Nearly complete rewrite of cgpt tool.

cgpt/cgpt_common.c

+/* Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ *
+ * Utility for ChromeOS-specific GPT partitions, Please see corresponding .c
+ * files for more details.
+ */
+
+#include "cgpt.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <assert.h>
+#include <stdarg.h>
+
+#include "cgptlib_internal.h"
+#include "crc32.h"
+
+
+void Error(const char *format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  fprintf(stderr, "ERROR: %s %s: ", progname, command);
+  vfprintf(stderr, format, ap);
+  va_end(ap);
+}
+
+
+int CheckValid(const struct drive *drive) {
+  if ((drive->gpt.valid_headers != MASK_BOTH) ||
+      (drive->gpt.valid_entries != MASK_BOTH)) {
+    fprintf(stderr, "\nWARNING: one of the GPT header/entries is invalid, "
+           "please run '%s repair'\n", progname);
+    return CGPT_FAILED;
+  }
+  return CGPT_OK;
+}
+
+/* Loads sectors from 'fd'.
+ * *buf is pointed to an allocated memory when returned, and should be
+ * freed by cgpt_close().
+ *
+ *   fd -- file descriptot.
+ *   buf -- pointer to buffer pointer
+ *   sector -- offset of starting sector (in sectors)
+ *   sector_bytes -- bytes per sector
+ *   sector_count -- number of sectors to load
+ *
+ * Returns CGPT_OK for successful. Aborts if any error occurs.
+ */
+static int Load(const int fd, uint8_t **buf,
+                const uint64_t sector,
+                const uint64_t sector_bytes,
+                const uint64_t sector_count) {
+  int count;  /* byte count to read */
+  int nread;
+
+  assert(buf);
+  count = sector_bytes * sector_count;
+  *buf = malloc(count);
+  assert(*buf);
+
+  if (-1 == lseek(fd, sector * sector_bytes, SEEK_SET))
+    goto error_free;
+
+  nread = read(fd, *buf, count);
+  if (nread < count)
+    goto error_free;
+
+  return CGPT_OK;
+
+error_free:
+  free(*buf);
+  *buf = 0;
+  return CGPT_FAILED;
+}
+
+
+int ReadPMBR(struct drive *drive) {
+  if (-1 == lseek(drive->fd, 0, SEEK_SET))
+    return CGPT_FAILED;
+
+  int nread = read(drive->fd, &drive->pmbr, sizeof(struct pmbr));
+  if (nread != sizeof(struct pmbr))
+    return CGPT_FAILED;
+
+  return CGPT_OK;
+}
+
+int WritePMBR(struct drive *drive) {
+  if (-1 == lseek(drive->fd, 0, SEEK_SET))
+    return CGPT_FAILED;
+
+  int nwrote = write(drive->fd, &drive->pmbr, sizeof(struct pmbr));
+  if (nwrote != sizeof(struct pmbr))
+    return CGPT_FAILED;
+
+  return CGPT_OK;
+}
+
+/* Saves sectors to 'fd'.
+ *
+ *   fd -- file descriptot.
+ *   buf -- pointer to buffer
+ *   sector -- starting sector offset
+ *   sector_bytes -- bytes per sector
+ *   sector_count -- number of sector to save
+ *
+ * Returns CGPT_OK for successful, CGPT_FAILED for failed.
+ */
+static int Save(const int fd, const uint8_t *buf,
+                const uint64_t sector,
+                const uint64_t sector_bytes,
+                const uint64_t sector_count) {
+  int count;  /* byte count to write */
+  int nwrote;
+
+  assert(buf);
+  count = sector_bytes * sector_count;
+
+  if (-1 == lseek(fd, sector * sector_bytes, SEEK_SET))
+    return CGPT_FAILED;
+
+  nwrote = write(fd, buf, count);
+  if (nwrote < count)
+    return CGPT_FAILED;
+
+  return CGPT_OK;
+}
+
+
+// Opens a block device or file, loads raw GPT data from it.
+// 
+// Returns CGPT_FAILED if any error happens.
+// Returns CGPT_OK if success and information are stored in 'drive'. */
+int DriveOpen(const char *drive_path, struct drive *drive) {
+  struct stat stat;
+
+  assert(drive_path);
+  assert(drive);
+
+  // Clear struct for proper error handling.
