sys/kern/vfs_conf.c

   1 /*-
   2  * Copyright (c) 1999 Michael Smith
   3  * All rights reserved.
   4  * Copyright (c) 1999 Poul-Henning Kamp
   5  * All rights reserved.
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  * 2. Redistributions in binary form must reproduce the above copyright
  13  *    notice, this list of conditions and the following disclaimer in the
  14  *    documentation and/or other materials provided with the distribution.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  26  * SUCH DAMAGE.
  27  *
  28  *      $FreeBSD: src/sys/kern/vfs_conf.c,v 1.49.2.5 2003/01/07 11:56:53 joerg Exp $
  29  *      $DragonFly: src/sys/kern/vfs_conf.c,v 1.34 2008/05/24 19:08:28 dillon Exp $
  30  */
  31
  32 /*
  33  * Locate and mount the root filesystem.
  34  *
  35  * The root filesystem is detailed in the kernel environment variable
  36  * vfs.root.mountfrom, which is expected to be in the general format
  37  *
  38  * <vfsname>:[<path>]
  39  * vfsname   := the name of a VFS known to the kernel and capable
  40  *              of being mounted as root
  41  * path      := disk device name or other data used by the filesystem
  42  *              to locate its physical store
  43  *
  44  */
  45
  46 #include "opt_rootdevname.h"
  47
  48 #include <sys/param.h>
  49 #include <sys/kernel.h>
  50 #include <sys/systm.h>
  51 #include <sys/proc.h>
  52 #include <sys/vnode.h>
  53 #include <sys/mount.h>
  54 #include <sys/malloc.h>
  55 #include <sys/reboot.h>
  56 #include <sys/diskslice.h>
  57 #include <sys/conf.h>
  58 #include <sys/cons.h>
  59 #include <sys/device.h>
  60 #include <sys/disk.h>
  61 #include <sys/namecache.h>
  62 #include <sys/paths.h>
  63 #include <sys/thread2.h>
  64 #include <sys/nlookup.h>
  65 #include <sys/devfs.h>
  66 #include <sys/sysctl.h>
  67
  68 #include "opt_ddb.h"
  69 #ifdef DDB
  70 #include <ddb/ddb.h>
  71 #endif
  72
  73 MALLOC_DEFINE(M_MOUNT, "mount", "vfs mount structure");
  74
  75 #define ROOTNAME        "root_device"
  76
  77 struct vnode    *rootvnode;
  78 struct nchandle rootnch;
  79
  80 /*
  81  * The root specifiers we will try if RB_CDROM is specified.  Note that
  82  * with DEVFS we do not use the compatibility slice's whole-disk 'c'
  83  * partition.  Instead we just use the whole disk, e.g. cd0 or cd0s0.
  84  */
  85 static char *cdrom_rootdevnames[] = {
  86         "cd9660:cd0",   /* SCSI (including AHCI and SILI) */
  87         "cd9660:acd0",  /* NATA */
  88         "cd9660:cd1",   /* SCSI (including AHCI and SILI) */
  89         "cd9660:acd1",  /* NATA */
  90         "cd9660:cd8",   /* USB */
  91         "cd9660:cd9",   /* USB */
  92         NULL
  93 };
  94
  95 int vfs_mountroot_devfs(void);
  96 static void     vfs_mountroot(void *junk);
  97 static int      vfs_mountroot_try(const char *mountfrom);
  98 static int      vfs_mountroot_ask(void);
  99 static int      getline(char *cp, int limit);
 100
 101 /* legacy find-root code */
 102 char            *rootdevnames[2] = {NULL, NULL};
 103 static int      setrootbyname(char *name);
 104
 105 SYSINIT(mountroot, SI_SUB_MOUNT_ROOT, SI_ORDER_SECOND, vfs_mountroot, NULL);
 106
 107 /*
 108  * Find and mount the root filesystem
 109  */
 110 static void
 111 vfs_mountroot(void *junk)
 112 {
 113         cdev_t  save_rootdev = rootdev;
 114         int     i;
 115         int     dummy;
 116
 117         /*
 118          * Make sure all disk devices created so far have also been probed,
 119          * and also make sure that the newly created device nodes for
 120          * probed disks are ready, too.
