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 Michael Kerrisk <mtk.manpages@gmail.com>
   4 .\"
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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 device name, but can also be a directory name
  56 or a dummy) to the directory specified by
  57 .IR target .
  58
  59 Appropriate privilege (Linux: the
  60 .B CAP_SYS_ADMIN
  61 capability) is required to mount filesystems.
  62
  63 Since Linux 2.4 a single filesystem can be visible at
  64 multiple mount points, and multiple mounts can be stacked
  65 on the same mount point.
  66 .\" Multiple mounts on same mount point: since 2.3.99pre7.
  67
  68 Values for the
  69 .I filesystemtype
  70 argument supported by the kernel are listed in
  71 .I /proc/filesystems
  72 (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs",
  73 "msdos", "proc", "nfs", "iso9660").
  74 Further types may become available when the appropriate modules
  75 are loaded.
  76
  77 The
  78 .I mountflags
  79 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
  80 in the top 16 bits (this was required in kernel versions prior to 2.4, but
  81 is no longer required and ignored if specified),
  82 and various mount flags
  83 .\" (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
  84 .\" and in \fI<sys/mount.h>\fP for glibc2)
  85 in the low order 16 bits:
  86 .\" FIXME 2.6.15 added flags for "shared subtree" functionality:
  87 .\"
  88 .\"     MS_PRIVATE
  89 .\"             All mounts are private by default. Previously shared mounts
  90 .\"             can be re-marked PRIVATE.
  91 .\"     MS_SHARED
  92 .\"             Mount points that are marked SHARED propagate mount events
  93 .\"             to one another after being cloned.
  94 .\"                     mount --make-rshared ==> MS_SHARED | MS_REC
  95 .\"     MS_SLAVE
  96 .\"             A previously shared mount point can be marked SLAVE, meaning
  97 .\"             it receives propagated events, but does not propagate events.
  98 .\"     MS_UNBINDABLE
  99 .\"             mounts cannot be bound into other places, and will not be
 100 .\"             propagated into new subtrees
 101 .\"
 102 .\" These settings are visible in /proc/$$/mountinfo
 103 .\"
 104 .\" These need to be documented on this page.
 105 .\" See:
 106 .\"
 107 .\" * Documentation/filesystems/sharedsubtree.txt
 108 .\"
 109 .\" * http://lwn.net/Articles/159077/
 110 .\"
 111 .\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
 112 .\"       Shared-Subtree Concept, Implementation, and Applications in Linux
 113 .\"       Al Viro viro@ftp.linux.org.uk
 114 .\"       Ram Pai linuxram@us.ibm.com
 115 .\"
 116 .\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
 117 .\"       Applying mount namespaces
 118 .\"
 119 .\"       Uncover practical applications for advanced Linux mounts features
 120 .\"       Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
 121 .\"       Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
 122 .\"       Date:  17 Sep 2007
 123 .\"
 124 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 125 .\"
 126 .TP
 127 .BR MS_BIND " (Linux 2.4 onward)"
 128 .\" since 2.4.0-test9
 129 Perform a bind mount, making a file or a directory subtree visible at
 130 another point within a filesystem.
 131 Bind mounts may cross filesystem boundaries and span
 132 .BR chroot (2)
 133 jails.
 134 The
 135 .IR filesystemtype
 136 and
 137 .IR data
 138 arguments are ignored.
 139 Up until Linux 2.6.26,
 140 .I mountflags
 141 was also ignored
 142 .\" with the exception of the "hidden" MS_REC mountflags bit
 143 (the bind mount has the same mount options as
 144 the underlying mount point).
 145 .TP
 146 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 147 Make directory changes on this filesystem synchronous.
 148 (This property can be obtained for individual directories
 149 or subtrees using
 150 .BR chattr (1).)
 151 .TP
 152 .BR MS_LAZYTIME " (since Linux 4.0)"
 153 .\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
 154 .\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
 155 .\" commit a26f49926da938f47561f386be56a83dd37a496d
 156 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
 157 by maintaining these changes only in memory.
