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Re: [Libguestfs] [PATCH 1/1] hivexml: Base64-encode non-printable data



I'm not familiar with iconv. However, I have a routine that will test to see if it is valid UTF-8. The XML you are generating should specify UTF-8 as the encoding mode. You should then check to make sure that all keys and values are UTF-8 and, if they are, just pass them through. If they are not you should escape them as BASE64 and indicate that they are escaped with the encoding='base64' attribute. If encoding= is ambigious you may wish to use key_encoding= and value_encoding= attributes (or is that encoding_key= and encoding_value=?).

This is what I'm doing with the other DFXML values and it seems to provide the best flexibility in practice.

Your "stricter" passage of everything through isprint() is not correct.  isprint() is not guaranteed to return true for valid UTF-8, and it will force encoding of many values that may commonly appear which can be properly encoded in UTF-8, for example, currency symbols (e.g. £). 

An option that you may wish to consider is automatically detecting UTF-16 and transcoding it as UTF-8. I believe that this will make the library dramatically more useful, because filenames are commonly stored in the registry and (I believe) are frequently encoded as UTF-16. Forcing these to be encoded as BASE64 is just silly when we can recode them as UTF-8. However, you may wish to note in the XML some kind of notation that they were recoded as UTF-8 from UTF-16.

Below you note that string values are treated "like they could be UTF-16".  I think that you should determine if they ARE or ARE NOT UTF-16. It should be pretty easy to distinguish the two for the majority of cases. For example, if there are a significant number of \000 characters and they always fall on either EVEN or ODD bytes, then there is a high probability of UTF-16. 


On Sep 17, 2011, at 12:53 PM, Alex Nelson wrote:

