block/qcow2-snapshot.c

   1 /*
   2  * Block driver for the QCOW version 2 format
   3  *
   4  * Copyright (c) 2004-2006 Fabrice Bellard
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24
  25 #include "qemu-common.h"
  26 #include "block/block_int.h"
  27 #include "block/qcow2.h"
  28
  29 typedef struct QEMU_PACKED QCowSnapshotHeader {
  30     /* header is 8 byte aligned */
  31     uint64_t l1_table_offset;
  32
  33     uint32_t l1_size;
  34     uint16_t id_str_size;
  35     uint16_t name_size;
  36
  37     uint32_t date_sec;
  38     uint32_t date_nsec;
  39
  40     uint64_t vm_clock_nsec;
  41
  42     uint32_t vm_state_size;
  43     uint32_t extra_data_size; /* for extension */
  44     /* extra data follows */
  45     /* id_str follows */
  46     /* name follows  */
  47 } QCowSnapshotHeader;
  48
  49 typedef struct QEMU_PACKED QCowSnapshotExtraData {
  50     uint64_t vm_state_size_large;
  51     uint64_t disk_size;
  52 } QCowSnapshotExtraData;
  53
  54 void qcow2_free_snapshots(BlockDriverState *bs)
  55 {
  56     BDRVQcowState *s = bs->opaque;
  57     int i;
  58
  59     for(i = 0; i < s->nb_snapshots; i++) {
  60         g_free(s->snapshots[i].name);
  61         g_free(s->snapshots[i].id_str);
  62     }
  63     g_free(s->snapshots);
  64     s->snapshots = NULL;
  65     s->nb_snapshots = 0;
  66 }
  67
  68 int qcow2_read_snapshots(BlockDriverState *bs)
  69 {
  70     BDRVQcowState *s = bs->opaque;
  71     QCowSnapshotHeader h;
  72     QCowSnapshotExtraData extra;
  73     QCowSnapshot *sn;
  74     int i, id_str_size, name_size;
  75     int64_t offset;
  76     uint32_t extra_data_size;
  77     int ret;
  78
  79     if (!s->nb_snapshots) {
  80         s->snapshots = NULL;
  81         s->snapshots_size = 0;
  82         return 0;
  83     }
  84
  85     offset = s->snapshots_offset;
  86     s->snapshots = g_malloc0(s->nb_snapshots * sizeof(QCowSnapshot));
  87
  88     for(i = 0; i < s->nb_snapshots; i++) {
  89         /* Read statically sized part of the snapshot header */
  90         offset = align_offset(offset, 8);
  91         ret = bdrv_pread(bs->file, offset, &h, sizeof(h));
  92         if (ret < 0) {
  93             goto fail;
  94         }
  95
  96         offset += sizeof(h);
  97         sn = s->snapshots + i;
  98         sn->l1_table_offset = be64_to_cpu(h.l1_table_offset);
  99         sn->l1_size = be32_to_cpu(h.l1_size);
 100         sn->vm_state_size = be32_to_cpu(h.vm_state_size);
 101         sn->date_sec = be32_to_cpu(h.date_sec);
 102         sn->date_nsec = be32_to_cpu(h.date_nsec);
 103         sn->vm_clock_nsec = be64_to_cpu(h.vm_clock_nsec);
 104         extra_data_size = be32_to_cpu(h.extra_data_size);
 105
 106         id_str_size = be16_to_cpu(h.id_str_size);
 107         name_size = be16_to_cpu(h.name_size);
 108
 109         /* Read extra data */
 110         ret = bdrv_pread(bs->file, offset, &extra,
 111                          MIN(sizeof(extra), extra_data_size));
 112         if (ret < 0) {
 113             goto fail;
 114         }
 115         offset += extra_data_size;
 116
 117         if (extra_data_size >= 8) {
 118             sn->vm_state_size = be64_to_cpu(extra.vm_state_size_large);
 119         }
 120
 121         if (extra_data_size >= 16) {
 122             sn->disk_size = be64_to_cpu(extra.disk_size);
 123         } else {
 124             sn->disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
 125         }
 126
 127         /* Read snapshot ID */
 128         sn->id_str = g_malloc(id_str_size + 1);
 129         ret = bdrv_pread(bs->file, offset, sn->id_str, id_str_size);
 130         if (ret < 0) {
 131             goto fail;
 132         }
 133         offset += id_str_size;
 134         sn->id_str[id_str_size] = '\0';
 135
 136         /* Read snapshot name */
 137         sn->name = g_malloc(name_size + 1);
 138         ret = bdrv_pread(bs->file, offset, sn->name, name_size);
 139         if (ret < 0) {
 140             goto fail;