 158 The on-disk timestamps are updated only when:
 159 .RS
 160 .IP (a) 5
 161 the inode needs to be updated for some change unrelated to file timestamps;
 162 .IP (b)
 163 the application employs
 164 .BR fsync (2),
 165 .BR syncfs (2),
 166 or
 167 .BR sync (2);
 168 .IP (c)
 169 an undeleted inode is evicted from memory; or
 170 .IP (d)
 171 more than 24 hours have passed since the inode was written to disk.
 172 .RE
 173 .IP
 174 This mount option significantly reduces writes
 175 needed to update the inode's timestamps, especially mtime and atime.
 176 However, in the event of a system crash, the atime and mtime fields
 177 on disk might be out of date by up to 24 hours.
 178
 179 Examples of workloads where this option could be of significant benefit
 180 include frequent random writes to preallocated files,
 181 as well as cases where the
 182 .B MS_STRICTATIME
 183 mount option is also enabled.
 184 (The advantage of combining
 185 .BR MS_STRICTATIME
 186 and
 187 .BR MS_LAZYTIME
 188 is that
 189 .BR stat (2)
 190 will return the correctly updated atime, but the atime updates
 191 will be flushed to disk only in the cases listed above.)
 192 .TP
 193 .B MS_MANDLOCK
 194 Permit mandatory locking on files in this filesystem.
 195 (Mandatory locking must still be enabled on a per-file basis,
 196 as described in
 197 .BR fcntl (2).)
 198 .\" FIXME Describe the MS_MOVE flag in more detail
 199 .TP
 200 .B MS_MOVE
 201 Move a subtree.
 202 .I source
 203 specifies an existing mount point and
 204 .I target
 205 specifies the new location.
 206 The move is atomic: at no point is the subtree unmounted.
 207 The
 208 .IR filesystemtype ", " mountflags ", and " data
 209 arguments are ignored.
 210 .TP
 211 .B MS_NOATIME
 212 Do not update access times for (all types of) files on this filesystem.
 213 .TP
 214 .B MS_NODEV
 215 Do not allow access to devices (special files) on this filesystem.
 216 .TP
 217 .B MS_NODIRATIME
 218 Do not update access times for directories on this filesystem.
 219 This flag provides a subset of the functionality provided by
 220 .BR MS_NOATIME ;
 221 that is,
 222 .BR MS_NOATIME
 223 implies
 224 .BR MS_NODIRATIME .
 225 .TP
 226 .B MS_NOEXEC
 227 Do not allow programs to be executed from this filesystem.
 228 .\" (Possibly useful for a filesystem that contains non-Linux executables.
 229 .\" Often used as a security feature, e.g., to make sure that restricted
 230 .\" users cannot execute files uploaded using ftp or so.)
 231 .TP
 232 .B MS_NOSUID
 233 Do not honor set-user-ID and set-group-ID bits when executing
 234 programs from this filesystem.
 235 .\" (This is a security feature to prevent users executing set-user-ID and
 236 .\" set-group-ID programs from removable disk devices.)
 237 .TP
 238 .B MS_RDONLY
 239 Mount filesystem read-only.
 240 .\"
 241 .\" FIXME Document MS_REC, available since 2.4.11.
 242 .\" This flag has meaning in conjunction with MS_BIND and
 243 .\" also with the shared subtree flags.
 244 .TP
 245 .BR MS_RELATIME " (since Linux 2.6.20)"
 246 When a file on this filesystem is accessed,
 247 update the file's last access time (atime) only if the current value
 248 of atime is less than or equal to the file's last modification time (mtime)
 249 or last status change time (ctime).
 250 This option is useful for programs, such as
 251 .BR mutt (1),
 252 that need to know when a file has been read since it was last modified.
 253 Since Linux 2.6.30, the kernel defaults to the behavior provided
 254 by this flag (unless
 255 .BR MS_NOATIME
 256 was specified), and the
 257 .B MS_STRICTATIME
 258 flag is required to obtain traditional semantics.
 259 In addition, since Linux 2.6.30,
 260 the file's last access time is always updated if it
 261 is more than 1 day old.
 262 .\" Matthew Garrett notes in the patch that added this behavior
 263 .\" that this lets utilities such as tmpreaper (which deletes
 264 .\" files based on last access time) work correctly.
 265 .TP
 266 .B MS_REMOUNT
 267 Remount an existing mount.
 268 This allows you to change the
 269 .I mountflags
 270 and
 271 .I data
 272 of an existing mount without having to unmount and remount the filesystem.