> Basically, if any of the characters passed to a hivexml.c function aren't isprint()-able, they're base64-encoded.
> 
> The characters passed rely on the iconv library.  Whatever comes out of the processing of iconv() in "UTF-8 or UTF-16" mode is what's passed to hivexml.  I checked all the UTF-processing points and have extensive notes in the postscript, but essentially:
> * Key and value names aren't run through iconv
> * Value string data and the hive header name are
> * The patch below runs a stricter sanitization than iconv, also running everything through isprint.
> 
> --Alex
> 
> Notes:
> * The hive's header-embedded name is just checked as UTF-16, though not passed onto hivexml (yet) (lib/hivex.c:hivex_open).  The name is converted with lib/hivex.c:windows_utf16_to_utf8, which relies on the iconv library, so ASCII going in should be ASCII coming out.
> * For node and value names, there is no UTF-n check (lib/hivex.c:hivex_node_name, hivex_value_key), because nobody developing hivex has seen something besides plain ASCII in the names yet.  I can probably identify somewhere where this isn't the case, but it'd take some machine-hours.  So far all I've seen in one corpus is a Registered sign, 0xae.
> * String values in general are treated like they could be UTF-16.  All are passed to windows_utf16_to_utf8 (lib/hivex.c:hivex_value_string, lib/hivex.c:hivex_value_multiple_strings).
> * If the hivex_value_string or hivex_value_multiple_strings function fails to process the data, the value_string_invalid_utf16 function of the hivex library invoker is called (lib/hivex.c:hivex__visit_node), which in hivexml's case just base64-encodes the output (xml/hivexml.c:value_string_invalid_utf16).
> 
> 
> On Sep 17, 2011, at 03:17 , Simson Garfinkel wrote:
> 
>> What does this do when there is UTF8 in the hive?
>> 
>> Sent from my iPad
>> 
>> On Sep 17, 2011, at 12:30 AM, Alex Nelson <ajnelson cs ucsc edu> wrote:
>> 
>>> Some of the data in names and string values were being unsafely printed,
>>> causing some types of XML processors to fail (e.g. Python's Expat).
>>> This patch checks for printability of each character and outputs base64
>>> with an encoding attribute for unsafe data.
>>> ---
>>> xml/hivexml.c |   75 ++++++++++++++++++++++++++++++++++++++++++++++++--------
>>> 1 files changed, 64 insertions(+), 11 deletions(-)
>>> 
>>> diff --git a/xml/hivexml.c b/xml/hivexml.c
>>> index cf11676..110c8fb 100644
>>> --- a/xml/hivexml.c
>>> +++ b/xml/hivexml.c
>>> @@ -27,6 +27,7 @@
>>> #include <errno.h>
>>> #include <time.h>
>>> #include <locale.h>
>>> +#include <ctype.h>
>>> 
>>> #ifdef HAVE_LIBINTL_H
>>> #include <libintl.h>
>>> @@ -201,6 +202,52 @@ filetime_to_8601 (int64_t windows_ticks)
>>> return ret;
>>> }
>>> 
>>> +/* Caller need not free return value afterwards. */
>>> +static char *
>>> +encoding_recommendation (const char *data)
>>> +{
>>> +  /* Note that this function assumes data is null-terminated. */
>>> +  //See if the data are printable
>>> +  int is_printable = 0;
>>> +  size_t i;
>>> +  size_t data_len = strlen(data);
>>> +
>>> +  for (i=0; i < data_len; i++) {
>>> +    is_printable = isprint (data[i]);
>>> +    if (!is_printable) {
>>> +      fprintf (stderr, "encoding_recommendation: Non-printable character found at data index %zu (c=%i)\n", i, data[i]);
>>> +      break;
>>> +    }
>>> +  }
>>> +
>>> +  return is_printable ? "none" : "base64";
>>> +}
>>> +
>>> +static int
>>> +safe_print_string_attribute (hive_h *h, void *writer_v, const char *attr_name, const char *attr_encoding, const char *attr_data)
>>> +{
>>> +  int ret = 0;
>>> +  char *encoding_to_use = NULL;
>>> +  if (attr_name && attr_data && attr_encoding) {
>>> +    xmlTextWriterPtr writer = (xmlTextWriterPtr) writer_v;
>>> +    encoding_to_use = encoding_recommendation (attr_data);
>>> +
>>> +    if (strcmp (encoding_to_use, "none") == 0)
>>> +      XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST attr_name, BAD_CAST attr_data));
>>> +    else if (strcmp (encoding_to_use, "base64") == 0) {
>>> +      XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST attr_encoding, BAD_CAST "base64"));
>>> +      XML_CHECK (xmlTextWriterStartAttribute, (writer, BAD_CAST attr_name));
>>> +      XML_CHECK (xmlTextWriterWriteBase64, (writer, BAD_CAST attr_data, 0, strlen(attr_data)));
>>> +      XML_CHECK (xmlTextWriterEndAttribute, (writer));
>>> +    } else {
>>> +      fprintf (stderr, "safe_print_string_attribute: Unexpected encoding to use (won't print here).\n");
>>> +      ret = -1;
>>> +    }
>>> +  } else
>>> +    ret = -1;
>>> +  return ret;
>>> +}
>>> +
>>> static int
>>> node_start (hive_h *h, void *writer_v, hive_node_h node, const char *name)
>>> {
>>> @@ -210,7 +257,10 @@ node_start (hive_h *h, void *writer_v, hive_node_h node, const char *name)
>>> 
>>> xmlTextWriterPtr writer = (xmlTextWriterPtr) writer_v;
>>> XML_CHECK (xmlTextWriterStartElement, (writer, BAD_CAST "node"));
>>> -  XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "name", BAD_CAST name));
>>> +
>>> +  ret = safe_print_string_attribute (h, writer_v, "name", "name_encoding", name);
>>> +  if (ret)
>>> +    fprintf (stderr, "Warning: node_start: safe_print_string_attribute failed, but we're continuing.\n");
>>> 
>>> if (node == hivex_root (h)) {
>>>   XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "root", BAD_CAST "1"));
>>> @@ -227,7 +277,7 @@ node_start (hive_h *h, void *writer_v, hive_node_h node, const char *name)
>>>   }
>>> }
>>> 
>>> -  return 0;
>>> +  return ret;
>>> }
>>> 
>>> static int
>>> @@ -242,13 +292,16 @@ static void
>>> start_value (xmlTextWriterPtr writer,
>>>            const char *key, const char *type, const char *encoding)
>>> {
>>> +  int ret = 0;
>>> XML_CHECK (xmlTextWriterStartElement, (writer, BAD_CAST "value"));
>>> XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "type", BAD_CAST type));
>>> if (encoding)
>>>   XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "encoding", BAD_CAST encoding));
>>> -  if (*key)
>>> -    XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "key", BAD_CAST key));
>>> -  else                          /* default key */
>>> +  if (*key) {
>>> +    ret = safe_print_string_attribute (NULL, writer, "key", "key_encoding", key);
>>> +    if (ret)
>>> +      fprintf (stderr, "Warning: start_value: safe_print_string_attribute failed, but we're continuing.\n");
>>> +  } else                          /* default key */
>>>   XML_CHECK (xmlTextWriterWriteAttribute, (writer, BAD_CAST "default", BAD_CAST "1"));
>>> }
>>> 
>>> @@ -264,6 +317,7 @@ value_string (hive_h *h, void *writer_v, hive_node_h node, hive_value_h value,
>>> {
>>> xmlTextWriterPtr writer = (xmlTextWriterPtr) writer_v;
>>> const char *type;
>>> +  int ret = 0;
>>> 
>>> switch (t) {
>>> case hive_t_string: type = "string"; break;
>>> @@ -286,11 +340,9 @@ value_string (hive_h *h, void *writer_v, hive_node_h node, hive_value_h value,
>>> }
>>> 
>>> start_value (writer, key, type, NULL);
>>> -  XML_CHECK (xmlTextWriterStartAttribute, (writer, BAD_CAST "value"));
>>> -  XML_CHECK (xmlTextWriterWriteString, (writer, BAD_CAST str));
>>> -  XML_CHECK (xmlTextWriterEndAttribute, (writer));
>>> +  ret = safe_print_string_attribute (h, writer_v, "value", "value_encoding", str);
>>> end_value (writer);
>>> -  return 0;
>>> +  return ret;
>>> }
>>> 
>>> static int
>>> @@ -299,17 +351,18 @@ value_multiple_strings (hive_h *h, void *writer_v, hive_node_h node,
>>>                       const char *key, char **argv)
>>> {
>>> xmlTextWriterPtr writer = (xmlTextWriterPtr) writer_v;
>>> +  int ret = 0;
>>> start_value (writer, key, "string-list", NULL);
>>> 
>>> size_t i;
>>> for (i = 0; argv[i] != NULL; ++i) {
>>>   XML_CHECK (xmlTextWriterStartElement, (writer, BAD_CAST "string"));
>>> -    XML_CHECK (xmlTextWriterWriteString, (writer, BAD_CAST argv[i]));
>>> +    ret = safe_print_string_attribute (h, writer_v, "value", "value_encoding", argv[i]);
>>>   XML_CHECK (xmlTextWriterEndElement, (writer));
>>> }
>>> 
>>> end_value (writer);
>>> -  return 0;
>>> +  return ret;
>>> }
>>> 
>>> static int
>>> -- 
>>> 1.7.4.4
>>> 
>>> 
> 



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