+  memset(drive, 0, sizeof(struct drive));
+
+  drive->fd = open(drive_path, O_RDWR | O_LARGEFILE);
+  if (drive->fd == -1) {
+    Error("Can't open %s: %s\n", drive_path, strerror(errno));
+    return CGPT_FAILED;
+  }
+
+  if (fstat(drive->fd, &stat) == -1) {
+    goto error_close;
+  }
+  if ((stat.st_mode & S_IFMT) != S_IFREG) {
+    if (ioctl(drive->fd, BLKGETSIZE64, &drive->size) < 0) {
+      Error("Can't read drive size from %s: %s\n", drive_path, strerror(errno));
+      goto error_close;
+    }
+    if (ioctl(drive->fd, BLKSSZGET, &drive->gpt.sector_bytes) < 0) {
+      Error("Can't read sector size from %s: %s\n",
+            drive_path, strerror(errno));
+      goto error_close;
+    }
+  } else {
+    drive->gpt.sector_bytes = 512;  /* bytes */
+    drive->size = stat.st_size;
+  }
+  if (drive->size % drive->gpt.sector_bytes) {
+    Error("Media size (%llu) is not a multiple of sector size(%d)\n",
+          (long long unsigned int)drive->size, drive->gpt.sector_bytes);
+    goto error_close;
+  }
+  drive->gpt.drive_sectors = drive->size / drive->gpt.sector_bytes;
+
+  // Read the data.
+  if (CGPT_OK != Load(drive->fd, &drive->gpt.primary_header,
+                      GPT_PMBR_SECTOR,
+                      drive->gpt.sector_bytes, GPT_HEADER_SECTOR)) {
+    goto error_close;
+  }
+  if (CGPT_OK != Load(drive->fd, &drive->gpt.secondary_header,
+                      drive->gpt.drive_sectors - GPT_PMBR_SECTOR,
+                      drive->gpt.sector_bytes, GPT_HEADER_SECTOR)) {
+    goto error_close;
+  }
+  if (CGPT_OK != Load(drive->fd, &drive->gpt.primary_entries,
+                      GPT_PMBR_SECTOR + GPT_HEADER_SECTOR,
+                      drive->gpt.sector_bytes, GPT_ENTRIES_SECTORS)) {
+        goto error_close;
+  }
+  if (CGPT_OK != Load(drive->fd, &drive->gpt.secondary_entries,
+                      drive->gpt.drive_sectors - GPT_HEADER_SECTOR
+                      - GPT_ENTRIES_SECTORS,
+                      drive->gpt.sector_bytes, GPT_ENTRIES_SECTORS)) {
+    goto error_close;
+  }
+    
+  // We just load the data. Caller must validate it.
+  return CGPT_OK;
+
+error_close:
+  (void) DriveClose(drive, 0);
+  return CGPT_FAILED;
+}
+
+
+int DriveClose(struct drive *drive, int update_as_needed) {
+  int errors = 0;
+
+  if (update_as_needed) {
+    if (drive->gpt.modified & GPT_MODIFIED_HEADER1) {
+      if (CGPT_OK != Save(drive->fd, drive->gpt.primary_header,
+                          GPT_PMBR_SECTOR,
+                          drive->gpt.sector_bytes, GPT_HEADER_SECTOR)) {
+        errors++;
+        Error("Cannot write primary header: %s\n", strerror(errno));
+      }
+    }
+      
+    if (drive->gpt.modified & GPT_MODIFIED_HEADER2) {
+      if(CGPT_OK != Save(drive->fd, drive->gpt.secondary_header,
+                         drive->gpt.drive_sectors - GPT_PMBR_SECTOR,
+                         drive->gpt.sector_bytes, GPT_HEADER_SECTOR)) {
+        errors++;
+        Error("Cannot write secondary header: %s\n", strerror(errno));
+      }
+    }
+    if (drive->gpt.modified & GPT_MODIFIED_ENTRIES1) {
+      if (CGPT_OK != Save(drive->fd, drive->gpt.primary_entries,
+                          GPT_PMBR_SECTOR + GPT_HEADER_SECTOR,
