.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH fsconfig 2 2025-10-01 "Linux man-pages (unreleased)"
.SH NAME
fsconfig \- configure new or existing filesystem context
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/mount.h>
.P
.BI "int fsconfig(int " fd ", unsigned int " cmd ,
.BI "             const char *_Nullable " key ,
.BI "             const void *_Nullable " value ", int " aux );
.fi
.SH DESCRIPTION
The
.BR fsconfig ()
system call is part of
the suite of file-descriptor-based mount facilities in Linux.
.P
.BR fsconfig ()
is used to supply parameters to
and issue commands against
the filesystem configuration context
associated with the file descriptor
.IR fd .
Filesystem configuration contexts can be created with
.BR fsopen (2)
or be instantiated from an extant filesystem instance with
.BR fspick (2).
.P
The
.I cmd
argument indicates the command to be issued.
Some commands supply parameters to the context
(equivalent to mount options specified with
.BR mount (8)),
while others are meta-operations on the filesystem context.
The list of valid
.I cmd
values are:
.RS
.TP
.B FSCONFIG_SET_FLAG
Set the flag parameter named by
.IR key .
.I value
must be NULL,
and
.I aux
must be 0.
.TP
.B FSCONFIG_SET_STRING
Set the string parameter named by
.I key
to the value specified by
.IR value .
.I value
points to a null-terminated string,
and
.I aux
must be 0.
.TP
.B FSCONFIG_SET_BINARY
Set the blob parameter named by
.I key
to the contents of the binary blob
specified by
.IR value .
.I value
points to
the start of a buffer
that is
.I aux
bytes in length.
.TP
.B FSCONFIG_SET_FD
Set the file parameter named by
.I key
to the open file description
referenced by the file descriptor
.IR aux .
.I value
must be NULL.
.IP
You may also use
.B \%FSCONFIG_SET_STRING
for file parameters,
with
.I value
set to a null-terminated string
containing a base-10 representation
of the file descriptor number.
This mechanism is primarily intended for compatibility
with older
.BR mount (2)-based
programs,
and only works for parameters
that
.I only
accept file descriptor arguments.
.TP
.B FSCONFIG_SET_PATH
Set the path parameter named by
.I key
to the object at a provided path,
resolved in a similar manner to
.BR openat (2).
.I value
points to a null-terminated pathname string,
and
.I aux
is equivalent to the
.I dirfd
argument to
.BR openat (2).
See
.BR openat (2)
for an explanation of the need for
.BR \%FSCONFIG_SET_PATH .
.IP
You may also use
.B \%FSCONFIG_SET_STRING
for path parameters,
the behaviour of which is equivalent to
.B \%FSCONFIG_SET_PATH
with
.I aux
set to
.BR \%AT_FDCWD .
.TP
.B FSCONFIG_SET_PATH_EMPTY
As with
.BR \%FSCONFIG_SET_PATH ,
except that if
.I value
is an empty string,
the file descriptor specified by
.I aux
is operated on directly
and may be any type of file
(not just a directory).
This is equivalent to the behaviour of
.B \%AT_EMPTY_PATH
with most "*at()" system calls.
If
.I aux
is
.BR \%AT_FDCWD ,
the parameter will be set to
the current working directory
of the calling process.
.TP
.B FSCONFIG_CMD_CREATE
This command instructs the filesystem driver
to instantiate an instance of the filesystem in the kernel
with the parameters specified in the filesystem configuration context.
.I key
and
.I value
must be NULL,
and
.I aux
must be 0.
.IP
This command can only be issued once
in the lifetime of a filesystem context.
If the operation succeeds,
the filesystem context
associated with file descriptor
.I fd
now references the created filesystem instance,
and is placed into a special "awaiting-mount" mode
that allows you to use
.BR fsmount (2)
to create a mount object from the filesystem instance.
.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
If the operation fails,
in most cases
the filesystem context is placed in a failed mode
and cannot be used for any further
.BR fsconfig ()
operations
(though you may still retrieve diagnostic messages
through the message retrieval interface,
as described in
the corresponding subsection of
.BR fsopen (2)).
.IP
This command can only be issued against
filesystem configuration contexts
that were created with
.BR fsopen (2).
In order to create a filesystem instance,
the calling process must have the
.B \%CAP_SYS_ADMIN
capability.
.IP
An important thing to be aware of is that
the Linux kernel will
.I silently
reuse extant filesystem instances
depending on the filesystem type
and the configured parameters
(each filesystem driver has
its own policy for
how filesystem instances are reused).
This means that
the filesystem instance "created" by
.B \%FSCONFIG_CMD_CREATE
may, in fact, be a reference
to an extant filesystem instance in the kernel.
(For reference,
this behaviour also applies to
.BR mount (2).)
.IP
One side-effect of this behaviour is that
if an extant filesystem instance is reused,
.I all
parameters configured
for this filesystem configuration context
are
.I silently ignored
(with the exception of the
.I ro
and
.I rw
flag parameters;
if the state of the read-only flag in the
extant filesystem instance and the filesystem configuration context
do not match, this operation will return
.BR EBUSY ).
