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_LAZYTIME ,
 325 .BR MS_MANDLOCK ,
 326 .BR MS_NOATIME ,
 327 .BR MS_NODEV ,
 328 .BR MS_NODIRATIME ,
 329 .BR MS_NOEXEC ,
 330 .BR MS_NOSUID ,
 331 .BR MS_RELATIME ,
 332 .BR MS_RDONLY ,
 333 and
 334 .BR MS_SYNCHRONOUS .
 335 Attempts to change the setting of the
 336 .\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
 337 .BR MS_DIRSYNC
 338 flag during a remount are silently ignored.
 339
 340 Since Linux 3.17,
 341 .\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
 342 if none of
 343 .BR MS_NOATIME ,
 344 .BR MS_NODIRATIME ,
 345 .BR MS_RELATIME ,
 346 or
 347 .BR MS_STRICTATIME
 348 is specified in
 349 .IR mountflags ,
 350 then the remount operation preserves the existing values of these flags
 351 (rather than defaulting to
 352 .BR MS_RELATIME ).
 353
 354 Since Linux 2.6.26, this flag can also be used to make
 355 .\" See https://lwn.net/Articles/281157/
 356 an existing bind mount read-only by specifying
 357 .IR mountflags
 358 as:
 359
 360     MS_REMOUNT | MS_BIND | MS_RDONLY
 361
 362 Note that only the
 363 .BR MS_RDONLY
 364 setting of the bind mount can be changed in this manner.
 365 .\"
 366 .SS Creating a bind mount
 367 If
 368 .I mountflags
 369 includes
 370 .BR MS_BIND
 371 (available since Linux 2.4),
 372 .\" since 2.4.0-test9
 373 then perform a bind mount.
 374 A bind mount makes a file or a directory subtree visible at
 375 another point within the single directory hierarchy.
 376 Bind mounts may cross filesystem boundaries and span
 377 .BR chroot (2)
 378 jails.
 379
 380 The
 381 .IR filesystemtype
 382 and
 383 .IR data
 384 arguments are ignored.
 385
 386 The remaining bits in the
 387 .I mountflags
 388 argument are also ignored, with the exception of
 389 .BR MS_REC .
 390 (The bind mount has the same mount options as
 391 the underlying mount point.)
 392 However, see the discussion of remounting above,
 393 for a method of making an existing bind mount read-only.
 394
 395 By default, when a directory is bind mounted,
 396 only that directory is mounted;
 397 if there are any submounts under the directory tree,
 398 they are not bind mounted.
 399 If the
 400 .BR MS_REC
 401 flag is also specified, then a recursive bind mount operation is performed:
 402 all submounts under the
 403 .I source
 404 subtree (other than unbindable mounts)
 405 are also bind mounted at the corresponding location in the
 406 .I target
 407 subtree.
 408 .\"
 409 .SS Changing the propagation type of an existing mount
 410 If
 411 .IR mountflags
 412 includes one of
 413 .BR MS_SHARED ,
 414 .BR MS_PRIVATE ,
 415 .BR MS_SLAVE ,
 416 or
 417 .BR MS_UNBINDABLE
 418 (all available since Linux 2.6.15),
 419 then the propagation type of an existing mount is changed.
 420 If more than one of these flags is specified, an error results.
 421
 422 The only flags that can be used with changing the propagation type are
 423 .BR MS_REC
 424 and
 425 .BR MS_SILENT .
 426
 427 The
 428 .IR source ,
 429 .IR filesystemtype ,
 430 and
 431 .IR data
 432 arguments are ignored.
 433
 434 The meanings of the propagation type flags are as follows:
 435 .TP
 436 .BR MS_SHARED
 437 Make this mount point shared.
 438 Mount and unmount events immediately under this mount point will propagate
 439 to the other mount points that are members of this mount's peer group.
 440 Propagation here means that the same mount or unmount will automatically
 441 occur under all of the other mount points in the peer group.
 442 Conversely, mount and unmount events that take place under
 443 peer mount points will propagate to this mount point.
 444 .TP
 445 .BR MS_PRIVATE
 446 Make this mount point private.
 447 Mount and unmount events do not propagate into or out of this mount point.
 448 This is the default propagation type for newly created mount points.
 449 .TP
 450 .BR MS_SLAVE
 451 If this is a shared mount point that is a member of a peer group
 452 that contains other members, convert it to a slave mount.
 453 If this is a shared mount point that is a member of a peer group
 454 that contains no other members, convert it to a private mount.
 455 Otherwise, the propagation type of the mount point is left unchanged.
 456
 457 When a mount point is a slave,
 458 mount and unmount events propagate into this mount point from
 459 the (master) shared peer group of which it was formerly a member.
 460 Mount and unmount events under this mount point do not propagate to any peer.
 461
 462 A mount point can be the slave of another peer group
 463 while at the same time sharing mount and unmount events
 464 with a peer group of which it is a member.
