nbd/client.c

   1 /*
   2  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
   3  *
   4  *  Network Block Device Client Side
   5  *
   6  *  This program is free software; you can redistribute it and/or modify
   7  *  it under the terms of the GNU General Public License as published by
   8  *  the Free Software Foundation; under version 2 of the License.
   9  *
  10  *  This program is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  *  GNU General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU General Public License
  16  *  along with this program; if not, see <http://www.gnu.org/licenses/>.
  17  */
  18
  19 #include "qemu/osdep.h"
  20 #include "qapi/error.h"
  21 #include "nbd-internal.h"
  22
  23 static int nbd_errno_to_system_errno(int err)
  24 {
  25     switch (err) {
  26     case NBD_SUCCESS:
  27         return 0;
  28     case NBD_EPERM:
  29         return EPERM;
  30     case NBD_EIO:
  31         return EIO;
  32     case NBD_ENOMEM:
  33         return ENOMEM;
  34     case NBD_ENOSPC:
  35         return ENOSPC;
  36     case NBD_EINVAL:
  37     default:
  38         return EINVAL;
  39     }
  40 }
  41
  42 /* Definitions for opaque data types */
  43
  44 static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
  45
  46 /* That's all folks */
  47
  48 /* Basic flow for negotiation
  49
  50    Server         Client
  51    Negotiate
  52
  53    or
  54
  55    Server         Client
  56    Negotiate #1
  57                   Option
  58    Negotiate #2
  59
  60    ----
  61
  62    followed by
  63
  64    Server         Client
  65                   Request
  66    Response
  67                   Request
  68    Response
  69                   ...
  70    ...
  71                   Request (type == 2)
  72
  73 */
  74
  75
  76 /* If type represents success, return 1 without further action.
  77  * If type represents an error reply, consume the rest of the packet on ioc.
  78  * Then return 0 for unsupported (so the client can fall back to
  79  * other approaches), or -1 with errp set for other errors.
  80  */
  81 static int nbd_handle_reply_err(QIOChannel *ioc, uint32_t opt, uint32_t type,
  82                                 Error **errp)
  83 {
  84     uint32_t len;
  85     char *msg = NULL;
  86     int result = -1;
  87
  88     if (!(type & (1 << 31))) {
  89         return 1;
  90     }
  91
  92     if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
  93         error_setg(errp, "failed to read option length");
  94         return -1;
  95     }
  96     len = be32_to_cpu(len);
  97     if (len) {
  98         if (len > NBD_MAX_BUFFER_SIZE) {
  99             error_setg(errp, "server's error message is too long");
 100             goto cleanup;
 101         }
 102         msg = g_malloc(len + 1);
 103         if (read_sync(ioc, msg, len) != len) {
 104             error_setg(errp, "failed to read option error message");
 105             goto cleanup;
 106         }
 107         msg[len] = '\0';
 108     }
 109
 110     switch (type) {
 111     case NBD_REP_ERR_UNSUP:
 112         TRACE("server doesn't understand request %d, attempting fallback",
 113               opt);
 114         result = 0;
 115         goto cleanup;
 116
 117     case NBD_REP_ERR_POLICY:
 118         error_setg(errp, "Denied by server for option %x", opt);
 119         break;
 120
 121     case NBD_REP_ERR_INVALID:
 122         error_setg(errp, "Invalid data length for option %x", opt);
 123         break;
 124
 125     case NBD_REP_ERR_TLS_REQD:
 126         error_setg(errp, "TLS negotiation required before option %x", opt);
 127         break;
 128
 129     default:
 130         error_setg(errp, "Unknown error code when asking for option %x", opt);
 131         break;
 132     }
 133
 134     if (msg) {
 135         error_append_hint(errp, "%s\n", msg);
 136     }
 137
 138  cleanup:
 139     g_free(msg);
 140     return result;
 141 }
 142
 143 static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
 144 {
 145     uint64_t magic;
 146     uint32_t opt;
 147     uint32_t type;
 148     uint32_t len;
 149     uint32_t namelen;
 150     int error;
 151
 152     *name = NULL;