 121          *
 122          * Messages can fly around here so get good synchronization
 123          * coverage.
 124          *
 125          * XXX - Delay an additional 2 seconds to help drivers which pickup
 126          *       devices asynchronously and are not caught by CAM's initial
 127          *       probe.
 128          */
 129         sync_devs();
 130         tsleep(&dummy, 0, "syncer", hz*2);
 131
 132
 133         /*
 134          * The root filesystem information is compiled in, and we are
 135          * booted with instructions to use it.
 136          */
 137 #ifdef ROOTDEVNAME
 138         if ((boothowto & RB_DFLTROOT) &&
 139             !vfs_mountroot_try(ROOTDEVNAME))
 140                 return;
 141 #endif
 142         /*
 143          * We are booted with instructions to prompt for the root filesystem,
 144          * or to use the compiled-in default when it doesn't exist.
 145          */
 146         if (boothowto & (RB_DFLTROOT | RB_ASKNAME)) {
 147                 if (!vfs_mountroot_ask())
 148                         return;
 149         }
 150
 151         /*
 152          * We've been given the generic "use CDROM as root" flag.  This is
 153          * necessary because one media may be used in many different
 154          * devices, so we need to search for them.
 155          */
 156         if (boothowto & RB_CDROM) {
 157                 for (i = 0; cdrom_rootdevnames[i] != NULL; i++) {
 158                         if (!vfs_mountroot_try(cdrom_rootdevnames[i]))
 159                                 return;
 160                 }
 161         }
 162
 163         /*
 164          * Try to use the value read by the loader from /etc/fstab, or
 165          * supplied via some other means.  This is the preferred
 166          * mechanism.
 167          */
 168         if (!vfs_mountroot_try(kgetenv("vfs.root.mountfrom")))
 169                 return;
 170
 171         /*
 172          * If a vfs set rootdev, try it (XXX VINUM HACK!)
 173          */
 174         if (save_rootdev != NULL) {
 175                 rootdev = save_rootdev;
 176                 if (!vfs_mountroot_try(""))
 177                         return;
 178         }
 179
 180         /*
 181          * Try values that may have been computed by the machine-dependant
 182          * legacy code.
 183          */
 184         if (rootdevnames[0] && !vfs_mountroot_try(rootdevnames[0]))
 185                 return;
 186         if (rootdevnames[1] && !vfs_mountroot_try(rootdevnames[1]))
 187                 return;
 188
 189         /*
 190          * If we have a compiled-in default, and haven't already tried it, try
 191          * it now.
 192          */
 193 #ifdef ROOTDEVNAME
 194         if (!(boothowto & RB_DFLTROOT))
 195                 if (!vfs_mountroot_try(ROOTDEVNAME))
 196                         return;
 197 #endif
 198
 199         /*
 200          * Everything so far has failed, prompt on the console if we haven't
 201          * already tried that.
 202          */
 203         if (!(boothowto & (RB_DFLTROOT | RB_ASKNAME)) && !vfs_mountroot_ask())
 204                 return;
 205         panic("Root mount failed, startup aborted.");
 206 }
 207
 208
 209 int
 210 vfs_mountroot_devfs(void)
 211 {
 212         struct vnode *vp;
 213         struct nchandle nch;
 214         struct nlookupdata nd;
 215         struct mount *mp;
 216         struct vfsconf *vfsp;
 217         int error;
 218         struct ucred *cred = proc0.p_ucred;
 219
 220         /*
 221          * Lookup the requested path and extract the nch and vnode.
