man2/mount.2

   1 .\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
   2 .\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
   3 .\" and Copyright (C) 2002, 2005, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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  26 .\"
  27 .\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
  28 .\" Modified 2001-10-13 by Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added note on historical behavior of MS_NOSUID
  30 .\" Modified 2002-05-16 by Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\"     Extensive changes and additions
  32 .\" Modified 2002-05-27 by aeb
  33 .\" Modified 2002-06-11 by Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
  35 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
  36 .\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
  37 .\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
  38 .\" 2008-10-06, mtk: Add discussion of namespaces.
  39 .\"
  40 .TH MOUNT 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 mount \- mount filesystem
  43 .SH SYNOPSIS
  44 .nf
  45 .B "#include <sys/mount.h>"
  46 .sp
  47 .BI "int mount(const char *" source ", const char *" target ,
  48 .BI "          const char *" filesystemtype ", unsigned long " mountflags ,
  49 .BI "          const void *" data );
  50 .fi
  51 .SH DESCRIPTION
  52 .BR mount ()
  53 attaches the filesystem specified by
  54 .I source
  55 (which is often a pathname referring to a device,
  56 but can also be the pathname of a directory or file,
  57 or a dummy string) to the location (a directory or file)
  58 specified by the pathname in
  59 .IR target .
  60
  61 Appropriate privilege (Linux: the
  62 .B CAP_SYS_ADMIN
  63 capability) is required to mount filesystems.
  64
  65 Values for the
  66 .I filesystemtype
  67 argument supported by the kernel are listed in
  68 .I /proc/filesystems
  69 (e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse",
  70 "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660").
  71 Further types may become available when the appropriate modules
  72 are loaded.
  73
  74 The
  75 .I data
  76 argument is interpreted by the different filesystems.
  77 Typically it is a string of comma-separated options
  78 understood by this filesystem.
  79 See
  80 .BR mount (8)
  81 for details of the options available for each filesystem type.
  82
  83 A call to
  84 .BR mount ()
  85 performs one of a number of general types of operation.
  86 depending on the bits specified in
  87 .IR mountflags .
  88 The choice of operation is determined by testing the bits set in
  89 .IR mountflags ,
  90 with the tests being conducted in the order listed here:
  91 .IP * 3
  92 Remount an existing mount:
  93 .IR mountflags
  94 includes
  95 .BR MS_REMOUNT .
  96 .IP *
  97 Create a bind mount:
  98 .IR mountflags
  99 includes
 100 .BR MS_BIND .
 101 .IP *
 102 Change the propagation type of an existing mount:
 103 .IR mountflags
 104 includes one of
 105 .BR MS_SHARED ,
 106 .BR MS_PRIVATE ,
 107 .BR MS_SLAVE ,
 108 or
 109 .BR MS_UNBINDABLE .
 110 .IP *
 111 Move an existing mount to a new location:
 112 .IR mountflags
 113 includes
 114 .BR MS_MOVE .
 115 .IP *
 116 Create a new mount:
 117 .IR mountflags
 118 includes none of the above flags.
 119 .PP
 120 Each of these operations is detailed later in this page.
 121 Further flags may be specified in
 122 .IR mountflags
 123 to modify the behavior of
 124 .BR mount (),
 125 as described below.
 126 .\"
 127 .SS Additional mount flags
 128 The list below describes the additional flags that can be specified in
 129 .IR mountflags .
 130 Note that some operation types ignore some or all of these flags,
 131 as described later in this page.
 132 .\"
 133 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 134 .\"
 135 .TP
 136 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 137 Make directory changes on this filesystem synchronous.
 138 (This property can be obtained for individual directories
 139 or subtrees using
 140 .BR chattr (1).)
 141 .TP
 142 .BR MS_LAZYTIME " (since Linux 4.0)"
 143 .\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
 144 .\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
 145 .\" commit a26f49926da938f47561f386be56a83dd37a496d
 146 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
 147 by maintaining these changes only in memory.