This also means that
.B \%FSCONFIG_CMD_RECONFIGURE
commands issued against
the "created" filesystem instance
will also affect any mount objects associated with
the extant filesystem instance.
.IP
Programs that need to ensure
that they create a new filesystem instance
with specific parameters
(notably, security-related parameters
such as
.I acl
to enable POSIX ACLs\[em]\c
as described in
.BR acl (5))
should use
.B \%FSCONFIG_CMD_CREATE_EXCL
instead.
.TP
.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
.\" commit 84ab1277ce5a90a8d1f377707d662ac43cc0918a
As with
.BR \%FSCONFIG_CMD_CREATE ,
except that the kernel is instructed
to not reuse extant filesystem instances.
If the operation
would be forced to
reuse an extant filesystem instance,
this operation will return
.B EBUSY
instead.
.IP
As a result (unlike
.BR \%FSCONFIG_CMD_CREATE ),
if this operation succeeds
then the calling process can be sure that
all of the parameters successfully configured with
.BR fsconfig ()
will actually be applied
to the created filesystem instance.
.TP
.B FSCONFIG_CMD_RECONFIGURE
This command instructs the filesystem driver
to apply the parameters specified in the filesystem configuration context
to the extant filesystem instance
referenced by the filesystem configuration context.
.I key
and
.I value
must be NULL,
and
.I aux
must be 0.
.IP
This is primarily intended for use with
.BR fspick (2),
but may also be used to modify
the parameters of a filesystem instance
after
.B \%FSCONFIG_CMD_CREATE
was used to create it
and a mount object was created using
.BR fsmount (2).
In order to reconfigure an extant filesystem instance,
the calling process must have the
.B CAP_SYS_ADMIN
capability.
.IP
If the operation succeeds,
the filesystem context is reset
but remains in reconfiguration mode
and thus can be reused for subsequent
.B \%FSCONFIG_CMD_RECONFIGURE
commands.
If the operation fails,
in most cases
the filesystem context is placed in a failed mode
and cannot be used for any further
.BR fsconfig ()
operations
(though you may still retrieve diagnostic messages
through the message retrieval interface,
as described in
the corresponding subsection of
.BR fsopen (2)).
.RE
.P
Parameters specified with
.BI FSCONFIG_SET_ *
do not take effect
until a corresponding
.B \%FSCONFIG_CMD_CREATE
or
.B \%FSCONFIG_CMD_RECONFIGURE
command is issued.
.SH RETURN VALUE
On success,
.BR fsconfig ()
returns 0.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
If an error occurs, the filesystem driver may provide
additional information about the error
through the message retrieval interface for filesystem configuration contexts.
This additional information can be retrieved at any time by calling
.BR read (2)
on the filesystem instance or filesystem configuration context
referenced by the file descriptor
.IR fd .
(See the "Message retrieval interface" subsection in
.BR fsopen (2)
for more details on the message format.)
.P
Even after an error occurs,
the filesystem configuration context is
.I not
invalidated,
and thus can still be used with other
.BR fsconfig ()
commands.
This means that users can probe support for filesystem parameters
on a per-parameter basis,
and adjust which parameters they wish to set.
.P
The error values given below result from
filesystem type independent errors.
Each filesystem type may have its own special errors
and its own special behavior.
See the Linux kernel source code for details.
.TP
.B EACCES
A component of a path
provided as a path parameter
was not searchable.
(See also
.BR path_resolution (7).)
.TP
.B EACCES
.B \%FSCONFIG_CMD_CREATE
was attempted
for a read-only filesystem
without specifying the
.RB ' ro '
flag parameter.
.TP
.B EACCES
A specified block device parameter
is located on a filesystem
mounted with the
.B \%MS_NODEV
option.
.TP
.B EBADF
The file descriptor given by
.I fd
(or possibly by
.IR aux ,
depending on the command)
is invalid.
.TP
.B EBUSY
The filesystem context associated with
.I fd
is in the wrong state
for the given command.
.TP
.B EBUSY
The filesystem instance cannot be reconfigured as read-only
with
.B \%FSCONFIG_CMD_RECONFIGURE
because some programs
still hold files open for writing.
.TP
.B EBUSY
A new filesystem instance was requested with
.B \%FSCONFIG_CMD_CREATE_EXCL
but a matching superblock already existed.
.TP
.B EFAULT
One of the pointer arguments
points to a location
outside the calling process's accessible address space.
.TP
.B EINVAL
.I fd
does not refer to
a filesystem configuration context
or filesystem instance.
.TP
.B EINVAL
One of the values of
.IR key ,
.IR value ,
and/or
.I aux
were set to a non-zero value when
.I cmd
required that they be zero
(or NULL).
.TP
.B EINVAL
The parameter named by
.I key
cannot be set
using the type specified with
.IR cmd .