 222          */
 223         error = nlookup_init_raw(&nd,
 224              "/dev", UIO_SYSSPACE, NLC_FOLLOW,
 225              cred, &rootnch);
 226
 227         if (error == 0) {
 228                 devfs_debug(DEVFS_DEBUG_DEBUG, "vfs_mountroot_devfs: nlookup_init is ok...\n");
 229                 if ((error = nlookup(&nd)) == 0) {
 230                         devfs_debug(DEVFS_DEBUG_DEBUG, "vfs_mountroot_devfs: nlookup is ok...\n");
 231                         if (nd.nl_nch.ncp->nc_vp == NULL) {
 232                                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: nlookup: simply not found\n");
 233                                 error = ENOENT;
 234                         }
 235                 }
 236         }
 237         if (error) {
 238                 nlookup_done(&nd);
 239                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: nlookup failed, error: %d\n", error);
 240                 return (error);
 241         }
 242
 243         /*
 244          * Extract the locked+refd ncp and cleanup the nd structure
 245          */
 246         nch = nd.nl_nch;
 247         cache_zero(&nd.nl_nch);
 248         nlookup_done(&nd);
 249
 250         /*
 251          * now we have the locked ref'd nch and unreferenced vnode.
 252          */
 253         vp = nch.ncp->nc_vp;
 254         if ((error = vget(vp, LK_EXCLUSIVE)) != 0) {
 255                 cache_put(&nch);
 256                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: vget failed\n");
 257                 return (error);
 258         }
 259         cache_unlock(&nch);
 260
 261         if ((error = vinvalbuf(vp, V_SAVE, 0, 0)) != 0) {
 262                 cache_drop(&nch);
 263                 vput(vp);
 264                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: vinvalbuf failed\n");
 265                 return (error);
 266         }
 267         if (vp->v_type != VDIR) {
 268                 cache_drop(&nch);
 269                 vput(vp);
 270                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: vp is not VDIR\n");
 271                 return (ENOTDIR);
 272         }
 273
 274         vfsp = vfsconf_find_by_name("devfs");
 275         vsetflags(vp, VMOUNT);
 276
 277         /*
 278          * Allocate and initialize the filesystem.
 279          */
 280         mp = kmalloc(sizeof(struct mount), M_MOUNT, M_ZERO|M_WAITOK);
 281         mount_init(mp);
 282         vfs_busy(mp, LK_NOWAIT);
 283         mp->mnt_op = vfsp->vfc_vfsops;
 284         mp->mnt_vfc = vfsp;
 285         vfsp->vfc_refcount++;
 286         mp->mnt_stat.f_type = vfsp->vfc_typenum;
 287         mp->mnt_flag |= vfsp->vfc_flags & MNT_VISFLAGMASK;
 288         strncpy(mp->mnt_stat.f_fstypename, vfsp->vfc_name, MFSNAMELEN);
 289         mp->mnt_stat.f_owner = cred->cr_uid;
 290         vn_unlock(vp);
 291
 292         /*
 293          * Mount the filesystem.
 294          */
 295         error = VFS_MOUNT(mp, "/dev", NULL, cred);
 296
 297         vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
 298
 299         /*
 300          * Put the new filesystem on the mount list after root.  The mount
 301          * point gets its own mnt_ncmountpt (unless the VFS already set one
 302          * up) which represents the root of the mount.  The lookup code
 303          * detects the mount point going forward and checks the root of
 304          * the mount going backwards.
 305          *
 306          * It is not necessary to invalidate or purge the vnode underneath
 307          * because elements under the mount will be given their own glue
 308          * namecache record.