 148 The on-disk timestamps are updated only when:
 149 .RS
 150 .IP (a) 5
 151 the inode needs to be updated for some change unrelated to file timestamps;
 152 .IP (b)
 153 the application employs
 154 .BR fsync (2),
 155 .BR syncfs (2),
 156 or
 157 .BR sync (2);
 158 .IP (c)
 159 an undeleted inode is evicted from memory; or
 160 .IP (d)
 161 more than 24 hours have passed since the inode was written to disk.
 162 .RE
 163 .IP
 164 This mount option significantly reduces writes
 165 needed to update the inode's timestamps, especially mtime and atime.
 166 However, in the event of a system crash, the atime and mtime fields
 167 on disk might be out of date by up to 24 hours.
 168
 169 Examples of workloads where this option could be of significant benefit
 170 include frequent random writes to preallocated files,
 171 as well as cases where the
 172 .B MS_STRICTATIME
 173 mount option is also enabled.
 174 (The advantage of combining
 175 .BR MS_STRICTATIME
 176 and
 177 .BR MS_LAZYTIME
 178 is that
 179 .BR stat (2)
 180 will return the correctly updated atime, but the atime updates
 181 will be flushed to disk only in the cases listed above.)
 182 .TP
 183 .B MS_MANDLOCK
 184 Permit mandatory locking on files in this filesystem.
 185 (Mandatory locking must still be enabled on a per-file basis,
 186 as described in
 187 .BR fcntl (2).)
 188 Since Linux 4.5,
 189 .\" commit 95ace75414f312f9a7b93d873f386987b92a5301
 190 this mount option requires the
 191 .B CAP_SYS_ADMIN
 192 capability.
 193 .\" FIXME Describe the MS_MOVE flag in more detail
 194 .TP
 195 .B MS_NOATIME
 196 Do not update access times for (all types of) files on this filesystem.
 197 .TP
 198 .B MS_NODEV
 199 Do not allow access to devices (special files) on this filesystem.
 200 .TP
 201 .B MS_NODIRATIME
 202 Do not update access times for directories on this filesystem.
 203 This flag provides a subset of the functionality provided by
 204 .BR MS_NOATIME ;
 205 that is,
 206 .BR MS_NOATIME
 207 implies
 208 .BR MS_NODIRATIME .
 209 .TP
 210 .B MS_NOEXEC
 211 Do not allow programs to be executed from this filesystem.
 212 .\" (Possibly useful for a filesystem that contains non-Linux executables.
 213 .\" Often used as a security feature, e.g., to make sure that restricted
 214 .\" users cannot execute files uploaded using ftp or so.)
 215 .TP
 216 .B MS_NOSUID
 217 Do not honor set-user-ID and set-group-ID bits or file capabilities
 218 when executing programs from this filesystem.
 219 .\" (This is a security feature to prevent users executing set-user-ID and
 220 .\" set-group-ID programs from removable disk devices.)
 221 .TP
 222 .B MS_RDONLY
 223 Mount filesystem read-only.
 224 .TP
 225 .BR MS_REC " (since Linux 2.4.11)"
 226 Used in conjunction with
 227 .BR MS_BIND
 228 to create a recursive bind mount,
 229 and in conjunction with the propagation type flags to recursively change
 230 the propagation type of all of the mounts in a subtree.
 231 See below for further details.
 232 .TP
 233 .BR MS_RELATIME " (since Linux 2.6.20)"
 234 When a file on this filesystem is accessed,
 235 update the file's last access time (atime) only if the current value
 236 of atime is less than or equal to the file's last modification time (mtime)
 237 or last status change time (ctime).
 238 This option is useful for programs, such as
 239 .BR mutt (1),
 240 that need to know when a file has been read since it was last modified.
 241 Since Linux 2.6.30, the kernel defaults to the behavior provided
 242 by this flag (unless
 243 .BR MS_NOATIME
 244 was specified), and the
 245 .B MS_STRICTATIME
 246 flag is required to obtain traditional semantics.
 247 In addition, since Linux 2.6.30,
 248 the file's last access time is always updated if it
 249 is more than 1 day old.
 250 .\" Matthew Garrett notes in the patch that added this behavior
 251 .\" that this lets utilities such as tmpreaper (which deletes
 252 .\" files based on last access time) work correctly.
 253 .TP
 254 .BR MS_SILENT " (since Linux 2.6.17)"
 255 Suppress the display of certain
 256 .RI ( printk ())
 257 warning messages in the kernel log.
