lib/libsys/mount.2

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.Dd March 30, 2020
.Dt MOUNT 2
.Os
.Sh NAME
.Nm mount ,
.Nm nmount ,
.Nm unmount
.Nd mount or dismount a file system
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/param.h
.In sys/mount.h
.Ft int
.Fn mount "const char *type" "const char *dir" "int flags" "void *data"
.Ft int
.Fn unmount "const char *dir" "int flags"
.In sys/uio.h
.Ft int
.Fn nmount "struct iovec *iov" "u_int niov" "int flags"
.Sh DESCRIPTION
The
.Fn mount
system call grafts
a file system object onto the system file tree
at the point
.Fa dir .
The argument
.Fa data
describes the file system object to be mounted.
The argument
.Fa type
tells the kernel how to interpret
.Fa data
(See
.Fa type
below).
The contents of the file system
become available through the new mount point
.Fa dir .
Any files in
.Fa dir
at the time
of a successful mount are swept under the carpet so to speak, and
are unavailable until the file system is unmounted.
.Pp
The
.Fn nmount
system call behaves similarly to
.Fn mount ,
except that the mount options (file system type name, device to mount,
mount-point name, etc.) are passed as an array of name-value pairs
in the array
.Fa iov ,
containing
.Fa niov
elements.
The following options are required by all file systems:
.Bl -column fstype -offset indent
.It
.Li fstype Ta file system type name (e.g., Dq Li procfs )
.It
.Li fspath Ta mount point pathname (e.g., Dq Li /proc )
.El
.Pp
Depending on the file system type, other options may be
recognized or required;
for example, most disk-based file systems require a
.Dq Li from
option containing the pathname of a special device
in addition to the options listed above.
.Pp
By default only the super-user may call the
.Fn mount
system call.
This restriction can be removed by setting the
.Va vfs.usermount
.Xr sysctl 8
variable
to a non-zero value; see the BUGS section for more information.
.Pp
The following
.Fa flags
may be specified to
suppress default semantics which affect file system access.
.Bl -tag -width MNT_SYNCHRONOUS
.It Dv MNT_RDONLY
The file system should be treated as read-only;
even the super-user may not write on it.
Specifying MNT_UPDATE without this option will upgrade
a read-only file system to read/write.
.It Dv MNT_NOEXEC
Do not allow files to be executed from the file system.
.It Dv MNT_NOSUID
Do not honor setuid or setgid bits on files when executing them.
This flag is set automatically when the caller is not the super-user.
.It Dv MNT_NOATIME
Disable update of file access times.
.It Dv MNT_SNAPSHOT
Create a snapshot of the file system.
This is currently only supported on UFS2 file systems, see
.Xr mksnap_ffs 8
for more information.
.It Dv MNT_SUIDDIR
Directories with the SUID bit set chown new files to their own owner.
This flag requires the SUIDDIR option to have been compiled into the kernel
to have any effect.
See the
.Xr mount 8
and
.Xr chmod 2
pages for more information.
.It Dv MNT_SYNCHRONOUS
All I/O to the file system should be done synchronously.
.It Dv MNT_ASYNC
All I/O to the file system should be done asynchronously.
.It Dv MNT_FORCE
Force a read-write mount even if the file system appears to be unclean.
Dangerous.
Together with
.Dv MNT_UPDATE
and
.Dv MNT_RDONLY ,
specify that the file system is to be forcibly downgraded to a read-only
mount even if some files are open for writing.
.It Dv MNT_NOCLUSTERR
Disable read clustering.
.It Dv MNT_NOCLUSTERW
Disable write clustering.
.It Dv MNT_NOCOVER
Do not mount over the root of another mount point.
.It Dv MNT_EMPTYDIR
Require an empty directory for the mount point directory.
.El
.Pp
The flag
.Dv MNT_UPDATE
indicates that the mount command is being applied
to an already mounted file system.
This allows the mount flags to be changed without requiring
that the file system be unmounted and remounted.
Some file systems may not allow all flags to be changed.
For example,
many file systems will not allow a change from read-write to read-only.
.Pp
The flag
.Dv MNT_RELOAD
causes the vfs subsystem to update its data structures pertaining to
the specified already mounted file system.
.Pp
The
.Fa type
argument names the file system.
The types of file systems known to the system can be obtained with
.Xr lsvfs 1 .
.Pp
The
.Fa data
argument
is a pointer to a structure that contains the type
specific arguments to mount.
The format for these argument structures is described in the
manual page for each file system.
By convention file system manual pages are named
by prefixing ``mount_'' to the name of the file system as returned by
.Xr lsvfs 1 .
Thus the
.Tn NFS
file system is described by the
.Xr mount_nfs 8
manual page.