 309          */
 310         if (!error) {
 311                 if (mp->mnt_ncmountpt.ncp == NULL) {
 312                         /*
 313                          * allocate, then unlock, but leave the ref intact
 314                          */
 315                         cache_allocroot(&mp->mnt_ncmountpt, mp, NULL);
 316                         cache_unlock(&mp->mnt_ncmountpt);
 317                 }
 318                 mp->mnt_ncmounton = nch;                /* inherits ref */
 319                 nch.ncp->nc_flag |= NCF_ISMOUNTPT;
 320
 321                 /* XXX get the root of the fs and cache_setvp(mnt_ncmountpt...) */
 322                 vclrflags(vp, VMOUNT);
 323                 mountlist_insert(mp, MNTINS_LAST);
 324                 vn_unlock(vp);
 325                 //checkdirs(&mp->mnt_ncmounton, &mp->mnt_ncmountpt);
 326                 error = vfs_allocate_syncvnode(mp);
 327                 if (error) {
 328                         devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: vfs_allocate_syncvnode failed\n");
 329                 }
 330                 vfs_unbusy(mp);
 331                 error = VFS_START(mp, 0);
 332                 vrele(vp);
 333         } else {
 334                 vfs_rm_vnodeops(mp, NULL, &mp->mnt_vn_coherency_ops);
 335                 vfs_rm_vnodeops(mp, NULL, &mp->mnt_vn_journal_ops);
 336                 vfs_rm_vnodeops(mp, NULL, &mp->mnt_vn_norm_ops);
 337                 vfs_rm_vnodeops(mp, NULL, &mp->mnt_vn_spec_ops);
 338                 vfs_rm_vnodeops(mp, NULL, &mp->mnt_vn_fifo_ops);
 339                 vclrflags(vp, VMOUNT);
 340                 mp->mnt_vfc->vfc_refcount--;
 341                 vfs_unbusy(mp);
 342                 kfree(mp, M_MOUNT);
 343                 cache_drop(&nch);
 344                 vput(vp);
 345                 devfs_debug(DEVFS_DEBUG_SHOW, "vfs_mountroot_devfs: mount failed\n");
 346         }
 347
 348         devfs_debug(DEVFS_DEBUG_DEBUG, "rootmount_devfs done with error: %d\n", error);
 349         return (error);
 350 }
 351
 352
 353 /*
 354  * Mount (mountfrom) as the root filesystem.
 355  */
 356 static int
 357 vfs_mountroot_try(const char *mountfrom)
 358 {
 359         struct mount    *mp, *mp2;
 360         char            *vfsname, *devname;
 361         int             error;
 362         char            patt[32];
 363         int             mountfromlen, len;
 364         const char      *cp, *ep;
 365         char            *mf;
 366
 367         vfsname = NULL;
 368         devname = NULL;
 369         mp      = NULL;
 370         mp2             = NULL;
 371         error   = EINVAL;
 372
 373         if (mountfrom == NULL)
 374                 return(error);          /* don't complain */
 375
 376         crit_enter();
 377         kprintf("Mounting root from %s\n", mountfrom);
 378         crit_exit();
 379
 380         mountfromlen = strlen(mountfrom);
 381         cp = mountfrom;
 382         /* parse vfs name and devname */
 383         vfsname = kmalloc(MFSNAMELEN, M_MOUNT, M_WAITOK);
 384         devname = kmalloc(MNAMELEN, M_MOUNT, M_WAITOK);
 385         mf = kmalloc(MFSNAMELEN+MNAMELEN, M_MOUNT, M_WAITOK);
 386         for(;;) {
 387                 for (ep = cp; (*ep != 0) && (*ep != ';'); ep++);
 388                 len = ep - cp;
 389                 bzero(vfsname, MFSNAMELEN);
 390                 bzero(devname, MNAMELEN);
 391                 bzero(mf, MFSNAMELEN+MNAMELEN);
 392                 strncpy(mf, cp, MFSNAMELEN+MNAMELEN);
 393
 394                 vfsname[0] = devname[0] = 0;
 395                 ksprintf(patt, "%%%d[a-z0-9]:%%%ds", MFSNAMELEN, MNAMELEN);
 396                 if (ksscanf(mf, patt, vfsname, devname) < 1)
 397                         goto end;
 398
 399                 /* allocate a root mount */
 400                 error = vfs_rootmountalloc(vfsname,
 401                                 devname[0] != 0 ? devname : ROOTNAME, &mp);
 402                 if (error != 0) {
 403                         kprintf("Can't allocate root mount for filesystem '%s': %d\n",
 404                                vfsname, error);
 405                         goto end;
 406                 }
 407                 mp->mnt_flag |= MNT_ROOTFS;
 408
 409                 /* do our best to set rootdev */
 410                 if ((strcmp(vfsname, "hammer") != 0) && (devname[0] != 0) &&
 411                     setrootbyname(devname))
 412                         kprintf("setrootbyname failed\n");
 413
 414                 /* If the root device is a type "memory disk", mount RW */
 415                 if (rootdev != NULL && dev_is_good(rootdev) &&
 416                     (dev_dflags(rootdev) & D_MEMDISK)) {
 417                         mp->mnt_flag &= ~MNT_RDONLY;
 418                 }
 419
 420                 error = VFS_MOUNT(mp, NULL, NULL, proc0.p_ucred);
 421