+                          drive->gpt.sector_bytes, GPT_ENTRIES_SECTORS)) {
+        errors++;
+        Error("Cannot write primary entries: %s\n", strerror(errno));
+      }
+    }
+    if (drive->gpt.modified & GPT_MODIFIED_ENTRIES2) {
+      if (CGPT_OK != Save(drive->fd, drive->gpt.secondary_entries,
+                          drive->gpt.drive_sectors - GPT_HEADER_SECTOR
+                          - GPT_ENTRIES_SECTORS,
+                          drive->gpt.sector_bytes, GPT_ENTRIES_SECTORS)) {
+        errors++;
+        Error("Cannot write secondary entries: %s\n", strerror(errno));
+      }
+    }
+  }
+
+  close(drive->fd);
+
+  if (drive->gpt.primary_header)
+    free(drive->gpt.primary_header);
+  drive->gpt.primary_header = 0;
+  if (drive->gpt.primary_entries)
+    free(drive->gpt.primary_entries);
+  drive->gpt.primary_entries = 0;
+  if (drive->gpt.secondary_header)
+    free(drive->gpt.secondary_header);
+  drive->gpt.secondary_header = 0;
+  if (drive->gpt.secondary_entries)
+    free(drive->gpt.secondary_entries);
+  drive->gpt.secondary_entries = 0;
+
+  return errors ? CGPT_FAILED : CGPT_OK;
+}
+
+
+
+/* GUID conversion functions. Accepted format:
+ *
+ *   "C12A7328-F81F-11D2-BA4B-00A0C93EC93B"
+ *
+ * Returns CGPT_OK if parsing is successful; otherwise CGPT_FAILED.
+ */
+int StrToGuid(const char *str, Guid *guid) {
+  uint32_t time_low;
+  uint16_t time_mid;
+  uint16_t time_high_and_version;
+  unsigned int chunk[11];
+
+  if (11 != sscanf(str, "%08X-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X",
+                   chunk+0,
+                   chunk+1,
+                   chunk+2,
+                   chunk+3,
+                   chunk+4,
+                   chunk+5,
+                   chunk+6,
+                   chunk+7,
+                   chunk+8,
+                   chunk+9,
+                   chunk+10)) {                   
+    printf("FAILED\n");
+    return CGPT_FAILED;
+  }
+
+  time_low = chunk[0] & 0xffffffff;
+  time_mid = chunk[1] & 0xffff;
+  time_high_and_version = chunk[2] & 0xffff;
+
+  guid->u.Uuid.time_low = htole32(time_low);
+  guid->u.Uuid.time_mid = htole16(time_mid);
+  guid->u.Uuid.time_high_and_version = htole16(time_high_and_version);
+
+  guid->u.Uuid.clock_seq_high_and_reserved = chunk[3] & 0xff;
+  guid->u.Uuid.clock_seq_low = chunk[4] & 0xff;
+  guid->u.Uuid.node[0] = chunk[5] & 0xff;
+  guid->u.Uuid.node[1] = chunk[6] & 0xff;
+  guid->u.Uuid.node[2] = chunk[7] & 0xff;
+  guid->u.Uuid.node[3] = chunk[8] & 0xff;
+  guid->u.Uuid.node[4] = chunk[9] & 0xff;
+  guid->u.Uuid.node[5] = chunk[10] & 0xff;
+
+  return CGPT_OK;
+}
+void GuidToStr(const Guid *guid, char *str) {
+  sprintf(str, "%08X-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X",
+          le32toh(guid->u.Uuid.time_low), le16toh(guid->u.Uuid.time_mid),
+          le16toh(guid->u.Uuid.time_high_and_version),
+          guid->u.Uuid.clock_seq_high_and_reserved, guid->u.Uuid.clock_seq_low,
+          guid->u.Uuid.node[0], guid->u.Uuid.node[1], guid->u.Uuid.node[2],
+          guid->u.Uuid.node[3], guid->u.Uuid.node[4], guid->u.Uuid.node[5]);
+}
+
+/* Convert UTF16 string to UTF8. Rewritten from gpt utility.
