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 2015-04-19 "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 bieing cloned.
  94
  95 .\"                     mount --make-rshared ==> MS_SHARED | MS_REC
  96
  97 .\"     MS_SLAVE
  98 .\"             A previously shared mount point can be marked SLAVE, meaning
  99 .\"             it receives propagated events, but does not propagate events.
 100 .\"     MS_UNBINDABLE
 101 .\"             mounts cannot be bound into other places, and will not be
 102 .\"             propagated into new subtrees
 103 .\"
 104 .\" These settings are visible in /proc/$$/mountinfo
 105 .\"
 106 .\" These need to be documented on this page.
 107 .\" See:
 108 .\"
 109 .\" * Documentation/filesystems/sharedsubtree.txt
 110 .\"
 111 .\" * http://lwn.net/Articles/159077/
 112 .\"
 113 .\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
 114 .\"       Shared-Subtree Concept, Implementation, and Applications in Linux
 115 .\"       Al Viro viro@ftp.linux.org.uk
 116 .\"       Ram Pai linuxram@us.ibm.com
 117 .\"
 118 .\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
 119 .\"       Applying mount namespaces
 120 .\"
 121 .\"       Uncover practical applications for advanced Linux mounts features
 122 .\"       Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
 123 .\"       Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
 124 .\"       Date:  17 Sep 2007
 125 .\"
 126 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 127 .\"
 128 .TP
 129 .BR MS_BIND " (Linux 2.4 onward)"
 130 .\" since 2.4.0-test9
 131 Perform a bind mount, making a file or a directory subtree visible at
 132 another point within a filesystem.
 133 Bind mounts may cross filesystem boundaries and span
 134 .BR chroot (2)
 135 jails.
 136 The
 137 .IR filesystemtype
 138 and
 139 .IR data
 140 arguments are ignored.
 141 Up until Linux 2.6.26,
 142 .I mountflags
 143 was also ignored
 144 .\" with the exception of the "hidden" MS_REC mountflags bit
 145 (the bind mount has the same mount options as
 146 the underlying mount point).
 147 .TP
 148 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 149 Make directory changes on this filesystem synchronous.
 150 (This property can be obtained for individual directories
 151 or subtrees using
 152 .BR chattr (1).)
 153 .TP
 154 .BR MS_LAZYTIME " (since Linux 4.0)"
 155 .\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
 156 .\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
 157 .\" commit a26f49926da938f47561f386be56a83dd37a496d
 158 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
 159 by maintaining these changes only in memory.
 160 The on-disk timestamps are updated only when:
 161 .RS
 162 .IP (a) 5
 163 the inode needs to be updated for some change unrelated to file timestamps;
 164 .IP (b)
 165 the application employs
 166 .BR fsync (2),
 167 .BR syncfs (2),
 168 or
 169 .BR sync (2);
 170 .IP (c)
 171 an undeleted inode is evicted from memory; or
 172 .IP (d)
 173 more than 24 hours have passed since the inode was written to disk.
 174 .RE
 175 .IP
 176 This mount option significantly reduces writes
 177 needed to update the inode's timestamps, especially mtime and atime.
 178 However, in the event of a system crash, the atime and mtime fields
 179 on disk might be out of date by up to 24 hours.
 180
 181 Examples of workloads where this option could be of significant benefit
 182 include frequent random writes to preallocated files,
 183 as well as cases where the
 184 .B MS_STRICTATIME
 185 mount option is also enabled.
 186 (The advantage of combining
 187 .BR MS_STRICTATIME
 188 and
 189 .BR MS_LAZYTIME
 190 is that
 191 .BR stat (2)
 192 will return the correctly updated atime, but the atime updates
 193 will be flushed to disk only in the cases listed above.)
 194 .TP
 195 .B MS_MANDLOCK
 196 Permit mandatory locking on files in this filesystem.
 197 (Mandatory locking must still be enabled on a per-file basis,
 198 as described in
 199 .BR fcntl (2).)
 200 .\" FIXME Describe the MS_MOVE flag in more detail
 201 .TP
 202 .B MS_MOVE
 203 Move a subtree.
 204 .I source
 205 specifies an existing mount point and
 206 .I target
 207 specifies the new location.
 208 The move is atomic: at no point is the subtree unmounted.
 209 The
 210 .IR filesystemtype ", " mountflags ", and " data
 211 arguments are ignored.
 212 .TP
 213 .B MS_NOATIME
 214 Do not update access times for (all types of) files on this filesystem.
 215 .TP
 216 .B MS_NODEV
 217 Do not allow access to devices (special files) on this filesystem.
 218 .TP
 219 .B MS_NODIRATIME
 220 Do not update access times for directories on this filesystem.
 221 This flag provides a subset of the functionality provided by
 222 .BR MS_NOATIME ;
 223 that is,
 224 .BR MS_NOATIME
 225 implies
 226 .BR MS_NODIRATIME .
 227 .TP
 228 .B MS_NOEXEC
 229 Do not allow programs to be executed from this filesystem.
 230 .\" (Possibly useful for a filesystem that contains non-Linux executables.