 141         }
 142         offset += name_size;
 143         sn->name[name_size] = '\0';
 144     }
 145
 146     s->snapshots_size = offset - s->snapshots_offset;
 147     return 0;
 148
 149 fail:
 150     qcow2_free_snapshots(bs);
 151     return ret;
 152 }
 153
 154 /* add at the end of the file a new list of snapshots */
 155 static int qcow2_write_snapshots(BlockDriverState *bs)
 156 {
 157     BDRVQcowState *s = bs->opaque;
 158     QCowSnapshot *sn;
 159     QCowSnapshotHeader h;
 160     QCowSnapshotExtraData extra;
 161     int i, name_size, id_str_size, snapshots_size;
 162     struct {
 163         uint32_t nb_snapshots;
 164         uint64_t snapshots_offset;
 165     } QEMU_PACKED header_data;
 166     int64_t offset, snapshots_offset;
 167     int ret;
 168
 169     /* compute the size of the snapshots */
 170     offset = 0;
 171     for(i = 0; i < s->nb_snapshots; i++) {
 172         sn = s->snapshots + i;
 173         offset = align_offset(offset, 8);
 174         offset += sizeof(h);
 175         offset += sizeof(extra);
 176         offset += strlen(sn->id_str);
 177         offset += strlen(sn->name);
 178     }
 179     snapshots_size = offset;
 180
 181     /* Allocate space for the new snapshot list */
 182     snapshots_offset = qcow2_alloc_clusters(bs, snapshots_size);
 183     offset = snapshots_offset;
 184     if (offset < 0) {
 185         return offset;
 186     }
 187     ret = bdrv_flush(bs);
 188     if (ret < 0) {
 189         return ret;
 190     }
 191
 192     /* The snapshot list position has not yet been updated, so these clusters
 193      * must indeed be completely free */
 194     ret = qcow2_pre_write_overlap_check(bs, QCOW2_OL_DEFAULT, offset,
 195                                         s->snapshots_size);
 196     if (ret < 0) {
 197         return ret;
 198     }
 199
 200
 201     /* Write all snapshots to the new list */
 202     for(i = 0; i < s->nb_snapshots; i++) {
 203         sn = s->snapshots + i;
 204         memset(&h, 0, sizeof(h));
 205         h.l1_table_offset = cpu_to_be64(sn->l1_table_offset);
 206         h.l1_size = cpu_to_be32(sn->l1_size);
 207         /* If it doesn't fit in 32 bit, older implementations should treat it
 208          * as a disk-only snapshot rather than truncate the VM state */
 209         if (sn->vm_state_size <= 0xffffffff) {
 210             h.vm_state_size = cpu_to_be32(sn->vm_state_size);
 211         }
 212         h.date_sec = cpu_to_be32(sn->date_sec);
 213         h.date_nsec = cpu_to_be32(sn->date_nsec);
 214         h.vm_clock_nsec = cpu_to_be64(sn->vm_clock_nsec);
 215         h.extra_data_size = cpu_to_be32(sizeof(extra));
 216
 217         memset(&extra, 0, sizeof(extra));
 218         extra.vm_state_size_large = cpu_to_be64(sn->vm_state_size);
 219         extra.disk_size = cpu_to_be64(sn->disk_size);
 220
 221         id_str_size = strlen(sn->id_str);
 222         name_size = strlen(sn->name);
 223         h.id_str_size = cpu_to_be16(id_str_size);
 224         h.name_size = cpu_to_be16(name_size);
 225         offset = align_offset(offset, 8);
 226
 227         ret = bdrv_pwrite(bs->file, offset, &h, sizeof(h));
 228         if (ret < 0) {
 229             goto fail;
 230         }
 231         offset += sizeof(h);
 232
 233         ret = bdrv_pwrite(bs->file, offset, &extra, sizeof(extra));
 234         if (ret < 0) {
 235             goto fail;
 236         }
 237         offset += sizeof(extra);
 238
 239         ret = bdrv_pwrite(bs->file, offset, sn->id_str, id_str_size);
 240         if (ret < 0) {
 241             goto fail;
 242         }
 243         offset += id_str_size;
 244
 245         ret = bdrv_pwrite(bs->file, offset, sn->name, name_size);
 246         if (ret < 0) {
 247             goto fail;
 248         }
 249         offset += name_size;
 250     }
 251
 252     /*
 253      * Update the header to point to the new snapshot table. This requires the
 254      * new table and its refcounts to be stable on disk.