+ * Caller must prepare enough space for UTF8. The rough estimation is:
+ *
+ *   utf8 length = bytecount(utf16) * 1.5
+ */
+#define SIZEOF_GPTENTRY_NAME 36  /* sizeof(GptEntry.name[]) */
+void UTF16ToUTF8(const uint16_t *utf16, uint8_t *utf8)
+{
+  size_t s8idx, s16idx, s16len;
+  uint32_t utfchar;
+  unsigned int next_utf16;
+
+  for (s16len = 0; s16len < SIZEOF_GPTENTRY_NAME && utf16[s16len]; ++s16len);
+
+  *utf8 = s8idx = s16idx = 0;
+  while (s16idx < s16len) {
+    utfchar = le16toh(utf16[s16idx++]);
+    if ((utfchar & 0xf800) == 0xd800) {
+      next_utf16 = le16toh(utf16[s16idx]);
+      if ((utfchar & 0x400) != 0 || (next_utf16 & 0xfc00) != 0xdc00)
+        utfchar = 0xfffd;
+      else
+        s16idx++;
+    }
+    if (utfchar < 0x80) {
+      utf8[s8idx++] = utfchar;
+    } else if (utfchar < 0x800) {
+      utf8[s8idx++] = 0xc0 | (utfchar >> 6);
+      utf8[s8idx++] = 0x80 | (utfchar & 0x3f);
+    } else if (utfchar < 0x10000) {
+      utf8[s8idx++] = 0xe0 | (utfchar >> 12);
+      utf8[s8idx++] = 0x80 | ((utfchar >> 6) & 0x3f);
+      utf8[s8idx++] = 0x80 | (utfchar & 0x3f);
+    } else if (utfchar < 0x200000) {
+      utf8[s8idx++] = 0xf0 | (utfchar >> 18);
+      utf8[s8idx++] = 0x80 | ((utfchar >> 12) & 0x3f);
+      utf8[s8idx++] = 0x80 | ((utfchar >> 6) & 0x3f);
+      utf8[s8idx++] = 0x80 | (utfchar & 0x3f);
+    }
+  }
+  utf8[s8idx++] = 0;
+}
+
+/* Convert UTF8 string to UTF16. Rewritten from gpt utility.
+ * Caller must prepare enough space for UTF16. The conservative estimation is:
+ *
+ *   utf16 bytecount = bytecount(utf8) / 3 * 4
+ */
+void UTF8ToUTF16(const uint8_t *utf8, uint16_t *utf16)
+{
+  size_t s16idx, s8idx, s8len;
+  uint32_t utfchar;
+  unsigned int c, utfbytes;
+
+  for (s8len = 0; utf8[s8len]; ++s8len);
+
+  s8idx = s16idx = 0;
+  utfbytes = 0;
+  do {
+    c = utf8[s8idx++];
+    if ((c & 0xc0) != 0x80) {
+      /* Initial characters. */
+      if (utfbytes != 0) {
+        /* Incomplete encoding. */
+        utf16[s16idx++] = 0xfffd;
+      }
+      if ((c & 0xf8) == 0xf0) {
+        utfchar = c & 0x07;
+        utfbytes = 3;
+      } else if ((c & 0xf0) == 0xe0) {
+        utfchar = c & 0x0f;
+        utfbytes = 2;
+      } else if ((c & 0xe0) == 0xc0) {
+        utfchar = c & 0x1f;
+        utfbytes = 1;
+      } else {
+        utfchar = c & 0x7f;
+        utfbytes = 0;
+      }
+    } else {
+      /* Followup characters. */
+      if (utfbytes > 0) {
+        utfchar = (utfchar << 6) + (c & 0x3f);
+        utfbytes--;
+      } else if (utfbytes == 0)
+        utfbytes = -1;
+        utfchar = 0xfffd;
+    }
+    if (utfbytes == 0) {
+      if (utfchar >= 0x10000) {
+        utf16[s16idx++] = htole16(0xd800 | ((utfchar>>10)-0x40));
+        if (s16idx >= SIZEOF_GPTENTRY_NAME) break;
+        utf16[s16idx++] = htole16(0xdc00 | (utfchar & 0x3ff));
+      } else {
+        utf16[s16idx++] = htole16(utfchar);
+      }
+    }
+  } while (c != 0 && s16idx < SIZEOF_GPTENTRY_NAME);
+  if (s16idx < SIZEOF_GPTENTRY_NAME)
+    utf16[s16idx++] = 0;
+}
+
+struct {
+  Guid type;
+  char *name;
+  char *description;
+} supported_types[] = {
+  {GPT_ENT_TYPE_CHROMEOS_KERNEL, "kernel", "ChromeOS kernel"},
+  {GPT_ENT_TYPE_CHROMEOS_ROOTFS, "rootfs", "ChromeOS rootfs"},
+  {GPT_ENT_TYPE_LINUX_DATA, "data", "Linux data"},
+  {GPT_ENT_TYPE_CHROMEOS_RESERVED, "reserved", "ChromeOS reserved"},
+  {GPT_ENT_TYPE_EFI, "efi", "EFI System Partition"},
+  {GPT_ENT_TYPE_UNUSED, "unused", "Unused (nonexistent) partition"},
+};
+
+/* Resolves human-readable GPT type.