 422                 if (!error)
 423                         break;
 424 end:
 425                 if(*ep == 0)
 426                         break;
 427                 cp = ep + 1;
 428         }
 429
 430         if (vfsname != NULL)
 431                 kfree(vfsname, M_MOUNT);
 432         if (devname != NULL)
 433                 kfree(devname, M_MOUNT);
 434         if (mf != NULL)
 435                 kfree(mf, M_MOUNT);
 436         if (error == 0) {
 437                 /* register with list of mounted filesystems */
 438                 mountlist_insert(mp, MNTINS_FIRST);
 439
 440                 /* sanity check system clock against root fs timestamp */
 441                 inittodr(mp->mnt_time);
 442                 vfs_unbusy(mp);
 443                 if (mp->mnt_syncer == NULL) {
 444                         error = vfs_allocate_syncvnode(mp);
 445                         if (error)
 446                                 kprintf("Warning: no syncer vp for root!\n");
 447                         error = 0;
 448                 }
 449         } else {
 450                 if (mp != NULL) {
 451                         vfs_unbusy(mp);
 452                         kfree(mp, M_MOUNT);
 453                 }
 454                 kprintf("Root mount failed: %d\n", error);
 455         }
 456         return(error);
 457 }
 458
 459
 460 static void
 461 vfs_mountroot_ask_callback(cdev_t dev, void *arg __unused)
 462 {
 463         if (dev_is_good(dev) && (dev_dflags(dev) & D_DISK))
 464                 kprintf(" \"%s\" ", dev->si_name);
 465 }
 466
 467
 468 /*
 469  * Spin prompting on the console for a suitable root filesystem
 470  */
 471 static int
 472 vfs_mountroot_ask(void)
 473 {
 474         char name[128];
 475         int llimit = 100;
 476
 477         kprintf("\nManual root filesystem specification:\n");
 478         kprintf("  <fstype>:<device>  Specify root (e.g. ufs:da0s1a)\n");
 479         kprintf("  ?                  List valid disk boot devices\n");
 480         kprintf("  panic              Just panic\n");
 481         kprintf("  abort              Abort manual input\n");
 482         while (llimit--) {
 483                 kprintf("\nmountroot> ");
 484
 485                 if (getline(name, 128) < 0)
 486                         break;
 487                 if (name[0] == 0) {
 488                         ;
 489                 } else if (name[0] == '?') {
 490                         kprintf("Possibly valid devices for root FS:\n");
 491                         //enumerate all disk devices
 492                         devfs_scan_callback(vfs_mountroot_ask_callback, NULL);
 493                         kprintf("\n");
 494                         continue;
 495                 } else if (strcmp(name, "panic") == 0) {
 496                         panic("panic from console");
 497                 } else if (strcmp(name, "abort") == 0) {
 498                         break;
 499                 } else if (vfs_mountroot_try(name) == 0) {
 500                         return(0);
 501                 }
 502         }
 503         return(1);
 504 }
 505
 506
 507 static int
 508 getline(char *cp, int limit)
 509 {
 510         char *lp;
 511         int c;
 512
 513         lp = cp;
 514         for (;;) {
 515                 c = cngetc();
 516
 517                 switch (c) {
 518                 case -1:
 519                         return(-1);
 520                 case '\n':
 521                 case '\r':
 522                         kprintf("\n");
 523                         *lp++ = '\0';
 524                         return(0);
 525                 case '\b':
 526                 case '\177':
 527                         if (lp > cp) {
 528                                 kprintf("\b \b");
 529                                 lp--;
 530                         } else {
 531                                 kprintf("%c", 7);
 532                         }
 533                         continue;
 534                 case '#':
 535                         kprintf("#");
 536                         lp--;
 537                         if (lp < cp)
 538                                 lp = cp;
 539                         continue;
 540                 case '@':
 541                 case 'u' & 037:
 542                         lp = cp;
 543                         kprintf("%c", '\n');
 544                         continue;
 545                 default:
 546                         if (lp - cp >= limit - 1) {
 547                                 kprintf("%c", 7);
 548                         } else {
 549                                 kprintf("%c", c);
 550                                 *lp++ = c;
 551                         }
 552                         continue;
 553                 }
 554         }
 555 }
 556
 557 /*
 558  * Convert a given name to the cdev_t of the disk-like device
 559  * it refers to.
