There are several issues with the CDS FileMapHeader loading code:

1. FileMapInfo::get_base_archive_name_from_header() does not check CRC. As a
   result, if the header is corrupt, it may return an invalid path.

2. FileMapHeader::compute_crc() does not cover the base_archive_name. As a result,
   corruption of this part of the header cannot be detected with
   -XX:+VerifySharedSpaces.

3. Currently, the header has a fixed part (sizeof(FileMapHeader)) plus a variable
   part (the base_archive_name). Loading the variable parts require a separate
   lseek()/read() operation. This will become more complicated if we need to
   add more variable parts into this data structure in the future.

4. JDK-8261455 (Automatically generate the CDS archive if necessary) needs to read
   the header of an archive generated by a different version of HotSpot. The
   current design does not allow that.

We should restructure the header and refactor the loading operation to address
the above issues:

=== proposed data structures ===

// This portion of the archive file header must remain unchanged for
// (_version >= 12). This makes it possible to read important information
// from a CDS archive created by a different version of HotSpot, so that
// we can automatically regenerate the archive as necessary.
struct GenericCDSFileMapHeader {
  // identify file type
  unsigned int _magic;

  // header crc checksum
  int          _crc;

  // CURRENT_CDS_ARCHIVE_VERSION of the JDK that dumped this archive
  int          _version;

  // total size of the header, in bytes
  unsigned int _header_size;

  // If this is a static archive, this field is 0.
  // If this is a dynamic archive:
  //     _base_archive_path_offset == 0:
  //        The base archive is the default CDS archive.
  //     _base_archive_path_offset >  0:
  //        (((char*)this) + _base_archive_path_offset) points to
  //        a 0-terminated string for the pathname of the base archive.
  unsigned int _base_archive_path_offset;

  // size of the base archive name including NULL terminator, or 0
  // if _base_archive_path_offset == 0.
  unsigned int _base_archive_name_size;
};

// This type is used by the Serviceability Agent to access the contents of
// a memory-mapped CDS archive.
struct CDSFileMapHeaderBase {
  // We cannot inherit from GenericCDSFileMapHeader as this type may be used
  // by both C and C++ code.
  GenericCDSFileMapHeader _generic_header;
  struct CDSFileMapRegion _space[NUM_CDS_REGIONS];
};

=== proposed header loading sequence ===

1. open file
2. read sizeof(GenericCDSFileMapHeader)
3. check _magic
4. Read the full header into a malloc'ed buffer of _header_size bytes
5. Check CRC
6. Check the consistency of other fields (this must be done after CRC
   check -- if CRC fas failed, it's meaningless to read other fields as
   they may be corrupt).
6.1 First check the fields in GenericCDSFileMapHeader
6.2 if _version == CURRENT_CDS_ARCHIVE_VERSION, check the fields of
   the rest of the data structure.