking directly to C, but we
couldn't accept it upstream (well, unless it was fully generated and
included in generator.ml, but that seems unlikely to be possible).
I was thinking of basing the more Pythonic API on top of the current
hivex.Hivex class, not adding more functionality to that wrapper. If someone
would like to create a broken registry, (s)he then has the full power with the
low-level API. If on the other hand one is looking for a way to access a
registry without breaking, a nicer API would be nice. Something that prevents
a programmer from writing 1 byte to a DWORD type for example. Something that
makes traversing through registry keys easier (as demonstrated before).
Would there be interest for inclusion of such an API in hivex? Since it uses
the existing Python methods, breakage must not be possible unless you break
other programs relying on it.
Having said that ...
> 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()
... a possible exception would be if it just involves adding some
extra code to the existing hivex.py file, eg. adding a just the extra
classes with __setattr__ and __getattr__ functions.
Yes, the low-level binding is left intact, it's just a new Hivex2 class that
is being added. No more changes are needed in libhivexmod.
> 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'.
It's worth saying that encoding in the registry itself is not always
UTF-16LE. It's sometimes UTF-8, ASCII or (in a case I found last
week) an NLS like ISO-8859-1 or Big5. Essentially the consuming app
always has to know what encoding to use. Doing "clever" stuff in the
bindings is therefore almost always going to be wrong in some case.
(This is also why the C functions like hivex_value_string are
deprecated).
When doing a registry export (.reg), all strings like "Key"="Value"
appears to
be UTF-16 strings. Trying to push an UTF-8 string into the registry results in
Chinese characters (UTF-16?). Could you confirm/reject this against the
exports of your keys? Also, when the trailing NUL byte is missing in the
services values, a BSOD can be observed.
If it is necessary to support other encodings, it may be worth to add a
function to wrap the encoding, (type?) and value:
UTF_16_LE = "utf-16-le"
class RegistryString(object):
def __init__(self, type, value, encoding=UTF_16_LE):
...
def value(self):
return self.value.encode(self.encoding) + u"\0".encode(self.encoding)
(maybe introduce a wrapper function for this to avoid long lines)
Strings are always NUL-terminated, right? I recall reading something like that
in the MSDN documentation.
Kind regards,
Peter
https://lekensteyn.nl
7 a.m.
New subject: New Python API?
On Mon, Aug 11, 2014 at 01:11:54AM +0200, Peter Wu wrote:
Would there be interest for inclusion of such an API in hivex? Since
it uses
the existing Python methods, breakage must not be possible unless you break
other programs relying on it.
It really depends on the details, but I suspect what is just a second
API is only going to cause confusion. However you are certainly
welcome to maintain a nicer hivex API outside, and we can even point
to it in the documentation.
> It's worth saying that encoding in the registry itself is
not always
> UTF-16LE. It's sometimes UTF-8, ASCII or (in a case I found last
> week) an NLS like ISO-8859-1 or Big5. Essentially the consuming app
> always has to know what encoding to use. Doing "clever" stuff in the
> bindings is therefore almost always going to be wrong in some case.
> (This is also why the C functions like hivex_value_string are
> deprecated).
When doing a registry export (.reg), all strings like "Key"="Value"
appears to be UTF-16 strings. Trying to push an UTF-8 string into
the registry results in Chinese characters (UTF-16?). Could you
confirm/reject this against the exports of your keys? Also, when the
trailing NUL byte is missing in the services values, a BSOD can be
observed.
Well it depends on the OS you are using. Try it with Windows XP which
uses (some of the time) an old CodePage or ISO-8859-X encoding for at
least registry key names. Also it depends on the Windows application
that is reading the registry.
See:
http://git.annexia.org/?p=hivex-test-data.git;a=commit;h=2145ff5774ecbd4c...
http://git.annexia.org/?p=hivex-test-data.git;a=commit;h=e296fba552f57c63...
If it is necessary to support other encodings, it may be worth to add
a
function to wrap the encoding, (type?) and value:
UTF_16_LE = "utf-16-le"
class RegistryString(object):
def __init__(self, type, value, encoding=UTF_16_LE):
...
def value(self):
return self.value.encode(self.encoding) + u"\0".encode(self.encoding)
(maybe introduce a wrapper function for this to avoid long lines)
Right, this can be made easier to use in the common case, but it'll
break in other cases.
The fundamental problem here is the Registry format is not
well-specified. Consumers can put whatever junk their developers felt
like at the time. Consider it to be a store of arbitrary binary
strings, which sometimes happen to have a well-known encoding.
Strings are always NUL-terminated, right? I recall reading something
like that in the MSDN documentation.
Yup, in well-formed values, but I bet you can find places where that
is not the case.
Rich.
--
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