 255      */
 256     ret = bdrv_flush(bs);
 257     if (ret < 0) {
 258         goto fail;
 259     }
 260
 261     QEMU_BUILD_BUG_ON(offsetof(QCowHeader, snapshots_offset) !=
 262         offsetof(QCowHeader, nb_snapshots) + sizeof(header_data.nb_snapshots));
 263
 264     header_data.nb_snapshots        = cpu_to_be32(s->nb_snapshots);
 265     header_data.snapshots_offset    = cpu_to_be64(snapshots_offset);
 266
 267     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, nb_snapshots),
 268                            &header_data, sizeof(header_data));
 269     if (ret < 0) {
 270         goto fail;
 271     }
 272
 273     /* free the old snapshot table */
 274     qcow2_free_clusters(bs, s->snapshots_offset, s->snapshots_size,
 275                         QCOW2_DISCARD_SNAPSHOT);
 276     s->snapshots_offset = snapshots_offset;
 277     s->snapshots_size = snapshots_size;
 278     return 0;
 279
 280 fail:
 281     return ret;
 282 }
 283
 284 static void find_new_snapshot_id(BlockDriverState *bs,
 285                                  char *id_str, int id_str_size)
 286 {
 287     BDRVQcowState *s = bs->opaque;
 288     QCowSnapshot *sn;
 289     int i, id, id_max = 0;
 290
 291     for(i = 0; i < s->nb_snapshots; i++) {
 292         sn = s->snapshots + i;
 293         id = strtoul(sn->id_str, NULL, 10);
 294         if (id > id_max)
 295             id_max = id;
 296     }
 297     snprintf(id_str, id_str_size, "%d", id_max + 1);
 298 }
 299
 300 static int find_snapshot_by_id(BlockDriverState *bs, const char *id_str)
 301 {
 302     BDRVQcowState *s = bs->opaque;
 303     int i;
 304
 305     for(i = 0; i < s->nb_snapshots; i++) {
 306         if (!strcmp(s->snapshots[i].id_str, id_str))
 307             return i;
 308     }
 309     return -1;
 310 }
 311
 312 static int find_snapshot_by_id_or_name(BlockDriverState *bs, const char *name)
 313 {
 314     BDRVQcowState *s = bs->opaque;
 315     int i, ret;
 316
 317     ret = find_snapshot_by_id(bs, name);
 318     if (ret >= 0)
 319         return ret;
 320     for(i = 0; i < s->nb_snapshots; i++) {
 321         if (!strcmp(s->snapshots[i].name, name))
 322             return i;
 323     }
 324     return -1;
 325 }
 326
 327 /* if no id is provided, a new one is constructed */
 328 int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
 329 {
 330     BDRVQcowState *s = bs->opaque;
 331     QCowSnapshot *new_snapshot_list = NULL;
 332     QCowSnapshot *old_snapshot_list = NULL;
 333     QCowSnapshot sn1, *sn = &sn1;
 334     int i, ret;
 335     uint64_t *l1_table = NULL;
 336     int64_t l1_table_offset;
 337
 338     memset(sn, 0, sizeof(*sn));
 339
 340     /* Generate an ID if it wasn't passed */
 341     if (sn_info->id_str[0] == '\0') {
 342         find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str));
 343     }
 344
 345     /* Check that the ID is unique */
 346     if (find_snapshot_by_id(bs, sn_info->id_str) >= 0) {
 347         return -EEXIST;
 348     }
 349
 350     /* Populate sn with passed data */
 351     sn->id_str = g_strdup(sn_info->id_str);
 352     sn->name = g_strdup(sn_info->name);
 353
 354     sn->disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
 355     sn->vm_state_size = sn_info->vm_state_size;
 356     sn->date_sec = sn_info->date_sec;
 357     sn->date_nsec = sn_info->date_nsec;
 358     sn->vm_clock_nsec = sn_info->vm_clock_nsec;