 560  */
 561 struct kdbn_info {
 562         const char *name;
 563         int nlen;
 564         int minor;
 565         cdev_t dev;
 566 };
 567
 568
 569 cdev_t
 570 kgetdiskbyname(const char *name)
 571 {
 572         char *cp;
 573         cdev_t rdev;
 574
 575         /*
 576          * Get the base name of the device
 577          */
 578         if (strncmp(name, __SYS_PATH_DEV, sizeof(__SYS_PATH_DEV) - 1) == 0)
 579                 name += sizeof(__SYS_PATH_DEV) - 1;
 580         cp = __DECONST(char *, name);
 581
 582         /*
 583          * Locate the device
 584          */
 585         kprintf("tryroot %s\n", name);
 586         rdev = devfs_find_device_by_name(name);
 587         if (rdev == NULL) {
 588                 kprintf("no disk named '%s'\n", name);
 589         }
 590         /*
 591          * FOUND DEVICE
 592          */
 593         return(rdev);
 594 }
 595
 596 /*
 597  * Set rootdev to match (name), given that we expect it to
 598  * refer to a disk-like device.
 599  */
 600 static int
 601 setrootbyname(char *name)
 602 {
 603         cdev_t diskdev;
 604
 605         diskdev = kgetdiskbyname(name);
 606         if (diskdev != NULL) {
 607                 rootdev = diskdev;
 608                 return (0);
 609         }
 610         /* set to NULL if kgetdiskbyname() fails so that if the first rootdev is
 611          * found by fails to mount and the second one isn't found, mountroot_try
 612          * doesn't try again with the first one
 613          */
 614         rootdev = NULL;
 615         return (1);
 616 }
 617
 618 #ifdef DDB
 619 DB_SHOW_COMMAND(disk, db_getdiskbyname)
 620 {
 621         cdev_t dev;
 622
 623         if (modif[0] == '\0') {
 624                 db_error("usage: show disk/devicename");
 625                 return;
 626         }
 627         dev = kgetdiskbyname(modif);
 628         if (dev != NULL)
 629                 db_printf("cdev_t = %p\n", dev);
 630         else
 631                 db_printf("No disk device matched.\n");
 632 }
 633 #endif
 634
 635 static int
 636 vfs_sysctl_real_root(SYSCTL_HANDLER_ARGS)
 637 {
 638         char *real_root;
 639         size_t len;
 640         int error;
 641
 642         real_root = kgetenv("vfs.root.realroot");
 643
 644         if (real_root == NULL)
 645                 real_root = "";
 646
 647         len = strlen(real_root) + 1;
 648
 649         error = sysctl_handle_string(oidp, real_root, len, req);
 650
 651         return error;
 652 }
 653
 654 SYSCTL_PROC(_vfs, OID_AUTO, real_root,
 655             CTLTYPE_STRING | CTLFLAG_RD, 0, 0, vfs_sysctl_real_root,
 656             "A", "Real root mount string");