/* Copyright © 2013 Canonical Limited

 *

 * This library is free software; you can redistribute it and/or

 * modify it under the terms of the GNU Lesser General Public

 * License as published by the Free Software Foundation; either

 * version 2.1 of the License, or (at your option) any later version.

 *

 * This library is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * Lesser General Public License for more details.

 *

 * You should have received a copy of the GNU Lesser General

 * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.

 *

 * Author: Ryan Lortie <desrt@desrt.ca>

 */

#include "config.h"

#include "thumbnail-verify.h"

#include <string.h>

/* Begin code to check the validity of thumbnail files.  In order to do

 * that we need to parse enough PNG in order to get the Thumb::URI,

 * Thumb::MTime and Thumb::Size tags out of the file.  Fortunately this

 * is relatively easy.

 */

typedef struct

{

  const gchar *uri;

  guint64      mtime;

  guint64      size;

} ExpectedInfo;

/* We *require* matches on URI and MTime, but the Size field is optional

 * (as per the spec).

 *

 * http://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html

 */

#define MATCHED_URI    (1u << 0)

#define MATCHED_MTIME  (1u << 1)

#define MATCHED_ALL    (MATCHED_URI | MATCHED_MTIME)

static gboolean

check_integer_match (guint64      expected,

                     const gchar *value,

                     guint32      value_size)

{

  /* Would be nice to g_ascii_strtoll here, but we don't have a variant

   * that works on strings that are not nul-terminated.

   *

   * It's easy enough to do it ourselves...

   */

  if (expected == 0)  /* special case: "0" */

    return value_size == 1 && value[0] == '0';

  /* Check each digit, as long as we have data from both */

  while (expected && value_size)

    {

      /* Check the low-order digit */

      if (value[value_size - 1] != (gchar) ((expected % 10) + '0'))

        return FALSE;

      /* Move on... */

      expected /= 10;

      value_size--;

    }

  /* Make sure nothing is left over, on either side */

  return !expected && !value_size;

}

static gboolean

check_png_info_chunk (ExpectedInfo *expected_info,

                      const gchar  *key,

                      guint32       key_size,

                      const gchar  *value,

                      guint32       value_size,

                      guint        *required_matches)

{

  if (key_size == 10 && memcmp (key, "Thumb::URI", 10) == 0)

    {

      gsize expected_size;

      expected_size = strlen (expected_info->uri);

      if (expected_size != value_size)

        return FALSE;

      if (memcmp (expected_info->uri, value, value_size) != 0)

        return FALSE;

      *required_matches |= MATCHED_URI;

    }

  else if (key_size == 12 && memcmp (key, "Thumb::MTime", 12) == 0)

    {

      if (!check_integer_match (expected_info->mtime, value, value_size))

        return FALSE;

      *required_matches |= MATCHED_MTIME;

    }

  else if (key_size == 11 && memcmp (key, "Thumb::Size", 11) == 0)

    {

      /* A match on Thumb::Size is not required for success, but if we

       * find this optional field and it's wrong, we should reject the

       * thumbnail.

       */

      if (!check_integer_match (expected_info->size, value, value_size))

        return FALSE;

    }

  return TRUE;

}

static gboolean

check_thumbnail_validity (ExpectedInfo *expected_info,

                          const gchar  *contents,

                          gsize         size)

{

  guint required_matches = 0;

  /* Reference: http://www.w3.org/TR/PNG/ */

  if (size < 8)

    return FALSE;

  if (memcmp (contents, "\x89PNG\r\n\x1a\n", 8) != 0)

    return FALSE;

  contents += 8, size -= 8;

  /* We need at least 12 bytes to have a chunk... */

  while (size >= 12)

    {

      guint32 chunk_size_be;

      guint32 chunk_size;

      /* PNG is not an aligned file format so we have to be careful

       * about reading integers...

       */

      memcpy (&chunk_size_be, contents, 4);

      chunk_size = GUINT32_FROM_BE (chunk_size_be);

      contents += 4, size -= 4;

      /* After consuming the size field, we need to have enough bytes

       * for 4 bytes type field, chunk_size bytes for data, then 4 byte

       * for CRC (which we ignore)

       *

       * We just read chunk_size from the file, so it may be very large.

       * Make sure it won't wrap when we add 8 to it.

       */

      if (G_MAXUINT32 - chunk_size < 8 || size < chunk_size + 8)

        goto out;

      /* We are only interested in tEXt fields */

      if (memcmp (contents, "tEXt", 4) == 0)

        {

          const gchar *key = contents + 4;

          guint32 key_size;

          /* We need to find the nul separator character that splits the

           * key/value.  The value is not terminated.

           *

           * If we find no nul then we just ignore the field.

           *

           * value may contain extra nuls, but check_png_info_chunk()

           * can handle that.

           */

          for (key_size = 0; key_size < chunk_size; key_size++)

            {

              if (key[key_size] == '\0')

                {

                  const gchar *value;

                  guint32 value_size;

                  /* Since key_size < chunk_size, value_size is

                   * definitely non-negative.

                   */

                  value_size = chunk_size - key_size - 1;

                  value = key + key_size + 1;

                  /* We found the separator character. */

                  if (!check_png_info_chunk (expected_info,

                                             key, key_size,

                                             value, value_size,

                                             &required_matches))

                    return FALSE;

                }

            }

        }

      else

        {

          /* A bit of a hack: assume that all tEXt chunks will appear

           * together.  Therefore, if we have already seen both required

           * fields and then see a non-tEXt chunk then we can assume we

           * are done.

           *

           * The common case is that the tEXt chunks come at the start

           * of the file before any of the image data.  This trick means

           * that we will only fault in a single page (4k) whereas many

           * thumbnails (particularly the large ones) can approach 100k

           * in size.

           */

          if (required_matches == MATCHED_ALL)

            goto out;

        }

      /* skip to the next chunk, ignoring CRC. */

      contents += 4, size -= 4;                         /* type field */

      contents += chunk_size, size -= chunk_size;       /* data */

      contents += 4, size -= 4;                         /* CRC */

    }

out:

  return required_matches == MATCHED_ALL;

}

gboolean

thumbnail_verify (const char     *thumbnail_path,

                  const gchar    *file_uri,

                  const GLocalFileStat *file_stat_buf)

{

  gboolean thumbnail_is_valid = FALSE;

  ExpectedInfo expected_info;

  GMappedFile *file;

  if (file_stat_buf == NULL)

    return FALSE;

  expected_info.uri = file_uri;

  expected_info.mtime = file_stat_buf->st_mtime;

  expected_info.size = file_stat_buf->st_size;

  file = g_mapped_file_new (thumbnail_path, FALSE, NULL);

  if (file)

    {

      thumbnail_is_valid = check_thumbnail_validity (&expected_info,

                                                     g_mapped_file_get_contents (file),

                                                     g_mapped_file_get_length (file));

      g_mapped_file_unref (file);

    }

  return thumbnail_is_valid;

}