 153     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 154         error_setg(errp, "failed to read list option magic");
 155         return -1;
 156     }
 157     magic = be64_to_cpu(magic);
 158     if (magic != NBD_REP_MAGIC) {
 159         error_setg(errp, "Unexpected option list magic");
 160         return -1;
 161     }
 162     if (read_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 163         error_setg(errp, "failed to read list option");
 164         return -1;
 165     }
 166     opt = be32_to_cpu(opt);
 167     if (opt != NBD_OPT_LIST) {
 168         error_setg(errp, "Unexpected option type %x expected %x",
 169                    opt, NBD_OPT_LIST);
 170         return -1;
 171     }
 172
 173     if (read_sync(ioc, &type, sizeof(type)) != sizeof(type)) {
 174         error_setg(errp, "failed to read list option type");
 175         return -1;
 176     }
 177     type = be32_to_cpu(type);
 178     error = nbd_handle_reply_err(ioc, opt, type, errp);
 179     if (error <= 0) {
 180         return error;
 181     }
 182
 183     if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
 184         error_setg(errp, "failed to read option length");
 185         return -1;
 186     }
 187     len = be32_to_cpu(len);
 188
 189     if (type == NBD_REP_ACK) {
 190         if (len != 0) {
 191             error_setg(errp, "length too long for option end");
 192             return -1;
 193         }
 194     } else if (type == NBD_REP_SERVER) {
 195         if (read_sync(ioc, &namelen, sizeof(namelen)) != sizeof(namelen)) {
 196             error_setg(errp, "failed to read option name length");
 197             return -1;
 198         }
 199         namelen = be32_to_cpu(namelen);
 200         if (len != (namelen + sizeof(namelen))) {
 201             error_setg(errp, "incorrect option mame length");
 202             return -1;
 203         }
 204         if (namelen > 255) {
 205             error_setg(errp, "export name length too long %d", namelen);
 206             return -1;
 207         }
 208
 209         *name = g_new0(char, namelen + 1);
 210         if (read_sync(ioc, *name, namelen) != namelen) {
 211             error_setg(errp, "failed to read export name");
 212             g_free(*name);
 213             *name = NULL;
 214             return -1;
 215         }
 216         (*name)[namelen] = '\0';
 217     } else {
 218         error_setg(errp, "Unexpected reply type %x expected %x",
 219                    type, NBD_REP_SERVER);
 220         return -1;
 221     }
 222     return 1;
 223 }
 224
 225
 226 static int nbd_receive_query_exports(QIOChannel *ioc,
 227                                      const char *wantname,
 228                                      Error **errp)
 229 {
 230     uint64_t magic = cpu_to_be64(NBD_OPTS_MAGIC);
 231     uint32_t opt = cpu_to_be32(NBD_OPT_LIST);
 232     uint32_t length = 0;
 233     bool foundExport = false;
 234
 235     TRACE("Querying export list");
 236     if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 237         error_setg(errp, "Failed to send list option magic");
 238         return -1;
 239     }
 240
 241     if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 242         error_setg(errp, "Failed to send list option number");
 243         return -1;
 244     }
 245
 246     if (write_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 247         error_setg(errp, "Failed to send list option length");
 248         return -1;
 249     }
 250
 251     TRACE("Reading available export names");
 252     while (1) {
 253         char *name = NULL;
 254         int ret = nbd_receive_list(ioc, &name, errp);
 255
 256         if (ret < 0) {
 257             g_free(name);
 258             name = NULL;
 259             return -1;
 260         }
 261         if (ret == 0) {
 262             /* Server doesn't support export listing, so
 263              * we will just assume an export with our
 264              * wanted name exists */
 265             foundExport = true;
 266             break;
 267         }
 268         if (name == NULL) {
 269             TRACE("End of export name list");
 270             break;
 271         }
 272         if (g_str_equal(name, wantname)) {
 273             foundExport = true;