 258 This flag supersedes the misnamed and obsolete
 259 .BR MS_VERBOSE
 260 flag (available since Linux 2.4.12), which has the same meaning.
 261 .TP
 262 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 263 Always update the last access time (atime) when files on this
 264 filesystem are accessed.
 265 (This was the default behavior before Linux 2.6.30.)
 266 Specifying this flag overrides the effect of setting the
 267 .BR MS_NOATIME
 268 and
 269 .BR MS_RELATIME
 270 flags.
 271 .TP
 272 .B MS_SYNCHRONOUS
 273 Make writes on this filesystem synchronous (as though
 274 the
 275 .B O_SYNC
 276 flag to
 277 .BR open (2)
 278 was specified for all file opens to this filesystem).
 279 .PP
 280 From Linux 2.4 onward, the
 281 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 282 flags are settable on a per-mount-point basis.
 283 From kernel 2.6.16 onward,
 284 .B MS_NOATIME
 285 and
 286 .B MS_NODIRATIME
 287 are also settable on a per-mount-point basis.
 288 The
 289 .B MS_RELATIME
 290 flag is also settable on a per-mount-point basis.
 291 .\"
 292 .SS Remounting an existing mount
 293 An existing mount may be remounted by specifying
 294 .B MS_REMOUNT
 295 in
 296 .IR mountflags .
 297 This allows you to change the
 298 .I mountflags
 299 and
 300 .I data
 301 of an existing mount without having to unmount and remount the filesystem.
 302 .I target
 303 should be the same value specified in the initial
 304 .BR mount ()
 305 call.
 306
 307 The
 308 .I source
 309 and
 310 .I filesystemtype
 311 arguments are ignored.
 312
 313 The
 314 .I mountflags
 315 and
 316 .I data
 317 arguments should match the values used in the original
 318 .BR mount ()
 319 call, except for those parameters that are being deliberately changed.
 320
 321 The following
 322 .I mountflags
 323 can be changed:
 324 .BR MS_MANDLOCK ,
 325 .BR MS_NOATIME ,
 326 .BR MS_NODEV ,
 327 .BR MS_NODIRATIME ,
 328 .BR MS_NOEXEC ,
 329 .BR MS_NOSUID ,
 330 .BR MS_RDONLY ,
 331 and
 332 .BR MS_SYNCHRONOUS .
 333
 334 Since Linux 3.17,
 335 .\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
 336 if none of
 337 .BR MS_NOATIME ,
 338 .BR MS_NODIRATIME ,
 339 .BR MS_RELATIME ,
 340 or
 341 .BR MS_STRICTATIME
 342 is specified in
 343 .IR mountflags ,
 344 then the remount operation preserves the existing values of these flags
 345 (rather than defaulting to
 346 .BR MS_RELATIME ).
 347
 348 Since Linux 2.6.26, this flag can also be used to make
 349 .\" See https://lwn.net/Articles/281157/
 350 an existing bind mount read-only by specifying
 351 .IR mountflags
 352 as:
 353
 354     MS_REMOUNT | MS_BIND | MS_RDONLY
 355
 356 Note that only the
 357 .BR MS_RDONLY
 358 setting of the bind mount can be changed in this manner.
 359 .\"
 360 .SS Creating a bind mount
 361 If
 362 .I mountflags
 363 includes
 364 .BR MS_BIND
 365 (available since Linux 2.4),
 366 .\" since 2.4.0-test9
 367 then perform a bind mount.
 368 A bind mount makes a file or a directory subtree visible at
 369 another point within the single directory hierarchy.
 370 Bind mounts may cross filesystem boundaries and span
 371 .BR chroot (2)
 372 jails.
 373
 374 The
 375 .IR filesystemtype
 376 and
 377 .IR data
 378 arguments are ignored.
 379
 380 The remaining bits in the
 381 .I mountflags
 382 argument are also ignored, with the exception of
 383 .BR MS_REC .
 384 (The bind mount has the same mount options as
 385 the underlying mount point.)
 386 However, see the discussion of remounting above,
 387 for a method of making an existing bind mount read-only.
