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>
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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 2014-09-21 "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 .B MS_MANDLOCK
 155 Permit mandatory locking on files in this filesystem.
 156 (Mandatory locking must still be enabled on a per-file basis,
 157 as described in
 158 .BR fcntl (2).)
 159 .\" FIXME Describe the MS_MOVE flag in more detail
 160 .TP
 161 .B MS_MOVE
 162 Move a subtree.
 163 .I source
 164 specifies an existing mount point and
 165 .I target
 166 specifies the new location.
 167 The move is atomic: at no point is the subtree unmounted.
 168 The
 169 .IR filesystemtype ", " mountflags ", and " data
 170 arguments are ignored.
 171 .TP
 172 .B MS_NOATIME
 173 Do not update access times for (all types of) files on this filesystem.
 174 .TP
 175 .B MS_NODEV
 176 Do not allow access to devices (special files) on this filesystem.
 177 .TP
 178 .B MS_NODIRATIME
 179 Do not update access times for directories on this filesystem.
 180 This flag provides a subset of the functionality provided by
 181 .BR MS_NOATIME ;
 182 that is,
 183 .BR MS_NOATIME
 184 implies
 185 .BR MS_NODIRATIME .
 186 .TP
 187 .B MS_NOEXEC
 188 Do not allow programs to be executed from this filesystem.
 189 .\" (Possibly useful for a filesystem that contains non-Linux executables.
 190 .\" Often used as a security feature, e.g., to make sure that restricted
 191 .\" users cannot execute files uploaded using ftp or so.)
 192 .TP
 193 .B MS_NOSUID
 194 Do not honor set-user-ID and set-group-ID bits when executing
 195 programs from this filesystem.
 196 .\" (This is a security feature to prevent users executing set-user-ID and
 197 .\" set-group-ID programs from removable disk devices.)
 198 .TP
 199 .B MS_RDONLY
 200 Mount filesystem read-only.
 201 .\"
 202 .\" FIXME Document MS_REC, available since 2.4.11.
 203 .\" This flag has meaning in conjunction with MS_BIND and
 204 .\" also with the shared subtree flags.
 205 .TP
 206 .BR MS_RELATIME " (since Linux 2.6.20)"
 207 When a file on this filesystem is accessed,
 208 update the file's last access time (atime) only if the current value
 209 of atime is less than or equal to the file's last modification time (mtime)
 210 or last status change time (ctime).
 211 This option is useful for programs, such as
 212 .BR mutt (1),
 213 that need to know when a file has been read since it was last modified.
 214 Since Linux 2.6.30, the kernel defaults to the behavior provided
 215 by this flag (unless
 216 .BR MS_NOATIME
 217 was specified), and the
 218 .B MS_STRICTATIME
 219 flag is required to obtain traditional semantics.
 220 In addition, since Linux 2.6.30,
 221 the file's last access time is always updated if it
 222 is more than 1 day old.
 223 .\" Matthew Garrett notes in the patch that added this behavior
 224 .\" that this lets utilities such as tmpreaper (which deletes
 225 .\" files based on last access time) work correctly.
 226 .TP
 227 .B MS_REMOUNT
 228 Remount an existing mount.
 229 This allows you to change the
 230 .I mountflags
 231 and
 232 .I data
 233 of an existing mount without having to unmount and remount the filesystem.
 234 .I target
 235 should be the same value specified in the initial
 236 .BR mount ()
 237 call;
 238 .I source
 239 and
 240 .I filesystemtype
 241 are ignored.
 242 The
 243 .I mountflags
 244 and
 245 .I data
 246 arguments should match the values used in the original
 247 .BR mount ()
 248 call, except for those parameters that are being deliberately changed.