 231 .\" Often used as a security feature, e.g., to make sure that restricted
 232 .\" users cannot execute files uploaded using ftp or so.)
 233 .TP
 234 .B MS_NOSUID
 235 Do not honor set-user-ID and set-group-ID bits when executing
 236 programs from this filesystem.
 237 .\" (This is a security feature to prevent users executing set-user-ID and
 238 .\" set-group-ID programs from removable disk devices.)
 239 .TP
 240 .B MS_RDONLY
 241 Mount filesystem read-only.
 242 .\"
 243 .\" FIXME Document MS_REC, available since 2.4.11.
 244 .\" This flag has meaning in conjunction with MS_BIND and
 245 .\" also with the shared subtree flags.
 246 .TP
 247 .BR MS_RELATIME " (since Linux 2.6.20)"
 248 When a file on this filesystem is accessed,
 249 update the file's last access time (atime) only if the current value
 250 of atime is less than or equal to the file's last modification time (mtime)
 251 or last status change time (ctime).
 252 This option is useful for programs, such as
 253 .BR mutt (1),
 254 that need to know when a file has been read since it was last modified.
 255 Since Linux 2.6.30, the kernel defaults to the behavior provided
 256 by this flag (unless
 257 .BR MS_NOATIME
 258 was specified), and the
 259 .B MS_STRICTATIME
 260 flag is required to obtain traditional semantics.
 261 In addition, since Linux 2.6.30,
 262 the file's last access time is always updated if it
 263 is more than 1 day old.
 264 .\" Matthew Garrett notes in the patch that added this behavior
 265 .\" that this lets utilities such as tmpreaper (which deletes
 266 .\" files based on last access time) work correctly.
 267 .TP
 268 .B MS_REMOUNT
 269 Remount an existing mount.
 270 This allows you to change the
 271 .I mountflags
 272 and
 273 .I data
 274 of an existing mount without having to unmount and remount the filesystem.
 275 .I target
 276 should be the same value specified in the initial
 277 .BR mount ()
 278 call;
 279 .I source
 280 and
 281 .I filesystemtype
 282 are ignored.
 283 The
 284 .I mountflags
 285 and
 286 .I data
 287 arguments should match the values used in the original
 288 .BR mount ()
 289 call, except for those parameters that are being deliberately changed.
 290
 291 The following
 292 .I mountflags
 293 can be changed:
 294 .BR MS_RDONLY ,
 295 .BR MS_SYNCHRONOUS ,
 296 .BR MS_MANDLOCK ;
 297 before kernel 2.6.16, the following could also be changed:
 298 .B MS_NOATIME
 299 and
 300 .BR MS_NODIRATIME ;
 301 and, additionally, before kernel 2.4.10, the following could also be changed:
 302 .BR MS_NOSUID ,
 303 .BR MS_NODEV ,
 304 .BR MS_NOEXEC .
 305 .TP
 306 .BR MS_SILENT " (since Linux 2.6.17)"
 307 Suppress the display of certain
 308 .RI ( printk ())
 309 warning messages in the kernel log.
 310 This flag supersedes the misnamed and obsolete
 311 .BR MS_VERBOSE
 312 flag (available since Linux 2.4.12), which has the same meaning.
 313 .TP
 314 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 315 Always update the last access time (atime) when files on this
 316 filesystem are accessed.
 317 (This was the default behavior before Linux 2.6.30.)
 318 Specifying this flag overrides the effect of setting the
 319 .BR MS_NOATIME
 320 and
 321 .BR MS_RELATIME
 322 flags.
 323 .TP
 324 .B MS_SYNCHRONOUS
 325 Make writes on this filesystem synchronous (as though
 326 the
 327 .B O_SYNC
 328 flag to
 329 .BR open (2)
 330 was specified for all file opens to this filesystem).
 331 .PP
 332 From Linux 2.4 onward, the
 333 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 334 flags are settable on a per-mount-point basis.
 335 From kernel 2.6.16 onward,
 336 .B MS_NOATIME
 337 and
 338 .B MS_NODIRATIME
 339 are also settable on a per-mount-point basis.
 340 The
 341 .B MS_RELATIME
 342 flag is also settable on a per-mount-point basis.
 343 .PP
 344 The
 345 .I data
 346 argument is interpreted by the different filesystems.
 347 Typically it is a string of comma-separated options
 348 understood by this filesystem.
 349 See
 350 .BR mount (8)
 351 for details of the options available for each filesystem type.
 352 .SH RETURN VALUE
 353 On success, zero is returned.
 354 On error, \-1 is returned, and
 355 .I errno
 356 is set appropriately.
 357 .SH ERRORS
 358 The error values given below result from filesystem type independent
 359 errors.
 360 Each filesystem type may have its own special errors and its
 361 own special behavior.
 362 See the Linux kernel source code for details.
 363 .TP
 364 .B EACCES
 365 A component of a path was not searchable.
 366 (See also
 367 .BR path_resolution (7).)
 368 Or, mounting a read-only filesystem was attempted without giving the
 369 .B MS_RDONLY
 370 flag.
 371 Or, the block device
 372 .I source
 373 is located on a filesystem mounted with the
 374 .B MS_NODEV
 375 option.