 465 .TP
 466 .BR MS_UNBINDABLE
 467 Make this mount unbindable.
 468 This is like a private mount,
 469 and in addition this mount can't be bind mounted.
 470 When a recursive bind mount
 471 .RB ( mount (2)
 472 with the
 473 .BR MS_BIND
 474 and
 475 .BR MS_REC
 476 flags) is performed on a directory subtree,
 477 any bind mounts within the subtree are automatically pruned
 478 (i.e., not replicated)
 479 when replicating that subtree to produce the target subtree.
 480 .PP
 481 By default, changing the propagation type affects only the
 482 .I target
 483 mount point.
 484 If the
 485 .B MS_REC
 486 flag is also specified in
 487 .IR mountflags ,
 488 then the propagation type of all mount points under
 489 .IR target
 490 is also changed.
 491 .\"
 492 .SS Moving a mount
 493 If
 494 .I mountflags
 495 contains the flag
 496 .BR MS_MOVE
 497 (available since Linux 2.4.18),
 498 then move a subtree:
 499 .I source
 500 specifies an existing mount point and
 501 .I target
 502 specifies the new location to which that mount point is to be relocated.
 503 The move is atomic: at no point is the subtree unmounted.
 504
 505 The remaining bits in the
 506 .IR mountflags
 507 argument are ignored, as are the
 508 .IR filesystemtype
 509 and
 510 .IR data
 511 arguments.
 512 .\"
 513 .SS Creating a new mount point
 514 If none of
 515 .BR MS_REMOUNT ,
 516 .BR MS_BIND ,
 517 .BR MS_MOVE ,
 518 .BR MS_SHARED ,
 519 .BR MS_PRIVATE ,
 520 .BR MS_SLAVE ,
 521 or
 522 .BR MS_UNBINDABLE
 523 is specified in
 524 .IR mountflags ,
 525 then
 526 .BR mount ()
 527 performs its default action: creating a new mount point.
 528 .IR source
 529 specifies the source for the new mount point, and
 530 .IR target
 531 specifies the directory at which to create the mount point.
 532
 533 The
 534 .I filesystemtype
 535 and
 536 .I data
 537 arguments are employed, and further bits may be specified in
 538 .IR mountflags
 539 to modify the behavior of the call.
 540 .\"
 541 .SH RETURN VALUE
 542 On success, zero is returned.
 543 On error, \-1 is returned, and
 544 .I errno
 545 is set appropriately.
 546 .SH ERRORS
 547 The error values given below result from filesystem type independent
 548 errors.
 549 Each filesystem type may have its own special errors and its
 550 own special behavior.
 551 See the Linux kernel source code for details.
 552 .TP
 553 .B EACCES
 554 A component of a path was not searchable.
 555 (See also
 556 .BR path_resolution (7).)
 557 .TP
 558 .B EACCES
 559 Mounting a read-only filesystem was attempted without giving the
 560 .B MS_RDONLY
 561 flag.
 562 .TP
 563 .B EACCES
 564 The block device
 565 .I source
 566 is located on a filesystem mounted with the
 567 .B MS_NODEV
 568 option.
 569 .\" mtk: Probably: write permission is required for MS_BIND, with
 570 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 571 .TP
 572 .B EBUSY
 573 .I source
 574 is already mounted.
 575 .TP
 576 .B EBUSY
 577 .I source
 578 cannot be remounted read-only,
 579 because it still holds files open for writing.
 580 .TP
 581 .B EBUSY
 582 .I source
 583 cannot be mounted on
 584 .I target
 585 because
 586 .I target
 587 is still busy (it is the working directory of some thread,
 588 the mount point of another device, has open files, etc.).
 589 .TP
 590 .B EFAULT
 591 One of the pointer arguments points outside the user address space.
 592 .TP
 593 .B EINVAL
 594 .I source
 595 had an invalid superblock.
 596 .TP
 597 .B EINVAL
 598 A remount operation
 599 .RB ( MS_REMOUNT )
 600 was attempted, but
 601 .I source
 602 was not already mounted on
 603 .IR target .
 604 .TP
 605 .B EINVAL
 606 A move operation
 607 .RB ( MS_MOVE )
 608 was attempted, but
 609 .I source
 610 was not a mount point, or was \(aq/\(aq.
 611 .TP
 612 .B EINVAL
 613 .I mountflags
 614 includes more than one of
 615 .BR MS_SHARED ,
 616 .BR MS_PRIVATE ,
 617 .BR MS_SLAVE ,
 618 or
 619 .BR MS_UNBINDABLE .
 620 .TP
 621 .B EINVAL
 622 .I mountflags
 623 includes
 624 .BR MS_SHARED ,
 625 .BR MS_PRIVATE ,
 626 .BR MS_SLAVE ,
 627 or
 628 .BR MS_UNBINDABLE
 629 and also includes a flag other than
 630 .BR MS_REC
 631 or
 632 .BR MS_SILENT .