 249
 250 The following
 251 .I mountflags
 252 can be changed:
 253 .BR MS_RDONLY ,
 254 .BR MS_SYNCHRONOUS ,
 255 .BR MS_MANDLOCK ;
 256 before kernel 2.6.16, the following could also be changed:
 257 .B MS_NOATIME
 258 and
 259 .BR MS_NODIRATIME ;
 260 and, additionally, before kernel 2.4.10, the following could also be changed:
 261 .BR MS_NOSUID ,
 262 .BR MS_NODEV ,
 263 .BR MS_NOEXEC .
 264 .TP
 265 .BR MS_SILENT " (since Linux 2.6.17)"
 266 Suppress the display of certain
 267 .RI ( printk ())
 268 warning messages in the kernel log.
 269 This flag supersedes the misnamed and obsolete
 270 .BR MS_VERBOSE
 271 flag (available since Linux 2.4.12), which has the same meaning.
 272 .TP
 273 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 274 Always update the last access time (atime) when files on this
 275 filesystem are accessed.
 276 (This was the default behavior before Linux 2.6.30.)
 277 Specifying this flag overrides the effect of setting the
 278 .BR MS_NOATIME
 279 and
 280 .BR MS_RELATIME
 281 flags.
 282 .TP
 283 .B MS_SYNCHRONOUS
 284 Make writes on this filesystem synchronous (as though
 285 the
 286 .B O_SYNC
 287 flag to
 288 .BR open (2)
 289 was specified for all file opens to this filesystem).
 290 .PP
 291 From Linux 2.4 onward, the
 292 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 293 flags are settable on a per-mount-point basis.
 294 From kernel 2.6.16 onward,
 295 .B MS_NOATIME
 296 and
 297 .B MS_NODIRATIME
 298 are also settable on a per-mount-point basis.
 299 The
 300 .B MS_RELATIME
 301 flag is also settable on a per-mount-point basis.
 302 .PP
 303 The
 304 .I data
 305 argument is interpreted by the different filesystems.
 306 Typically it is a string of comma-separated options
 307 understood by this filesystem.
 308 See
 309 .BR mount (8)
 310 for details of the options available for each filesystem type.
 311 .SH "RETURN VALUE"
 312 On success, zero is returned.
 313 On error, \-1 is returned, and
 314 .I errno
 315 is set appropriately.
 316 .SH ERRORS
 317 The error values given below result from filesystem type independent
 318 errors.
 319 Each filesystem type may have its own special errors and its
 320 own special behavior.
 321 See the Linux kernel source code for details.
 322 .TP
 323 .B EACCES
 324 A component of a path was not searchable.
 325 (See also
 326 .BR path_resolution (7).)
 327 Or, mounting a read-only filesystem was attempted without giving the
 328 .B MS_RDONLY
 329 flag.
 330 Or, the block device
 331 .I source
 332 is located on a filesystem mounted with the
 333 .B MS_NODEV
 334 option.
 335 .\" mtk: Probably: write permission is required for MS_BIND, with
 336 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 337 .TP
 338 .B EBUSY
 339 .I source
 340 is already mounted.
 341 Or, it cannot be remounted read-only,
 342 because it still holds files open for writing.
 343 Or, it cannot be mounted on
 344 .I target
 345 because
 346 .I target
 347 is still busy (it is the working directory of some thread,
 348 the mount point of another device, has open files, etc.).
 349 .TP
 350 .B EFAULT
 351 One of the pointer arguments points outside the user address space.
 352 .TP
 353 .B EINVAL
 354 .I source
 355 had an invalid superblock.
 356 Or, a remount
 357 .RB ( MS_REMOUNT )
 358 was attempted, but
 359 .I source
 360 was not already mounted on
 361 .IR target .
 362 Or, a move
 363 .RB ( MS_MOVE )
 364 was attempted, but
 365 .I source
 366 was not a mount point, or was \(aq/\(aq.
 367 .TP
 368 .B ELOOP
 369 Too many links encountered during pathname resolution.
 370 Or, a move was attempted, while
 371 .I target
 372 is a descendant of
 373 .IR source .