 388
 389 By default, when a directory is bind mounted,
 390 only that directory is mounted;
 391 if there are any submounts under the directory tree,
 392 they are not bind mounted.
 393 If the
 394 .BR MS_REC
 395 flag is also specified, then a recursive bind mount operation is performed:
 396 all submounts under the
 397 .I source
 398 subtree (other than unbindable mounts)
 399 are also bind mounted at the corresponding location in the
 400 .I target
 401 subtree.
 402 .\"
 403 .SS Changing the propagation type of an existing mount
 404 If
 405 .IR mountflags
 406 includes one of
 407 .BR MS_SHARED ,
 408 .BR MS_PRIVATE ,
 409 .BR MS_SLAVE ,
 410 or
 411 .BR MS_UNBINDABLE
 412 (all available since Linux 2.6.15),
 413 then the propagation type of an existing mount is changed.
 414 If more than one of these flags is specified, an error results.
 415
 416 The only flags that can be used with changing the propagation type are
 417 .BR MS_REC
 418 and
 419 .BR MS_SILENT .
 420
 421 The
 422 .IR source ,
 423 .IR filesystemtype ,
 424 and
 425 .IR data
 426 arguments are ignored.
 427
 428 The meanings of the propagation type flags are as follows:
 429 .TP
 430 .BR MS_SHARED
 431 Make this mount point shared.
 432 Mount and unmount events immediately under this mount point will propagate
 433 to the other mount points that are members of this mount's peer group.
 434 Propagation here means that the same mount or unmount will automatically
 435 occur under all of the other mount points in the peer group.
 436 Conversely, mount and unmount events that take place under
 437 peer mount points will propagate to this mount point.
 438 .TP
 439 .BR MS_PRIVATE
 440 Make this mount point private.
 441 Mount and unmount events do not propagate into or out of this mount point.
 442 This is the default propagation type for newly created mount points.
 443 .TP
 444 .BR MS_SLAVE
 445 If this is a shared mount point that is a member of a peer group
 446 that contains other members, convert it to a slave mount.
 447 If this is a shared mount point that is a member of a peer group
 448 that contains no other members, convert it to a private mount.
 449 Otherwise, the propagation type of the mount point is left unchanged.
 450
 451 When a mount point is a slave,
 452 mount and unmount events propagate into this mount point from
 453 the (master) shared peer group of which it was formerly a member.
 454 Mount and unmount events under this mount point do not propagate to any peer.
 455
 456 A mount point can be the slave of another peer group
 457 while at the same time sharing mount and unmount events
 458 with a peer group of which it is a member.
 459 .TP
 460 .BR MS_UNBINDABLE
 461 Make this mount unbindable.
 462 This is like a private mount,
 463 and in addition this mount can't be bind mounted.
 464 When a recursive bind mount
 465 .RB ( mount (2)
 466 with the
 467 .BR MS_BIND
 468 and
 469 .BR MS_REC
 470 flags) is performed on a directory subtree,
 471 any bind mounts within the subtree are automatically pruned
 472 (i.e., not replicated)
 473 when replicating that subtree to produce the target subtree.
 474 .PP
 475 By default, changing the propagation type affects only the
 476 .I target
 477 mount point.
 478 If the
 479 .B MS_REC
 480 flag is also specified in
 481 .IR mountflags ,
 482 then the propagation type of all mount points under
 483 .IR target
 484 is also changed.
 485 .\"
 486 .SS Moving a mount
 487 If
 488 .I mountflags
 489 contains the flag
 490 .BR MS_MOVE
 491 (available since Linux 2.4.18),
 492 then move a subtree:
 493 .I source
 494 specifies an existing mount point and
 495 .I target
 496 specifies the new location to which that mount point is to be relocated.
 497 The move is atomic: at no point is the subtree unmounted.
 498
 499 The remaining bits in the
 500 .IR mountflags
 501 argument are ignored, as are the
 502 .IR filesystemtype
 503 and
 504 .IR data
 505 arguments.