 359
 360     /* Allocate the L1 table of the snapshot and copy the current one there. */
 361     l1_table_offset = qcow2_alloc_clusters(bs, s->l1_size * sizeof(uint64_t));
 362     if (l1_table_offset < 0) {
 363         ret = l1_table_offset;
 364         goto fail;
 365     }
 366
 367     sn->l1_table_offset = l1_table_offset;
 368     sn->l1_size = s->l1_size;
 369
 370     l1_table = g_malloc(s->l1_size * sizeof(uint64_t));
 371     for(i = 0; i < s->l1_size; i++) {
 372         l1_table[i] = cpu_to_be64(s->l1_table[i]);
 373     }
 374
 375     ret = qcow2_pre_write_overlap_check(bs, QCOW2_OL_DEFAULT,
 376             sn->l1_table_offset, s->l1_size * sizeof(uint64_t));
 377     if (ret < 0) {
 378         goto fail;
 379     }
 380
 381     ret = bdrv_pwrite(bs->file, sn->l1_table_offset, l1_table,
 382                       s->l1_size * sizeof(uint64_t));
 383     if (ret < 0) {
 384         goto fail;
 385     }
 386
 387     g_free(l1_table);
 388     l1_table = NULL;
 389
 390     /*
 391      * Increase the refcounts of all clusters and make sure everything is
 392      * stable on disk before updating the snapshot table to contain a pointer
 393      * to the new L1 table.
 394      */
 395     ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 1);
 396     if (ret < 0) {
 397         goto fail;
 398     }
 399
 400     /* Append the new snapshot to the snapshot list */
 401     new_snapshot_list = g_malloc((s->nb_snapshots + 1) * sizeof(QCowSnapshot));
 402     if (s->snapshots) {
 403         memcpy(new_snapshot_list, s->snapshots,
 404                s->nb_snapshots * sizeof(QCowSnapshot));
 405         old_snapshot_list = s->snapshots;
 406     }
 407     s->snapshots = new_snapshot_list;
 408     s->snapshots[s->nb_snapshots++] = *sn;
 409
 410     ret = qcow2_write_snapshots(bs);
 411     if (ret < 0) {
 412         g_free(s->snapshots);
 413         s->snapshots = old_snapshot_list;
 414         goto fail;
 415     }
 416
 417     g_free(old_snapshot_list);
 418
 419 #ifdef DEBUG_ALLOC
 420     {
 421       BdrvCheckResult result = {0};
 422       qcow2_check_refcounts(bs, &result, 0);
 423     }
 424 #endif
 425     return 0;
 426
 427 fail:
 428     g_free(sn->id_str);
 429     g_free(sn->name);
 430     g_free(l1_table);
 431
 432     return ret;
 433 }
 434
 435 /* copy the snapshot 'snapshot_name' into the current disk image */
 436 int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
 437 {
 438     BDRVQcowState *s = bs->opaque;
 439     QCowSnapshot *sn;
 440     int i, snapshot_index;
 441     int cur_l1_bytes, sn_l1_bytes;
 442     int ret;
 443     uint64_t *sn_l1_table = NULL;
 444
 445     /* Search the snapshot */
 446     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
 447     if (snapshot_index < 0) {
 448         return -ENOENT;
 449     }
 450     sn = &s->snapshots[snapshot_index];
 451
 452     if (sn->disk_size != bs->total_sectors * BDRV_SECTOR_SIZE) {
 453         error_report("qcow2: Loading snapshots with different disk "
 454             "size is not implemented");
 455         ret = -ENOTSUP;
 456         goto fail;
 457     }
 458
 459     /*
 460      * Make sure that the current L1 table is big enough to contain the whole
 461      * L1 table of the snapshot. If the snapshot L1 table is smaller, the
 462      * current one must be padded with zeros.