 374 .TP
 375 .B EMFILE
 376 (In case no block device is required:)
 377 Table of dummy devices is full.
 378 .TP
 379 .B ENAMETOOLONG
 380 A pathname was longer than
 381 .BR MAXPATHLEN .
 382 .TP
 383 .B ENODEV
 384 .I filesystemtype
 385 not configured in the kernel.
 386 .TP
 387 .B ENOENT
 388 A pathname was empty or had a nonexistent component.
 389 .TP
 390 .B ENOMEM
 391 The kernel could not allocate a free page to copy filenames or data into.
 392 .TP
 393 .B ENOTBLK
 394 .I source
 395 is not a block device (and a device was required).
 396 .TP
 397 .B ENOTDIR
 398 .IR target ,
 399 or a prefix of
 400 .IR source ,
 401 is not a directory.
 402 .TP
 403 .B ENXIO
 404 The major number of the block device
 405 .I source
 406 is out of range.
 407 .TP
 408 .B EPERM
 409 The caller does not have the required privileges.
 410 .SH VERSIONS
 411 The definitions of
 412 .BR MS_DIRSYNC ,
 413 .BR MS_MOVE ,
 414 .BR MS_REC ,
 415 .BR MS_RELATIME ,
 416 and
 417 .BR MS_STRICTATIME
 418 were added to glibc headers in version 2.12.
 419 .\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
 420 .\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
 421 .SH "CONFORMING TO"
 422 This function is Linux-specific and should not be used in
 423 programs intended to be portable.
 424 .SH NOTES
 425 The original
 426 .B MS_SYNC
 427 flag was renamed
 428 .B MS_SYNCHRONOUS
 429 in 1.1.69
 430 when a different
 431 .B MS_SYNC
 432 was added to \fI<mman.h>\fP.
 433 .LP
 434 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 435 on a filesystem mounted with
 436 .B MS_NOSUID
 437 would fail with
 438 .BR EPERM .
 439 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 440 just silently ignored in this case.
 441 .\" The change is in patch-2.4.0-prerelease.
 442 .SS Per-process namespaces
 443 Starting with kernel 2.4.19, Linux provides
 444 per-process mount namespaces.
 445 A mount namespace is the set of filesystem mounts that
 446 are visible to a process.
 447 Mount-point namespaces can be (and usually are)
 448 shared between multiple processes,
 449 and changes to the namespace (i.e., mounts and unmounts) by one process
 450 are visible to all other processes sharing the same namespace.
 451 (The pre-2.4.19 Linux situation can be considered as one in which
 452 a single namespace was shared by every process on the system.)
 453
 454 A child process created by
 455 .BR fork (2)
 456 shares its parent's mount namespace;
 457 the mount namespace is preserved across an
 458 .BR execve (2).
 459
 460 A process can obtain a private mount namespace if:
 461 it was created using the
 462 .BR clone (2)
 463 .BR CLONE_NEWNS
 464 flag,
 465 in which case its new namespace is initialized to be a
 466 .I copy
 467 of the namespace of the process that called
 468 .BR clone (2);
 469 or it calls
 470 .BR unshare (2)
 471 with the
 472 .BR CLONE_NEWNS
 473 flag,
 474 which causes the caller's mount namespace to obtain a private copy
 475 of the namespace that it was previously sharing with other processes,
 476 so that future mounts and unmounts by the caller are invisible
 477 to other processes (except child processes that the caller
 478 subsequently creates) and vice versa.
 479
 480 The Linux-specific
 481 .I /proc/PID/mounts
 482 file exposes the list of mount points in the mount
 483 namespace of the process with the specified ID; see
 484 .BR proc (5)
 485 for details.
 486 .SH "SEE ALSO"
 487 .BR umount (2),
 488 .BR namespaces (7),
 489 .BR path_resolution (7),
 490 .BR lsblk (8),
 491 .BR mount (8),
 492 .BR umount (8)