 633 .TP
 634 .BR EINVAL
 635 An attempt was made to bind mount an unbindable mount.
 636 .TP
 637 .B ELOOP
 638 Too many links encountered during pathname resolution.
 639 .TP
 640 .B ELOOP
 641 A move operation was attempted, and
 642 .I target
 643 is a descendant of
 644 .IR source .
 645 .TP
 646 .B EMFILE
 647 (In case no block device is required:)
 648 Table of dummy devices is full.
 649 .TP
 650 .B ENAMETOOLONG
 651 A pathname was longer than
 652 .BR MAXPATHLEN .
 653 .TP
 654 .B ENODEV
 655 .I filesystemtype
 656 not configured in the kernel.
 657 .TP
 658 .B ENOENT
 659 A pathname was empty or had a nonexistent component.
 660 .TP
 661 .B ENOMEM
 662 The kernel could not allocate a free page to copy filenames or data into.
 663 .TP
 664 .B ENOTBLK
 665 .I source
 666 is not a block device (and a device was required).
 667 .TP
 668 .B ENOTDIR
 669 .IR target ,
 670 or a prefix of
 671 .IR source ,
 672 is not a directory.
 673 .TP
 674 .B ENXIO
 675 The major number of the block device
 676 .I source
 677 is out of range.
 678 .TP
 679 .B EPERM
 680 The caller does not have the required privileges.
 681 .SH VERSIONS
 682 The definitions of
 683 .BR MS_DIRSYNC ,
 684 .BR MS_MOVE ,
 685 .BR MS_PRIVATE ,
 686 .BR MS_REC ,
 687 .BR MS_RELATIME ,
 688 .BR MS_SHARED ,
 689 .BR MS_SLAVED ,
 690 .BR MS_STRICTATIME
 691 and
 692 .BR MS_UNBINDABLE
 693 were added to glibc headers in version 2.12.
 694 .\"
 695 .SH CONFORMING TO
 696 This function is Linux-specific and should not be used in
 697 programs intended to be portable.
 698 .SH NOTES
 699 Since Linux 2.4 a single filesystem can be mounted at
 700 multiple mount points, and multiple mounts can be stacked
 701 on the same mount point.
 702 .\" Multiple mounts on same mount point: since 2.3.99pre7.
 703
 704 The
 705 .I mountflags
 706 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
 707 in the top 16 bits.
 708 (All of the other flags discussed in DESCRIPTION
 709 occupy the low order 16 bits of
 710 .IR mountflags .)
 711 Specifying
 712 .BR MS_MGC_VAL
 713 was required in kernel versions prior to 2.4,
 714 but since Linux 2.4 is no longer required and is ignored if specified.
 715
 716 The original
 717 .B MS_SYNC
 718 flag was renamed
 719 .B MS_SYNCHRONOUS
 720 in 1.1.69
 721 when a different
 722 .B MS_SYNC
 723 was added to \fI<mman.h>\fP.
 724 .LP
 725 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 726 on a filesystem mounted with
 727 .B MS_NOSUID
 728 would fail with
 729 .BR EPERM .
 730 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 731 just silently ignored in this case.
 732 .\" The change is in patch-2.4.0-prerelease.
 733 .\"
 734
 735 .SS Per-process namespaces
 736 Starting with kernel 2.4.19, Linux provides
 737 per-process mount namespaces.
 738 A mount namespace is the set of filesystem mounts that
 739 are visible to a process.
 740 Mount-point namespaces can be (and usually are)
 741 shared between multiple processes,
 742 and changes to the namespace (i.e., mounts and unmounts) by one process
 743 are visible to all other processes sharing the same namespace.
 744 (The pre-2.4.19 Linux situation can be considered as one in which
 745 a single namespace was shared by every process on the system.)
 746
 747 A child process created by
 748 .BR fork (2)
 749 shares its parent's mount namespace;
 750 the mount namespace is preserved across an
 751 .BR execve (2).
 752
 753 A process can obtain a private mount namespace if:
 754 it was created using the
 755 .BR clone (2)
 756 .BR CLONE_NEWNS
 757 flag,
 758 in which case its new namespace is initialized to be a
 759 .I copy
 760 of the namespace of the process that called
 761 .BR clone (2);
 762 or it calls
 763 .BR unshare (2)
 764 with the
 765 .BR CLONE_NEWNS
 766 flag,
 767 which causes the caller's mount namespace to obtain a private copy
 768 of the namespace that it was previously sharing with other processes,
 769 so that future mounts and unmounts by the caller are invisible
 770 to other processes (except child processes that the caller
 771 subsequently creates) and vice versa.
 772
 773 The Linux-specific
 774 .I /proc/PID/mounts
 775 file exposes the list of mount points in the mount
 776 namespace of the process with the specified ID; see
 777 .BR proc (5)
 778 for details.
 779 .SH SEE ALSO
 780 .BR umount (2),
 781 .BR namespaces (7),
 782 .BR path_resolution (7),
 783 .BR lsblk (8),
 784 .BR findmnt (8),
 785 .BR mount (8),
 786 .BR umount (8)