[Libguestfs] hivex: some issues (key encoding, ...) and suggested fixes

[Matthew Booth]

On 26/02/11 18:56, Török Edwin wrote:
> Hi,
>
> libhivex seems to do a great job at parsing hives most of the time, but
> there are some issues with a few registry keys.
>
> These can be worked around in the application that uses libhivex, but I
> think it'd be better if libhivex handled these itself.
>
> 1. UTF16 string in REG_SZ that has garbage after the \0\0
>
> There is code in hivex.c to handle this already but I think it has a typo:
>
>    /* Deal with the case where Windows has allocated a large buffer
>     * full of random junk, and only the first few bytes of the buffer
>     * contain a genuine UTF-16 string.
>     *
>     * In this case, iconv would try to process the junk bytes as UTF-16
>     * and inevitably find an illegal sequence (EILSEQ).  Instead, stop
>     * after we find the first \0\0.
>     *
>     * (Found by Hilko Bengen in a fresh Windows XP SOFTWARE hive).
>     */
>    size_t slen = utf16_string_len_in_bytes_max (data, len);
>    if (slen>  len)
>      len = slen;
>
>    char *ret = windows_utf16_to_utf8 (data, len);
>
> slen is only used to increase length of data, but I think it should be
> decreasing it (to stop earlier).

Yup, that certainly looks like a bug.

> 2. Non-ascii node names
>
> I found a node with a \xDC (Ü) in it:
> SOFTWARE\\ODBC\\ODBCINST.INI\\MS Code Page-\xDCbersetzer
>
> hivex.c has a comment like this:
>    /* AFAIK the node name is always plain ASCII, so no conversion
>     * to UTF-8 is necessary.  However we do need to nul-terminate
>     * the string.
>     */
>
> I think hivex should convert the node names from CP1252 (or is it
> ISO-8859-1?) to UTF-8.
>
> Workaround: I do the CP1252 ->  UTF8 conversion myself for now
>
> 3. node_get_child is slow
>
> Documentation issue, it should say that using node_get_child is slow
> (because registry doesn't have an index, and you do a linear search).
>
> Workaround: I create a map of node names to children of a node, a lookup
> in that is faster than using node_get_child repeatedly
>
> 4. hivexml output is not a well-formed XML
>
> See problem #1 and #2, if value_string and node_name are fixed to not
> dump the binary garbage and just return UTF8 then I think hivexml's
> output would pass xmllint.

As it happens, I opened a BZ on this just the other day. I think there's 
an additional element here: it seems that sometimes a registry key 
genuinely contains non-text data. An example is 
HKLM/SOFTWARE/Microsoft/MSDTC/Security/XAKey, which I'm guessing is a 
cryptographic key. This would require a CDATA section. However, it's not 
clear to me how the tool can reliably infer that a value is binary data 
without specific knowledge of the schema.

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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