 274             TRACE("Found desired export name '%s'", name);
 275         } else {
 276             TRACE("Ignored export name '%s'", name);
 277         }
 278         g_free(name);
 279     }
 280
 281     if (!foundExport) {
 282         error_setg(errp, "No export with name '%s' available", wantname);
 283         return -1;
 284     }
 285
 286     return 0;
 287 }
 288
 289 static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
 290                                         QCryptoTLSCreds *tlscreds,
 291                                         const char *hostname, Error **errp)
 292 {
 293     uint64_t magic = cpu_to_be64(NBD_OPTS_MAGIC);
 294     uint32_t opt = cpu_to_be32(NBD_OPT_STARTTLS);
 295     uint32_t length = 0;
 296     uint32_t type;
 297     QIOChannelTLS *tioc;
 298     struct NBDTLSHandshakeData data = { 0 };
 299
 300     TRACE("Requesting TLS from server");
 301     if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 302         error_setg(errp, "Failed to send option magic");
 303         return NULL;
 304     }
 305
 306     if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 307         error_setg(errp, "Failed to send option number");
 308         return NULL;
 309     }
 310
 311     if (write_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 312         error_setg(errp, "Failed to send option length");
 313         return NULL;
 314     }
 315
 316     TRACE("Getting TLS reply from server1");
 317     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 318         error_setg(errp, "failed to read option magic");
 319         return NULL;
 320     }
 321     magic = be64_to_cpu(magic);
 322     if (magic != NBD_REP_MAGIC) {
 323         error_setg(errp, "Unexpected option magic");
 324         return NULL;
 325     }
 326     TRACE("Getting TLS reply from server2");
 327     if (read_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 328         error_setg(errp, "failed to read option");
 329         return NULL;
 330     }
 331     opt = be32_to_cpu(opt);
 332     if (opt != NBD_OPT_STARTTLS) {
 333         error_setg(errp, "Unexpected option type %x expected %x",
 334                    opt, NBD_OPT_STARTTLS);
 335         return NULL;
 336     }
 337
 338     TRACE("Getting TLS reply from server");
 339     if (read_sync(ioc, &type, sizeof(type)) != sizeof(type)) {
 340         error_setg(errp, "failed to read option type");
 341         return NULL;
 342     }
 343     type = be32_to_cpu(type);
 344     if (type != NBD_REP_ACK) {
 345         error_setg(errp, "Server rejected request to start TLS %x",
 346                    type);
 347         return NULL;
 348     }
 349
 350     TRACE("Getting TLS reply from server");
 351     if (read_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 352         error_setg(errp, "failed to read option length");
 353         return NULL;
 354     }
 355     length = be32_to_cpu(length);
 356     if (length != 0) {
 357         error_setg(errp, "Start TLS reponse was not zero %x",
 358                    length);
 359         return NULL;
 360     }
 361
 362     TRACE("TLS request approved, setting up TLS");
 363     tioc = qio_channel_tls_new_client(ioc, tlscreds, hostname, errp);
 364     if (!tioc) {
 365         return NULL;
 366     }
 367     data.loop = g_main_loop_new(g_main_context_default(), FALSE);
 368     TRACE("Starting TLS hanshake");
 369     qio_channel_tls_handshake(tioc,
 370                               nbd_tls_handshake,
 371                               &data,
 372                               NULL);
 373
 374     if (!data.complete) {
 375         g_main_loop_run(data.loop);
 376     }
 377     g_main_loop_unref(data.loop);
 378     if (data.error) {
 379         error_propagate(errp, data.error);
 380         object_unref(OBJECT(tioc));
 381         return NULL;
 382     }
 383
 384     return QIO_CHANNEL(tioc);
 385 }
 386
 387
 388 int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
 389                           QCryptoTLSCreds *tlscreds, const char *hostname,
 390                           QIOChannel **outioc,
 391                           off_t *size, Error **errp)
 392 {
 393     char buf[256];
 394     uint64_t magic, s;
 395     int rc;
 396