 463      */
 464     ret = qcow2_grow_l1_table(bs, sn->l1_size, true);
 465     if (ret < 0) {
 466         goto fail;
 467     }
 468
 469     cur_l1_bytes = s->l1_size * sizeof(uint64_t);
 470     sn_l1_bytes = sn->l1_size * sizeof(uint64_t);
 471
 472     /*
 473      * Copy the snapshot L1 table to the current L1 table.
 474      *
 475      * Before overwriting the old current L1 table on disk, make sure to
 476      * increase all refcounts for the clusters referenced by the new one.
 477      * Decrease the refcount referenced by the old one only when the L1
 478      * table is overwritten.
 479      */
 480     sn_l1_table = g_malloc0(cur_l1_bytes);
 481
 482     ret = bdrv_pread(bs->file, sn->l1_table_offset, sn_l1_table, sn_l1_bytes);
 483     if (ret < 0) {
 484         goto fail;
 485     }
 486
 487     ret = qcow2_update_snapshot_refcount(bs, sn->l1_table_offset,
 488                                          sn->l1_size, 1);
 489     if (ret < 0) {
 490         goto fail;
 491     }
 492
 493     ret = qcow2_pre_write_overlap_check(bs,
 494             QCOW2_OL_DEFAULT & ~QCOW2_OL_ACTIVE_L1,
 495             s->l1_table_offset, cur_l1_bytes);
 496     if (ret < 0) {
 497         goto fail;
 498     }
 499
 500     ret = bdrv_pwrite_sync(bs->file, s->l1_table_offset, sn_l1_table,
 501                            cur_l1_bytes);
 502     if (ret < 0) {
 503         goto fail;
 504     }
 505
 506     /*
 507      * Decrease refcount of clusters of current L1 table.
 508      *
 509      * At this point, the in-memory s->l1_table points to the old L1 table,
 510      * whereas on disk we already have the new one.
 511      *
 512      * qcow2_update_snapshot_refcount special cases the current L1 table to use
 513      * the in-memory data instead of really using the offset to load a new one,
 514      * which is why this works.
 515      */
 516     ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset,
 517                                          s->l1_size, -1);
 518
 519     /*
 520      * Now update the in-memory L1 table to be in sync with the on-disk one. We
 521      * need to do this even if updating refcounts failed.
 522      */
 523     for(i = 0;i < s->l1_size; i++) {
 524         s->l1_table[i] = be64_to_cpu(sn_l1_table[i]);
 525     }
 526
 527     if (ret < 0) {
 528         goto fail;
 529     }
 530
 531     g_free(sn_l1_table);
 532     sn_l1_table = NULL;
 533
 534     /*
 535      * Update QCOW_OFLAG_COPIED in the active L1 table (it may have changed
 536      * when we decreased the refcount of the old snapshot.
 537      */
 538     ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
 539     if (ret < 0) {
 540         goto fail;
 541     }
 542
 543 #ifdef DEBUG_ALLOC
 544     {
 545         BdrvCheckResult result = {0};
 546         qcow2_check_refcounts(bs, &result, 0);
 547     }
 548 #endif
 549     return 0;
 550
 551 fail:
 552     g_free(sn_l1_table);
 553     return ret;
 554 }
 555
 556 int qcow2_snapshot_delete(BlockDriverState *bs, const char *snapshot_id)
 557 {
 558     BDRVQcowState *s = bs->opaque;
 559     QCowSnapshot sn;
 560     int snapshot_index, ret;
 561
 562     /* Search the snapshot */
 563     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
 564     if (snapshot_index < 0) {
 565         return -ENOENT;
 566     }
 567     sn = s->snapshots[snapshot_index];
 568
 569     /* Remove it from the snapshot list */
 570     memmove(s->snapshots + snapshot_index,
 571             s->snapshots + snapshot_index + 1,
 572             (s->nb_snapshots - snapshot_index - 1) * sizeof(sn));
 573     s->nb_snapshots--;
 574     ret = qcow2_write_snapshots(bs);
 575     if (ret < 0) {
 576         return ret;
 577     }
 578
 579     /*
 580      * The snapshot is now unused, clean up. If we fail after this point, we
 581      * won't recover but just leak clusters.