+ * Returns CGPT_OK if found.
+ * Returns CGPT_FAILED if no known type found. */
+int ResolveType(const Guid *type, char *buf) {
+  int i;
+  for (i = 0; i < ARRAY_COUNT(supported_types); ++i) {
+    if (!memcmp(type, &supported_types[i].type, sizeof(Guid))) {
+      strcpy(buf, supported_types[i].description);
+      return CGPT_OK;
+    }
+  }
+  return CGPT_FAILED;
+}
+
+int SupportedType(const char *name, Guid *type) {
+  int i;
+  for (i = 0; i < ARRAY_COUNT(supported_types); ++i) {
+    if (!strcmp(name, supported_types[i].name)) {
+      memcpy(type, &supported_types[i].type, sizeof(Guid));
+      return CGPT_OK;
+    }
+  }
+  return CGPT_FAILED;
+}
+
+void PrintTypes(void) {
+  int i;
+  printf("The partition type may also be given as one of these aliases:\n\n");
+  for (i = 0; i < ARRAY_COUNT(supported_types); ++i) {
+    printf("    %-10s  %s\n", supported_types[i].name,
+                          supported_types[i].description);
+  }
+  printf("\n");
+}
+
+uint32_t GetNumberOfEntries(const GptData *gpt) {
+  GptHeader *header = 0;
+  if (gpt->valid_headers & MASK_PRIMARY)
+    header = (GptHeader*)gpt->primary_header;
+  else if (gpt->valid_headers & MASK_SECONDARY)
+    header = (GptHeader*)gpt->secondary_header;
+  else
+    return 0;
+  return header->number_of_entries;
+}
+
+static uint32_t GetSizeOfEntries(const GptData *gpt) {
+  GptHeader *header = 0;
+  if (gpt->valid_headers & MASK_PRIMARY)
+    header = (GptHeader*)gpt->primary_header;
+  else if (gpt->valid_headers & MASK_SECONDARY)
+    header = (GptHeader*)gpt->secondary_header;
+  else
+    return 0;
+  return header->number_of_entries;
+}
+
+GptEntry *GetEntry(GptData *gpt, int secondary, int entry_index) {
+  uint8_t *entries;
+  int stride = GetSizeOfEntries(gpt);
+  if (!stride)
+    return 0;
+
+  if (secondary == PRIMARY) {
+    entries = gpt->primary_entries;
+  } else {
+    entries = gpt->secondary_entries;
+  }
+
+  return (GptEntry*)(&entries[stride * entry_index]);
+}
+
+void SetPriority(GptData *gpt, int secondary, int entry_index, int priority) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+
+  assert(priority >= 0 && priority <= CGPT_ATTRIBUTE_MAX_PRIORITY);
+  entry->attributes &= ~CGPT_ATTRIBUTE_PRIORITY_MASK;
+  entry->attributes |= (uint64_t)priority << CGPT_ATTRIBUTE_PRIORITY_OFFSET;
+}
+
+int GetPriority(GptData *gpt, int secondary, int entry_index) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+  return (entry->attributes & CGPT_ATTRIBUTE_PRIORITY_MASK) >>
+         CGPT_ATTRIBUTE_PRIORITY_OFFSET;
+}
+
+void SetTries(GptData *gpt, int secondary, int entry_index, int tries) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+
+  assert(tries >= 0 && tries <= CGPT_ATTRIBUTE_MAX_TRIES);
+  entry->attributes &= ~CGPT_ATTRIBUTE_TRIES_MASK;
+  entry->attributes |= (uint64_t)tries << CGPT_ATTRIBUTE_TRIES_OFFSET;
+}
+
+int GetTries(GptData *gpt, int secondary, int entry_index) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+  return (entry->attributes & CGPT_ATTRIBUTE_TRIES_MASK) >>
+         CGPT_ATTRIBUTE_TRIES_OFFSET;
+}
+
+void SetSuccessful(GptData *gpt, int secondary, int entry_index, int success) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+