 506 .\"
 507 .SS Creating a new mount point
 508 If none of
 509 .BR MS_REMOUNT ,
 510 .BR MS_BIND ,
 511 .BR MS_MOVE ,
 512 .BR MS_SHARED ,
 513 .BR MS_PRIVATE ,
 514 .BR MS_SLAVE ,
 515 or
 516 .BR MS_UNBINDABLE
 517 is specified in
 518 .IR mountflags ,
 519 then
 520 .BR mount ()
 521 performs its default action: creating a new mount point.
 522 .IR source
 523 specifies the source for the new mount point, and
 524 .IR target
 525 specifies the directory at which to create the mount point.
 526
 527 The
 528 .I filesystemtype
 529 and
 530 .I data
 531 arguments are employed, and further bits may be specified in
 532 .IR mountflags
 533 to modify the behavior of the call.
 534 .\"
 535 .SH RETURN VALUE
 536 On success, zero is returned.
 537 On error, \-1 is returned, and
 538 .I errno
 539 is set appropriately.
 540 .SH ERRORS
 541 The error values given below result from filesystem type independent
 542 errors.
 543 Each filesystem type may have its own special errors and its
 544 own special behavior.
 545 See the Linux kernel source code for details.
 546 .TP
 547 .B EACCES
 548 A component of a path was not searchable.
 549 (See also
 550 .BR path_resolution (7).)
 551 .TP
 552 .B EACCES
 553 Mounting a read-only filesystem was attempted without giving the
 554 .B MS_RDONLY
 555 flag.
 556 .TP
 557 .B EACCES
 558 The block device
 559 .I source
 560 is located on a filesystem mounted with the
 561 .B MS_NODEV
 562 option.
 563 .\" mtk: Probably: write permission is required for MS_BIND, with
 564 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 565 .TP
 566 .B EBUSY
 567 .I source
 568 is already mounted.
 569 .TP
 570 .B EBUSY
 571 .I source
 572 cannot be remounted read-only,
 573 because it still holds files open for writing.
 574 .TP
 575 .B EBUSY
 576 .I source
 577 cannot be mounted on
 578 .I target
 579 because
 580 .I target
 581 is still busy (it is the working directory of some thread,
 582 the mount point of another device, has open files, etc.).
 583 .TP
 584 .B EFAULT
 585 One of the pointer arguments points outside the user address space.
 586 .TP
 587 .B EINVAL
 588 .I source
 589 had an invalid superblock.
 590 .TP
 591 .B EINVAL
 592 A remount operation
 593 .RB ( MS_REMOUNT )
 594 was attempted, but
 595 .I source
 596 was not already mounted on
 597 .IR target .
 598 .TP
 599 .B EINVAL
 600 A move operation
 601 .RB ( MS_MOVE )
 602 was attempted, but
 603 .I source
 604 was not a mount point, or was \(aq/\(aq.
 605 .TP
 606 .B EINVAL
 607 .I mountflags
 608 includes more than one of
 609 .BR MS_SHARED ,
 610 .BR MS_PRIVATE ,
 611 .BR MS_SLAVE ,
 612 or
 613 .BR MS_UNBINDABLE .
 614 .TP
 615 .B EINVAL
 616 .I mountflags
 617 includes
 618 .BR MS_SHARED ,
 619 .BR MS_PRIVATE ,
 620 .BR MS_SLAVE ,
 621 or
 622 .BR MS_UNBINDABLE
 623 and also includes a flag other than
 624 .BR MS_REC
 625 or
 626 .BR MS_SILENT .
 627 .TP
 628 .BR EINVAL
 629 An attempt was made to bind mount an unbindable mount.
 630 .TP
 631 .B ELOOP
 632 Too many links encountered during pathname resolution.
 633 .TP
 634 .B ELOOP
 635 A move operation was attempted, and
 636 .I target
 637 is a descendant of
 638 .IR source .
 639 .TP
 640 .B EMFILE
 641 (In case no block device is required:)
 642 Table of dummy devices is full.
 643 .TP
 644 .B ENAMETOOLONG
 645 A pathname was longer than
 646 .BR MAXPATHLEN .
 647 .TP
 648 .B ENODEV
 649 .I filesystemtype
 650 not configured in the kernel.
 651 .TP
 652 .B ENOENT
 653 A pathname was empty or had a nonexistent component.
 654 .TP
 655 .B ENOMEM
 656 The kernel could not allocate a free page to copy filenames or data into.
 657 .TP
 658 .B ENOTBLK
 659 .I source
 660 is not a block device (and a device was required).