 397     TRACE("Receiving negotiation tlscreds=%p hostname=%s.",
 398           tlscreds, hostname ? hostname : "<null>");
 399
 400     rc = -EINVAL;
 401
 402     if (outioc) {
 403         *outioc = NULL;
 404     }
 405     if (tlscreds && !outioc) {
 406         error_setg(errp, "Output I/O channel required for TLS");
 407         goto fail;
 408     }
 409
 410     if (read_sync(ioc, buf, 8) != 8) {
 411         error_setg(errp, "Failed to read data");
 412         goto fail;
 413     }
 414
 415     buf[8] = '\0';
 416     if (strlen(buf) == 0) {
 417         error_setg(errp, "Server connection closed unexpectedly");
 418         goto fail;
 419     }
 420
 421     TRACE("Magic is %c%c%c%c%c%c%c%c",
 422           qemu_isprint(buf[0]) ? buf[0] : '.',
 423           qemu_isprint(buf[1]) ? buf[1] : '.',
 424           qemu_isprint(buf[2]) ? buf[2] : '.',
 425           qemu_isprint(buf[3]) ? buf[3] : '.',
 426           qemu_isprint(buf[4]) ? buf[4] : '.',
 427           qemu_isprint(buf[5]) ? buf[5] : '.',
 428           qemu_isprint(buf[6]) ? buf[6] : '.',
 429           qemu_isprint(buf[7]) ? buf[7] : '.');
 430
 431     if (memcmp(buf, "NBDMAGIC", 8) != 0) {
 432         error_setg(errp, "Invalid magic received");
 433         goto fail;
 434     }
 435
 436     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 437         error_setg(errp, "Failed to read magic");
 438         goto fail;
 439     }
 440     magic = be64_to_cpu(magic);
 441     TRACE("Magic is 0x%" PRIx64, magic);
 442
 443     if (magic == NBD_OPTS_MAGIC) {
 444         uint32_t clientflags = 0;
 445         uint32_t opt;
 446         uint32_t namesize;
 447         uint16_t globalflags;
 448         uint16_t exportflags;
 449         bool fixedNewStyle = false;
 450
 451         if (read_sync(ioc, &globalflags, sizeof(globalflags)) !=
 452             sizeof(globalflags)) {
 453             error_setg(errp, "Failed to read server flags");
 454             goto fail;
 455         }
 456         globalflags = be16_to_cpu(globalflags);
 457         *flags = globalflags << 16;
 458         TRACE("Global flags are %x", globalflags);
 459         if (globalflags & NBD_FLAG_FIXED_NEWSTYLE) {
 460             fixedNewStyle = true;
 461             TRACE("Server supports fixed new style");
 462             clientflags |= NBD_FLAG_C_FIXED_NEWSTYLE;
 463         }
 464         /* client requested flags */
 465         clientflags = cpu_to_be32(clientflags);
 466         if (write_sync(ioc, &clientflags, sizeof(clientflags)) !=
 467             sizeof(clientflags)) {
 468             error_setg(errp, "Failed to send clientflags field");
 469             goto fail;
 470         }
 471         if (tlscreds) {
 472             if (fixedNewStyle) {
 473                 *outioc = nbd_receive_starttls(ioc, tlscreds, hostname, errp);
 474                 if (!*outioc) {
 475                     goto fail;
 476                 }
 477                 ioc = *outioc;
 478             } else {
 479                 error_setg(errp, "Server does not support STARTTLS");
 480                 goto fail;
 481             }
 482         }
 483         if (!name) {
 484             TRACE("Using default NBD export name \"\"");
 485             name = "";
 486         }
 487         if (fixedNewStyle) {
 488             /* Check our desired export is present in the
 489              * server export list. Since NBD_OPT_EXPORT_NAME
 490              * cannot return an error message, running this
 491              * query gives us good error reporting if the
 492              * server required TLS
 493              */
 494             if (nbd_receive_query_exports(ioc, name, errp) < 0) {
 495                 goto fail;
 496             }
 497         }
 498         /* write the export name */
 499         magic = cpu_to_be64(magic);
 500         if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 501             error_setg(errp, "Failed to send export name magic");
 502             goto fail;
 503         }
 504         opt = cpu_to_be32(NBD_OPT_EXPORT_NAME);
 505         if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 506             error_setg(errp, "Failed to send export name option number");
 507             goto fail;