It should be noted that a manual page for default
file systems, known as UFS and UFS2, does not exist.
.Pp
The
.Fn unmount
system call disassociates the file system from the specified
mount point
.Fa dir .
.Pp
The
.Fa flags
argument may include
.Dv MNT_FORCE
to specify that the file system should be forcibly unmounted
even if files are still active.
Active special devices continue to work,
but any further accesses to any other active files result in errors
even if the file system is later remounted.
.Pp
If the
.Dv MNT_BYFSID
flag is specified,
.Fa dir
should instead be a file system ID encoded as
.Dq Li FSID : Ns Ar val0 : Ns Ar val1 ,
where
.Ar val0
and
.Ar val1
are the contents of the
.Vt fsid_t
.Va val[]
array in decimal.
The file system that has the specified file system ID will be unmounted.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn mount
and
.Fn nmount
system calls will fail when one of the following occurs:
.Bl -tag -width Er
.It Bq Er EPERM
The caller is neither the super-user nor the owner of
.Fa dir .
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or the entire length of a path name exceeded 1023 characters.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating a pathname.
.It Bq Er ENOENT
A component of
.Fa dir
does not exist.
.It Bq Er ENOTDIR
A component of
.Fa name
is not a directory,
or a path prefix of
.Fa special
is not a directory.
.It Bq Er EBUSY
Another process currently holds a reference to
.Fa dir .
.It Bq Er EBUSY
The
.Dv MNT_NOCOVER
option was given, and the requested mount point
is already the root of another mount point.
.It Bq Er EFAULT
The
.Fa dir
argument
points outside the process's allocated address space.
.It Bq Er EIO
An I/O error occurred while reading data from
.Fa special .
.It Bq Er EINTEGRITY
The backing store for
.Fa special
detected corrupted data while reading.
.El
.Pp
The following errors can occur for a
.Em ufs
file system mount:
.Bl -tag -width Er
.It Bq Er ENODEV
A component of ufs_args
.Fa fspec
does not exist.
.It Bq Er ENOTBLK
The
.Fa fspec
argument
is not a block device.
.It Bq Er ENOTEMPTY
The
.Dv MNT_EMPTYDIR
option was specified, and the requested mount point
is not an empty directory.
.It Bq Er ENXIO
The major device number of
.Fa fspec
is out of range (this indicates no device driver exists
for the associated hardware).
.It Bq Er EBUSY
.Fa fspec
is already mounted.
.It Bq Er EMFILE
No space remains in the mount table.
.It Bq Er EINVAL
The super block for the file system had a bad magic
number or an out of range block size.
.It Bq Er EINTEGRITY
The super block for the file system had a bad check hash.
The check hash can usually be corrected by running
.Xr fsck 8 .
.It Bq Er ENOMEM
Not enough memory was available to read the cylinder
group information for the file system.
.It Bq Er EIO
An I/O error occurred while reading the super block or
cylinder group information.
.It Bq Er EFAULT
The
.Fa fspec
argument
points outside the process's allocated address space.
.El
.Pp
The following errors can occur for a
.Em nfs
file system mount:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
.Em Nfs
timed out trying to contact the server.
.It Bq Er EFAULT
Some part of the information described by nfs_args
points outside the process's allocated address space.
.El
.Pp
The
.Fn unmount
system call may fail with one of the following errors:
.Bl -tag -width Er
.It Bq Er EPERM
The caller is neither the super-user nor the user who issued the corresponding
.Fn mount
call.
.It Bq Er ENAMETOOLONG
The length of the path name exceeded 1023 characters.
.It Bq Er EINVAL
The requested directory is not in the mount table.
.It Bq Er ENOENT
The file system ID specified using
.Dv MNT_BYFSID
was not found in the mount table.
.It Bq Er EINVAL
The file system ID specified using
.Dv MNT_BYFSID
could not be decoded.
.It Bq Er EINVAL
The specified file system is the root file system.
.It Bq Er EBUSY
A process is holding a reference to a file located
on the file system.
.It Bq Er EIO
An I/O error occurred while writing cached file system information.
.It Bq Er EFAULT
The
.Fa dir
argument
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr lsvfs 1 ,
.Xr mksnap_ffs 8 ,
.Xr mount 8 ,
.Xr umount 8
.Sh HISTORY
The
.Fn mount
and
.Fn unmount
functions appeared in
.At v1 .
The
.Fn nmount
system call first appeared in
.Fx 5.0 .
.Sh BUGS
Some of the error codes need translation to more obvious messages.
.Pp
Allowing untrusted users to mount arbitrary media, e.g. by enabling
.Va vfs.usermount ,
should not be considered safe.
Most file systems in
.Fx
were not built to safeguard against malicious devices.