+  assert(success >= 0 && success <= CGPT_ATTRIBUTE_MAX_SUCCESSFUL);
+  entry->attributes &= ~CGPT_ATTRIBUTE_SUCCESSFUL_MASK;
+  entry->attributes |= (uint64_t)success << CGPT_ATTRIBUTE_SUCCESSFUL_OFFSET;
+}
+
+int GetSuccessful(GptData *gpt, int secondary, int entry_index) {
+  GptEntry *entry;
+  entry = GetEntry(gpt, secondary, entry_index);
+  return (entry->attributes & CGPT_ATTRIBUTE_SUCCESSFUL_MASK) >>
+         CGPT_ATTRIBUTE_SUCCESSFUL_OFFSET;
+}
+
+
+#define TOSTRING(A) #A
+const char *GptError(int errnum) {
+  const char *error_string[] = {
+    TOSTRING(GPT_SUCCESS),
+    TOSTRING(GPT_ERROR_NO_VALID_KERNEL),
+    TOSTRING(GPT_ERROR_INVALID_HEADERS),
+    TOSTRING(GPT_ERROR_INVALID_ENTRIES),
+    TOSTRING(GPT_ERROR_INVALID_SECTOR_SIZE),
+    TOSTRING(GPT_ERROR_INVALID_SECTOR_NUMBER),
+    TOSTRING(GPT_ERROR_INVALID_UPDATE_TYPE)
+  };
+  if (errnum < 0 || errnum >= ARRAY_COUNT(error_string))
+    return "<illegal value>";
+  return error_string[errnum];
+}
+
+/*  Update CRC value if necessary.  */
+void UpdateCrc(GptData *gpt) {
+  GptHeader *primary_header, *secondary_header;
+
+  primary_header = (GptHeader*)gpt->primary_header;
+  secondary_header = (GptHeader*)gpt->secondary_header;
+
+  if (gpt->modified & GPT_MODIFIED_ENTRIES1) {
+    primary_header->entries_crc32 =
+        Crc32(gpt->primary_entries, TOTAL_ENTRIES_SIZE);
+  }
+  if (gpt->modified & GPT_MODIFIED_ENTRIES2) {
+    secondary_header->entries_crc32 =
+        Crc32(gpt->secondary_entries, TOTAL_ENTRIES_SIZE);
+  }
+  if (gpt->modified & GPT_MODIFIED_HEADER1) {
+    primary_header->header_crc32 = 0;
+    primary_header->header_crc32 = Crc32(
+        (const uint8_t *)primary_header, primary_header->size);
+  }
+  if (gpt->modified & GPT_MODIFIED_HEADER2) {
+    secondary_header->header_crc32 = 0;
+    secondary_header->header_crc32 = Crc32(
+        (const uint8_t *)secondary_header, secondary_header->size);
+  }
+}
+/* Two headers are NOT bitwise identical. For example, my_lba pointers to header
+ * itself so that my_lba in primary and secondary is definitely different.
+ * Only the following fields should be identical.
+ *
+ *   first_usable_lba
+ *   last_usable_lba
+ *   number_of_entries
+ *   size_of_entry
+ *   disk_uuid
+ *
+ * If any of above field are not matched, overwrite secondary with primary since
+ * we always trust primary.
+ * If any one of header is invalid, copy from another. */
+int IsSynonymous(const GptHeader* a, const GptHeader* b) {
+  if ((a->first_usable_lba == b->first_usable_lba) &&
+      (a->last_usable_lba == b->last_usable_lba) &&
+      (a->number_of_entries == b->number_of_entries) &&
+      (a->size_of_entry == b->size_of_entry) &&
+      (!memcmp(&a->disk_uuid, &b->disk_uuid, sizeof(Guid))))
+    return 1;
+  return 0;
+}
+
+/* Primary entries and secondary entries should be bitwise identical.
+ * If two entries tables are valid, compare them. If not the same,
+ * overwrites secondary with primary (primary always has higher priority),
+ * and marks secondary as modified.
+ * If only one is valid, overwrites invalid one.