 508         }
 509         namesize = cpu_to_be32(strlen(name));
 510         if (write_sync(ioc, &namesize, sizeof(namesize)) !=
 511             sizeof(namesize)) {
 512             error_setg(errp, "Failed to send export name length");
 513             goto fail;
 514         }
 515         if (write_sync(ioc, (char *)name, strlen(name)) != strlen(name)) {
 516             error_setg(errp, "Failed to send export name");
 517             goto fail;
 518         }
 519
 520         if (read_sync(ioc, &s, sizeof(s)) != sizeof(s)) {
 521             error_setg(errp, "Failed to read export length");
 522             goto fail;
 523         }
 524         *size = be64_to_cpu(s);
 525         TRACE("Size is %" PRIu64, *size);
 526
 527         if (read_sync(ioc, &exportflags, sizeof(exportflags)) !=
 528             sizeof(exportflags)) {
 529             error_setg(errp, "Failed to read export flags");
 530             goto fail;
 531         }
 532         exportflags = be16_to_cpu(exportflags);
 533         *flags |= exportflags;
 534         TRACE("Export flags are %x", exportflags);
 535     } else if (magic == NBD_CLIENT_MAGIC) {
 536         if (name) {
 537             error_setg(errp, "Server does not support export names");
 538             goto fail;
 539         }
 540         if (tlscreds) {
 541             error_setg(errp, "Server does not support STARTTLS");
 542             goto fail;
 543         }
 544
 545         if (read_sync(ioc, &s, sizeof(s)) != sizeof(s)) {
 546             error_setg(errp, "Failed to read export length");
 547             goto fail;
 548         }
 549         *size = be64_to_cpu(s);
 550         TRACE("Size is %" PRIu64, *size);
 551
 552         if (read_sync(ioc, flags, sizeof(*flags)) != sizeof(*flags)) {
 553             error_setg(errp, "Failed to read export flags");
 554             goto fail;
 555         }
 556         *flags = be32_to_cpup(flags);
 557     } else {
 558         error_setg(errp, "Bad magic received");
 559         goto fail;
 560     }
 561
 562     if (read_sync(ioc, &buf, 124) != 124) {
 563         error_setg(errp, "Failed to read reserved block");
 564         goto fail;
 565     }
 566     rc = 0;
 567
 568 fail:
 569     return rc;
 570 }
 571
 572 #ifdef __linux__
 573 int nbd_init(int fd, QIOChannelSocket *sioc, uint32_t flags, off_t size)
 574 {
 575     TRACE("Setting NBD socket");
 576
 577     if (ioctl(fd, NBD_SET_SOCK, sioc->fd) < 0) {
 578         int serrno = errno;
 579         LOG("Failed to set NBD socket");
 580         return -serrno;
 581     }
 582
 583     TRACE("Setting block size to %lu", (unsigned long)BDRV_SECTOR_SIZE);
 584
 585     if (ioctl(fd, NBD_SET_BLKSIZE, (size_t)BDRV_SECTOR_SIZE) < 0) {
 586         int serrno = errno;
 587         LOG("Failed setting NBD block size");
 588         return -serrno;
 589     }
 590
 591     TRACE("Setting size to %zd block(s)", (size_t)(size / BDRV_SECTOR_SIZE));
 592
 593     if (ioctl(fd, NBD_SET_SIZE_BLOCKS, (size_t)(size / BDRV_SECTOR_SIZE)) < 0) {
 594         int serrno = errno;
 595         LOG("Failed setting size (in blocks)");
 596         return -serrno;
 597     }
 598
 599     if (ioctl(fd, NBD_SET_FLAGS, flags) < 0) {
 600         if (errno == ENOTTY) {
 601             int read_only = (flags & NBD_FLAG_READ_ONLY) != 0;
 602             TRACE("Setting readonly attribute");
 603
 604             if (ioctl(fd, BLKROSET, (unsigned long) &read_only) < 0) {
 605                 int serrno = errno;
 606                 LOG("Failed setting read-only attribute");
 607                 return -serrno;
 608             }
 609         } else {
 610             int serrno = errno;
 611             LOG("Failed setting flags");
 612             return -serrno;
 613         }
 614     }
 615
 616     TRACE("Negotiation ended");
 617
 618     return 0;
 619 }
 620
 621 int nbd_client(int fd)
 622 {
 623     int ret;
 624     int serrno;
 625
 626     TRACE("Doing NBD loop");
 627
 628     ret = ioctl(fd, NBD_DO_IT);
 629     if (ret < 0 && errno == EPIPE) {
 630         /* NBD_DO_IT normally returns EPIPE when someone has disconnected
 631          * the socket via NBD_DISCONNECT.  We do not want to return 1 in
 632          * that case.