 661 .TP
 662 .B ENOTDIR
 663 .IR target ,
 664 or a prefix of
 665 .IR source ,
 666 is not a directory.
 667 .TP
 668 .B ENXIO
 669 The major number of the block device
 670 .I source
 671 is out of range.
 672 .TP
 673 .B EPERM
 674 The caller does not have the required privileges.
 675 .SH VERSIONS
 676 The definitions of
 677 .BR MS_DIRSYNC ,
 678 .BR MS_MOVE ,
 679 .BR MS_PRIVATE ,
 680 .BR MS_REC ,
 681 .BR MS_RELATIME ,
 682 .BR MS_SHARED ,
 683 .BR MS_SLAVED ,
 684 .BR MS_STRICTATIME
 685 and
 686 .BR MS_UNBINDABLE
 687 were added to glibc headers in version 2.12.
 688 .\"
 689 .SH CONFORMING TO
 690 This function is Linux-specific and should not be used in
 691 programs intended to be portable.
 692 .SH NOTES
 693 Since Linux 2.4 a single filesystem can be mounted at
 694 multiple mount points, and multiple mounts can be stacked
 695 on the same mount point.
 696 .\" Multiple mounts on same mount point: since 2.3.99pre7.
 697
 698 The
 699 .I mountflags
 700 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
 701 in the top 16 bits.
 702 (All of the other flags discussed in DESCRIPTION
 703 occupy the low order 16 bits of
 704 .IR mountflags .)
 705 Specifying
 706 .BR MS_MGC_VAL
 707 was required in kernel versions prior to 2.4,
 708 but since Linux 2.4 is no longer required and is ignored if specified.
 709
 710 The original
 711 .B MS_SYNC
 712 flag was renamed
 713 .B MS_SYNCHRONOUS
 714 in 1.1.69
 715 when a different
 716 .B MS_SYNC
 717 was added to \fI<mman.h>\fP.
 718 .LP
 719 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 720 on a filesystem mounted with
 721 .B MS_NOSUID
 722 would fail with
 723 .BR EPERM .
 724 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 725 just silently ignored in this case.
 726 .\" The change is in patch-2.4.0-prerelease.
 727 .\"
 728
 729 .SS Per-process namespaces
 730 Starting with kernel 2.4.19, Linux provides
 731 per-process mount namespaces.
 732 A mount namespace is the set of filesystem mounts that
 733 are visible to a process.
 734 Mount-point namespaces can be (and usually are)
 735 shared between multiple processes,
 736 and changes to the namespace (i.e., mounts and unmounts) by one process
 737 are visible to all other processes sharing the same namespace.
 738 (The pre-2.4.19 Linux situation can be considered as one in which
 739 a single namespace was shared by every process on the system.)
 740
 741 A child process created by
 742 .BR fork (2)
 743 shares its parent's mount namespace;
 744 the mount namespace is preserved across an
 745 .BR execve (2).
 746
 747 A process can obtain a private mount namespace if:
 748 it was created using the
 749 .BR clone (2)
 750 .BR CLONE_NEWNS
 751 flag,
 752 in which case its new namespace is initialized to be a
 753 .I copy
 754 of the namespace of the process that called
 755 .BR clone (2);
 756 or it calls
 757 .BR unshare (2)
 758 with the
 759 .BR CLONE_NEWNS
 760 flag,
 761 which causes the caller's mount namespace to obtain a private copy
 762 of the namespace that it was previously sharing with other processes,
 763 so that future mounts and unmounts by the caller are invisible
 764 to other processes (except child processes that the caller
 765 subsequently creates) and vice versa.
 766
 767 The Linux-specific
 768 .I /proc/PID/mounts
 769 file exposes the list of mount points in the mount
 770 namespace of the process with the specified ID; see
 771 .BR proc (5)
 772 for details.
 773 .SH SEE ALSO
 774 .BR umount (2),
 775 .BR namespaces (7),
 776 .BR path_resolution (7),
 777 .BR lsblk (8),
 778 .BR findmnt (8),
 779 .BR mount (8),
 780 .BR umount (8)