 273 .I target
 274 should be the same value specified in the initial
 275 .BR mount ()
 276 call;
 277 .I source
 278 and
 279 .I filesystemtype
 280 are ignored.
 281 The
 282 .I mountflags
 283 and
 284 .I data
 285 arguments should match the values used in the original
 286 .BR mount ()
 287 call, except for those parameters that are being deliberately changed.
 288
 289 The following
 290 .I mountflags
 291 can be changed:
 292 .BR MS_RDONLY ,
 293 .BR MS_SYNCHRONOUS ,
 294 .BR MS_MANDLOCK ;
 295 before kernel 2.6.16, the following could also be changed:
 296 .B MS_NOATIME
 297 and
 298 .BR MS_NODIRATIME ;
 299 and, additionally, before kernel 2.4.10, the following could also be changed:
 300 .BR MS_NOSUID ,
 301 .BR MS_NODEV ,
 302 .BR MS_NOEXEC .
 303 .TP
 304 .BR MS_SILENT " (since Linux 2.6.17)"
 305 Suppress the display of certain
 306 .RI ( printk ())
 307 warning messages in the kernel log.
 308 This flag supersedes the misnamed and obsolete
 309 .BR MS_VERBOSE
 310 flag (available since Linux 2.4.12), which has the same meaning.
 311 .TP
 312 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 313 Always update the last access time (atime) when files on this
 314 filesystem are accessed.
 315 (This was the default behavior before Linux 2.6.30.)
 316 Specifying this flag overrides the effect of setting the
 317 .BR MS_NOATIME
 318 and
 319 .BR MS_RELATIME
 320 flags.
 321 .TP
 322 .B MS_SYNCHRONOUS
 323 Make writes on this filesystem synchronous (as though
 324 the
 325 .B O_SYNC
 326 flag to
 327 .BR open (2)
 328 was specified for all file opens to this filesystem).
 329 .PP
 330 From Linux 2.4 onward, the
 331 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 332 flags are settable on a per-mount-point basis.
 333 From kernel 2.6.16 onward,
 334 .B MS_NOATIME
 335 and
 336 .B MS_NODIRATIME
 337 are also settable on a per-mount-point basis.
 338 The
 339 .B MS_RELATIME
 340 flag is also settable on a per-mount-point basis.
 341 .PP
 342 The
 343 .I data
 344 argument is interpreted by the different filesystems.
 345 Typically it is a string of comma-separated options
 346 understood by this filesystem.
 347 See
 348 .BR mount (8)
 349 for details of the options available for each filesystem type.
 350 .SH RETURN VALUE
 351 On success, zero is returned.
 352 On error, \-1 is returned, and
 353 .I errno
 354 is set appropriately.
 355 .SH ERRORS
 356 The error values given below result from filesystem type independent
 357 errors.
 358 Each filesystem type may have its own special errors and its
 359 own special behavior.
 360 See the Linux kernel source code for details.
 361 .TP
 362 .B EACCES
 363 A component of a path was not searchable.
 364 (See also
 365 .BR path_resolution (7).)
 366 Or, mounting a read-only filesystem was attempted without giving the
 367 .B MS_RDONLY
 368 flag.
 369 Or, the block device
 370 .I source
 371 is located on a filesystem mounted with the
 372 .B MS_NODEV
 373 option.
 374 .\" mtk: Probably: write permission is required for MS_BIND, with
 375 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 376 .TP
 377 .B EBUSY
 378 .I source
 379 is already mounted.
 380 Or, it cannot be remounted read-only,
 381 because it still holds files open for writing.
 382 Or, it cannot be mounted on
 383 .I target
 384 because
 385 .I target
 386 is still busy (it is the working directory of some thread,
 387 the mount point of another device, has open files, etc.).
 388 .TP
 389 .B EFAULT
 390 One of the pointer arguments points outside the user address space.
 391 .TP
 392 .B EINVAL
 393 .I source
 394 had an invalid superblock.
 395 Or, a remount
 396 .RB ( MS_REMOUNT )
 397 was attempted, but
 398 .I source
 399 was not already mounted on
 400 .IR target .