 376 .\" mtk: Probably: write permission is required for MS_BIND, with
 377 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 378 .TP
 379 .B EBUSY
 380 .I source
 381 is already mounted.
 382 Or, it cannot be remounted read-only,
 383 because it still holds files open for writing.
 384 Or, it cannot be mounted on
 385 .I target
 386 because
 387 .I target
 388 is still busy (it is the working directory of some thread,
 389 the mount point of another device, has open files, etc.).
 390 .TP
 391 .B EFAULT
 392 One of the pointer arguments points outside the user address space.
 393 .TP
 394 .B EINVAL
 395 .I source
 396 had an invalid superblock.
 397 Or, a remount
 398 .RB ( MS_REMOUNT )
 399 was attempted, but
 400 .I source
 401 was not already mounted on
 402 .IR target .
 403 Or, a move
 404 .RB ( MS_MOVE )
 405 was attempted, but
 406 .I source
 407 was not a mount point, or was \(aq/\(aq.
 408 .TP
 409 .B ELOOP
 410 Too many links encountered during pathname resolution.
 411 Or, a move was attempted, while
 412 .I target
 413 is a descendant of
 414 .IR source .
 415 .TP
 416 .B EMFILE
 417 (In case no block device is required:)
 418 Table of dummy devices is full.
 419 .TP
 420 .B ENAMETOOLONG
 421 A pathname was longer than
 422 .BR MAXPATHLEN .
 423 .TP
 424 .B ENODEV
 425 .I filesystemtype
 426 not configured in the kernel.
 427 .TP
 428 .B ENOENT
 429 A pathname was empty or had a nonexistent component.
 430 .TP
 431 .B ENOMEM
 432 The kernel could not allocate a free page to copy filenames or data into.
 433 .TP
 434 .B ENOTBLK
 435 .I source
 436 is not a block device (and a device was required).
 437 .TP
 438 .B ENOTDIR
 439 .IR target ,
 440 or a prefix of
 441 .IR source ,
 442 is not a directory.
 443 .TP
 444 .B ENXIO
 445 The major number of the block device
 446 .I source
 447 is out of range.
 448 .TP
 449 .B EPERM
 450 The caller does not have the required privileges.
 451 .SH VERSIONS
 452 The definitions of
 453 .BR MS_DIRSYNC ,
 454 .BR MS_MOVE ,
 455 .BR MS_REC ,
 456 .BR MS_RELATIME ,
 457 and
 458 .BR MS_STRICTATIME
 459 were added to glibc headers in version 2.12.
 460 .\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
 461 .\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
 462 .SH CONFORMING TO
 463 This function is Linux-specific and should not be used in
 464 programs intended to be portable.
 465 .SH NOTES
 466 The original
 467 .B MS_SYNC
 468 flag was renamed
 469 .B MS_SYNCHRONOUS
 470 in 1.1.69
 471 when a different
 472 .B MS_SYNC
 473 was added to \fI<mman.h>\fP.
 474 .LP
 475 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 476 on a filesystem mounted with
 477 .B MS_NOSUID
 478 would fail with
 479 .BR EPERM .
 480 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 481 just silently ignored in this case.
 482 .\" The change is in patch-2.4.0-prerelease.
 483 .SS Per-process namespaces
 484 Starting with kernel 2.4.19, Linux provides
 485 per-process mount namespaces.
 486 A mount namespace is the set of filesystem mounts that
 487 are visible to a process.
 488 Mount-point namespaces can be (and usually are)
 489 shared between multiple processes,
 490 and changes to the namespace (i.e., mounts and unmounts) by one process
 491 are visible to all other processes sharing the same namespace.
 492 (The pre-2.4.19 Linux situation can be considered as one in which
 493 a single namespace was shared by every process on the system.)
 494
 495 A child process created by
 496 .BR fork (2)
 497 shares its parent's mount namespace;
 498 the mount namespace is preserved across an
 499 .BR execve (2).
 500
 501 A process can obtain a private mount namespace if:
 502 it was created using the
 503 .BR clone (2)
 504 .BR CLONE_NEWNS
 505 flag,
 506 in which case its new namespace is initialized to be a
 507 .I copy
 508 of the namespace of the process that called
 509 .BR clone (2);
 510 or it calls
 511 .BR unshare (2)
 512 with the
 513 .BR CLONE_NEWNS
 514 flag,
 515 which causes the caller's mount namespace to obtain a private copy
 516 of the namespace that it was previously sharing with other processes,
 517 so that future mounts and unmounts by the caller are invisible
 518 to other processes (except child processes that the caller
 519 subsequently creates) and vice versa.
 520
 521 The Linux-specific
 522 .I /proc/PID/mounts
 523 file exposes the list of mount points in the mount
 524 namespace of the process with the specified ID; see
 525 .BR proc (5)
 526 for details.
 527 .SH SEE ALSO
 528 .BR umount (2),
 529 .BR namespaces (7),
 530 .BR path_resolution (7),
 531 .BR lsblk (8),
 532 .BR mount (8),
 533 .BR umount (8)