+ * If all are invalid, does nothing.
+ * This function returns bit masks for GptData.modified field.
+ * Note that CRC is NOT re-computed in this function.
+ */
+uint8_t RepairEntries(GptData *gpt, const uint32_t valid_entries) {
+  if (valid_entries == MASK_BOTH) {
+    if (memcmp(gpt->primary_entries, gpt->secondary_entries,
+               TOTAL_ENTRIES_SIZE)) {
+      memcpy(gpt->secondary_entries, gpt->primary_entries, TOTAL_ENTRIES_SIZE);
+      return GPT_MODIFIED_ENTRIES2;
+    }
+  } else if (valid_entries == MASK_PRIMARY) {
+    memcpy(gpt->secondary_entries, gpt->primary_entries, TOTAL_ENTRIES_SIZE);
+    return GPT_MODIFIED_ENTRIES2;
+  } else if (valid_entries == MASK_SECONDARY) {
+    memcpy(gpt->primary_entries, gpt->secondary_entries, TOTAL_ENTRIES_SIZE);
+    return GPT_MODIFIED_ENTRIES1;
+  }
+
+  return 0;
+}
+
+/* The above five fields are shared between primary and secondary headers.
+ * We can recover one header from another through copying those fields. */
+void CopySynonymousParts(GptHeader* target, const GptHeader* source) {
+  target->first_usable_lba = source->first_usable_lba;
+  target->last_usable_lba = source->last_usable_lba;
+  target->number_of_entries = source->number_of_entries;
+  target->size_of_entry = source->size_of_entry;
+  memcpy(&target->disk_uuid, &source->disk_uuid, sizeof(Guid));
+}
+
+/* This function repairs primary and secondary headers if possible.
+ * If both headers are valid (CRC32 is correct) but
+ *   a) indicate inconsistent usable LBA ranges,
+ *   b) inconsistent partition entry size and number,
+ *   c) inconsistent disk_uuid,
+ * we will use the primary header to overwrite secondary header.
+ * If primary is invalid (CRC32 is wrong), then we repair it from secondary.
+ * If secondary is invalid (CRC32 is wrong), then we repair it from primary.
+ * This function returns the bitmasks for modified header.
+ * Note that CRC value is NOT re-computed in this function. UpdateCrc() will
+ * do it later.
+ */
+uint8_t RepairHeader(GptData *gpt, const uint32_t valid_headers) {
+  GptHeader *primary_header, *secondary_header;
+
+  primary_header = (GptHeader*)gpt->primary_header;
+  secondary_header = (GptHeader*)gpt->secondary_header;
+
+  if (valid_headers == MASK_BOTH) {
+    if (!IsSynonymous(primary_header, secondary_header)) {
+      CopySynonymousParts(secondary_header, primary_header);
+      return GPT_MODIFIED_HEADER2;
+    }
+  } else if (valid_headers == MASK_PRIMARY) {
+    memcpy(secondary_header, primary_header, primary_header->size);
+    secondary_header->my_lba = gpt->drive_sectors - 1;  /* the last sector */
+    secondary_header->alternate_lba = primary_header->my_lba;
+    secondary_header->entries_lba = secondary_header->my_lba -
+        GPT_ENTRIES_SECTORS;
+    return GPT_MODIFIED_HEADER2;
+  } else if (valid_headers == MASK_SECONDARY) {
+    memcpy(primary_header, secondary_header, secondary_header->size);
+    primary_header->my_lba = GPT_PMBR_SECTOR;  /* the second sector on drive */
+    primary_header->alternate_lba = secondary_header->my_lba;
+    primary_header->entries_lba = primary_header->my_lba + GPT_HEADER_SECTOR;
+    return GPT_MODIFIED_HEADER1;
+  }
+
+  return 0;
+}
+
+
+int IsZero(const Guid *gp) {
+  return (0 == memcmp(gp, &guid_unused, sizeof(Guid)));
+}
+
+void PMBRToStr(struct pmbr *pmbr, char *str) {
+  char buf[256];
+  if (IsZero(&pmbr->boot_guid)) {
+    sprintf(str, "PMBR");
+  } else {
+    GuidToStr(&pmbr->boot_guid, buf);
+    sprintf(str, "PMBR (Boot GUID: %s)", buf);
+  }
+}
+