 401 Or, a move
 402 .RB ( MS_MOVE )
 403 was attempted, but
 404 .I source
 405 was not a mount point, or was \(aq/\(aq.
 406 .TP
 407 .B ELOOP
 408 Too many links encountered during pathname resolution.
 409 Or, a move was attempted, while
 410 .I target
 411 is a descendant of
 412 .IR source .
 413 .TP
 414 .B EMFILE
 415 (In case no block device is required:)
 416 Table of dummy devices is full.
 417 .TP
 418 .B ENAMETOOLONG
 419 A pathname was longer than
 420 .BR MAXPATHLEN .
 421 .TP
 422 .B ENODEV
 423 .I filesystemtype
 424 not configured in the kernel.
 425 .TP
 426 .B ENOENT
 427 A pathname was empty or had a nonexistent component.
 428 .TP
 429 .B ENOMEM
 430 The kernel could not allocate a free page to copy filenames or data into.
 431 .TP
 432 .B ENOTBLK
 433 .I source
 434 is not a block device (and a device was required).
 435 .TP
 436 .B ENOTDIR
 437 .IR target ,
 438 or a prefix of
 439 .IR source ,
 440 is not a directory.
 441 .TP
 442 .B ENXIO
 443 The major number of the block device
 444 .I source
 445 is out of range.
 446 .TP
 447 .B EPERM
 448 The caller does not have the required privileges.
 449 .SH VERSIONS
 450 The definitions of
 451 .BR MS_DIRSYNC ,
 452 .BR MS_MOVE ,
 453 .BR MS_REC ,
 454 .BR MS_RELATIME ,
 455 and
 456 .BR MS_STRICTATIME
 457 were added to glibc headers in version 2.12.
 458 .\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
 459 .\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
 460 .SH CONFORMING TO
 461 This function is Linux-specific and should not be used in
 462 programs intended to be portable.
 463 .SH NOTES
 464 The original
 465 .B MS_SYNC
 466 flag was renamed
 467 .B MS_SYNCHRONOUS
 468 in 1.1.69
 469 when a different
 470 .B MS_SYNC
 471 was added to \fI<mman.h>\fP.
 472 .LP
 473 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 474 on a filesystem mounted with
 475 .B MS_NOSUID
 476 would fail with
 477 .BR EPERM .
 478 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 479 just silently ignored in this case.
 480 .\" The change is in patch-2.4.0-prerelease.
 481 .SS Per-process namespaces
 482 Starting with kernel 2.4.19, Linux provides
 483 per-process mount namespaces.
 484 A mount namespace is the set of filesystem mounts that
 485 are visible to a process.
 486 Mount-point namespaces can be (and usually are)
 487 shared between multiple processes,
 488 and changes to the namespace (i.e., mounts and unmounts) by one process
 489 are visible to all other processes sharing the same namespace.
 490 (The pre-2.4.19 Linux situation can be considered as one in which
 491 a single namespace was shared by every process on the system.)
 492
 493 A child process created by
 494 .BR fork (2)
 495 shares its parent's mount namespace;
 496 the mount namespace is preserved across an
 497 .BR execve (2).
 498
 499 A process can obtain a private mount namespace if:
 500 it was created using the
 501 .BR clone (2)
 502 .BR CLONE_NEWNS
 503 flag,
 504 in which case its new namespace is initialized to be a
 505 .I copy
 506 of the namespace of the process that called
 507 .BR clone (2);
 508 or it calls
 509 .BR unshare (2)
 510 with the
 511 .BR CLONE_NEWNS
 512 flag,
 513 which causes the caller's mount namespace to obtain a private copy
 514 of the namespace that it was previously sharing with other processes,
 515 so that future mounts and unmounts by the caller are invisible
 516 to other processes (except child processes that the caller
 517 subsequently creates) and vice versa.
 518
 519 The Linux-specific
 520 .I /proc/PID/mounts
 521 file exposes the list of mount points in the mount
 522 namespace of the process with the specified ID; see
 523 .BR proc (5)
 524 for details.
 525 .SH SEE ALSO
 526 .BR umount (2),
 527 .BR namespaces (7),
 528 .BR path_resolution (7),
 529 .BR lsblk (8),
 530 .BR findmnt (8),
 531 .BR mount (8),
 532 .BR umount (8)