 633          */
 634         ret = 0;
 635     }
 636     serrno = errno;
 637
 638     TRACE("NBD loop returned %d: %s", ret, strerror(serrno));
 639
 640     TRACE("Clearing NBD queue");
 641     ioctl(fd, NBD_CLEAR_QUE);
 642
 643     TRACE("Clearing NBD socket");
 644     ioctl(fd, NBD_CLEAR_SOCK);
 645
 646     errno = serrno;
 647     return ret;
 648 }
 649 #else
 650 int nbd_init(int fd, QIOChannelSocket *ioc, uint32_t flags, off_t size)
 651 {
 652     return -ENOTSUP;
 653 }
 654
 655 int nbd_client(int fd)
 656 {
 657     return -ENOTSUP;
 658 }
 659 #endif
 660
 661 ssize_t nbd_send_request(QIOChannel *ioc, struct nbd_request *request)
 662 {
 663     uint8_t buf[NBD_REQUEST_SIZE];
 664     ssize_t ret;
 665
 666     TRACE("Sending request to server: "
 667           "{ .from = %" PRIu64", .len = %u, .handle = %" PRIu64", .type=%i}",
 668           request->from, request->len, request->handle, request->type);
 669
 670     cpu_to_be32w((uint32_t*)buf, NBD_REQUEST_MAGIC);
 671     cpu_to_be32w((uint32_t*)(buf + 4), request->type);
 672     cpu_to_be64w((uint64_t*)(buf + 8), request->handle);
 673     cpu_to_be64w((uint64_t*)(buf + 16), request->from);
 674     cpu_to_be32w((uint32_t*)(buf + 24), request->len);
 675
 676     ret = write_sync(ioc, buf, sizeof(buf));
 677     if (ret < 0) {
 678         return ret;
 679     }
 680
 681     if (ret != sizeof(buf)) {
 682         LOG("writing to socket failed");
 683         return -EINVAL;
 684     }
 685     return 0;
 686 }
 687
 688 ssize_t nbd_receive_reply(QIOChannel *ioc, struct nbd_reply *reply)
 689 {
 690     uint8_t buf[NBD_REPLY_SIZE];
 691     uint32_t magic;
 692     ssize_t ret;
 693
 694     ret = read_sync(ioc, buf, sizeof(buf));
 695     if (ret < 0) {
 696         return ret;
 697     }
 698
 699     if (ret != sizeof(buf)) {
 700         LOG("read failed");
 701         return -EINVAL;
 702     }
 703
 704     /* Reply
 705        [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)
 706        [ 4 ..  7]    error   (0 == no error)
 707        [ 7 .. 15]    handle
 708      */
 709
 710     magic = be32_to_cpup((uint32_t*)buf);
 711     reply->error  = be32_to_cpup((uint32_t*)(buf + 4));
 712     reply->handle = be64_to_cpup((uint64_t*)(buf + 8));
 713
 714     reply->error = nbd_errno_to_system_errno(reply->error);
 715
 716     TRACE("Got reply: "
 717           "{ magic = 0x%x, .error = %d, handle = %" PRIu64" }",
 718           magic, reply->error, reply->handle);
 719
 720     if (magic != NBD_REPLY_MAGIC) {
 721         LOG("invalid magic (got 0x%x)", magic);
 722         return -EINVAL;
 723     }
 724     return 0;
 725 }
 726