 582      */
 583     g_free(sn.id_str);
 584     g_free(sn.name);
 585
 586     /*
 587      * Now decrease the refcounts of clusters referenced by the snapshot and
 588      * free the L1 table.
 589      */
 590     ret = qcow2_update_snapshot_refcount(bs, sn.l1_table_offset,
 591                                          sn.l1_size, -1);
 592     if (ret < 0) {
 593         return ret;
 594     }
 595     qcow2_free_clusters(bs, sn.l1_table_offset, sn.l1_size * sizeof(uint64_t),
 596                         QCOW2_DISCARD_SNAPSHOT);
 597
 598     /* must update the copied flag on the current cluster offsets */
 599     ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
 600     if (ret < 0) {
 601         return ret;
 602     }
 603
 604 #ifdef DEBUG_ALLOC
 605     {
 606         BdrvCheckResult result = {0};
 607         qcow2_check_refcounts(bs, &result, 0);
 608     }
 609 #endif
 610     return 0;
 611 }
 612
 613 int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab)
 614 {
 615     BDRVQcowState *s = bs->opaque;
 616     QEMUSnapshotInfo *sn_tab, *sn_info;
 617     QCowSnapshot *sn;
 618     int i;
 619
 620     if (!s->nb_snapshots) {
 621         *psn_tab = NULL;
 622         return s->nb_snapshots;
 623     }
 624
 625     sn_tab = g_malloc0(s->nb_snapshots * sizeof(QEMUSnapshotInfo));
 626     for(i = 0; i < s->nb_snapshots; i++) {
 627         sn_info = sn_tab + i;
 628         sn = s->snapshots + i;
 629         pstrcpy(sn_info->id_str, sizeof(sn_info->id_str),
 630                 sn->id_str);
 631         pstrcpy(sn_info->name, sizeof(sn_info->name),
 632                 sn->name);
 633         sn_info->vm_state_size = sn->vm_state_size;
 634         sn_info->date_sec = sn->date_sec;
 635         sn_info->date_nsec = sn->date_nsec;
 636         sn_info->vm_clock_nsec = sn->vm_clock_nsec;
 637     }
 638     *psn_tab = sn_tab;
 639     return s->nb_snapshots;
 640 }
 641
 642 int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name)
 643 {
 644     int i, snapshot_index;
 645     BDRVQcowState *s = bs->opaque;
 646     QCowSnapshot *sn;
 647     uint64_t *new_l1_table;
 648     int new_l1_bytes;
 649     int ret;
 650
 651     assert(bs->read_only);
 652
 653     /* Search the snapshot */
 654     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_name);
 655     if (snapshot_index < 0) {
 656         return -ENOENT;
 657     }
 658     sn = &s->snapshots[snapshot_index];
 659
 660     /* Allocate and read in the snapshot's L1 table */
 661     new_l1_bytes = s->l1_size * sizeof(uint64_t);
 662     new_l1_table = g_malloc0(align_offset(new_l1_bytes, 512));
 663
 664     ret = bdrv_pread(bs->file, sn->l1_table_offset, new_l1_table, new_l1_bytes);
 665     if (ret < 0) {
 666         g_free(new_l1_table);
 667         return ret;
 668     }
 669
 670     /* Switch the L1 table */
 671     g_free(s->l1_table);
 672
 673     s->l1_size = sn->l1_size;
 674     s->l1_table_offset = sn->l1_table_offset;
 675     s->l1_table = new_l1_table;
 676
 677     for(i = 0;i < s->l1_size; i++) {
 678         be64_to_cpus(&s->l1_table[i]);
 679     }
 680
 681     return 0;
 682 }