.TP
.B EINVAL
One of the source parameters
referred to
an invalid superblock.
.TP
.B ELOOP
Too many links encountered
during pathname resolution
of a path argument.
.TP
.B ENAMETOOLONG
A path argument was longer than
.BR PATH_MAX .
.TP
.B ENOENT
A path argument had a non-existent component.
.TP
.B ENOENT
A path argument is an empty string,
but
.I cmd
is not
.BR \%FSCONFIG_SET_PATH_EMPTY .
.TP
.B ENOMEM
The kernel could not allocate sufficient memory to complete the operation.
.TP
.B ENOTBLK
The parameter named by
.I key
must be a block device,
but the provided parameter value was not a block device.
.TP
.B ENOTDIR
A component of the path prefix
of a path argument
was not a directory.
.TP
.B EOPNOTSUPP
The command given by
.I cmd
is not valid.
.TP
.B ENXIO
The major number
of a block device parameter
is out of range.
.TP
.B EPERM
The command given by
.I cmd
was
.BR \%FSCONFIG_CMD_CREATE ,
.BR \%FSCONFIG_CMD_CREATE_EXCL ,
or
.BR \%FSCONFIG_CMD_RECONFIGURE ,
but the calling process does not have the required
.B \%CAP_SYS_ADMIN
capability.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 5.2.
.\" commit ecdab150fddb42fe6a739335257949220033b782
.\" commit 400913252d09f9cfb8cce33daee43167921fc343
glibc 2.36.
.SH NOTES
.SS Generic filesystem parameters
Each filesystem driver is responsible for
parsing most parameters specified with
.BR fsconfig (),
meaning that individual filesystems
may have very different behaviour
when encountering parameters with the same name.
In general,
you should not assume that the behaviour of
.BR fsconfig ()
when specifying a parameter to one filesystem type
will match the behaviour of the same parameter
with a different filesystem type.
.P
However,
the following generic parameters
apply to all filesystems and have unified behaviour.
They are set using the listed
.BI \%FSCONFIG_SET_ *
command.
.TP
.BR ro \~and\~ rw \~( FSCONFIG_SET_FLAG )
Configure whether the filesystem instance is read-only.
.TP
.BR dirsync \~( FSCONFIG_SET_FLAG )
Make directory changes on this filesystem instance synchronous.
.TP
.BR sync \~and\~ async \~( FSCONFIG_SET_FLAG )
Configure whether writes on this filesystem instance
will be made synchronous
(as though the
.B O_SYNC
flag to
.BR open (2)
was specified for
all file opens in this filesystem instance).
.TP
.BR lazytime \~and\~ nolazytime \~( FSCONFIG_SET_FLAG )
Configure whether to reduce on-disk updates
of inode timestamps on this filesystem instance
(as described in the
.B \%MS_LAZYTIME
section of
.BR mount (2)).
.TP
.BR mand \~and\~ nomand \~( FSCONFIG_SET_FLAG )
Configure whether the filesystem instance should permit mandatory locking.
Since Linux 5.15,
.\" commit f7e33bdbd6d1bdf9c3df8bba5abcf3399f957ac3
mandatory locking has been deprecated
and setting this flag is a no-op.
.TP
.BR source \~( FSCONFIG_SET_STRING )
This parameter is equivalent to the
.I source
parameter passed to
.BR mount (2)
for the same filesystem type,
and is usually the pathname of a block device
containing the filesystem.
This parameter may only be set once
per filesystem configuration context transaction.
.P
In addition,
any filesystem parameters associated with
Linux Security Modules (LSMs)
are also generic with respect to the underlying filesystem.
See the documentation for the LSM you wish to configure for more details.
.SS Mount attributes and filesystem parameters
Some filesystem parameters
(traditionally associated with
.BR mount (8)-style
options)
have a sibling mount attribute
with superficially similar user-facing behaviour.
.P
For a description of the distinction between
mount attributes and filesystem parameters,
see the "Mount attributes and filesystem parameters" subsection of
.BR mount_setattr (2).
.SH CAVEATS
.SS Filesystem parameter types
As a result of
each filesystem driver being responsible for
parsing most parameters specified with
.BR fsconfig (),
some filesystem drivers
may have unintuitive behaviour
with regards to which
.BI \%FSCONFIG_SET_ *
commands are permitted
to configure a given parameter.
.P
In order for
filesystem parameters to be backwards compatible with
.BR mount (2),
they must be parseable as strings;
this almost universally means that
.B \%FSCONFIG_SET_STRING
can also be used to configure them.
.\" Aleksa Sarai
.\"   Theoretically, a filesystem could check fc->oldapi and refuse
.\"   FSCONFIG_SET_STRING if the operation is coming from the new API, but no
.\"   filesystems do this (and probably never will).
However, other
.BI \%FSCONFIG_SET_ *
commands need to be opted into
by each filesystem driver's parameter parser.
.P
One of the most user-visible instances of
this inconsistency is that
many filesystems do not support
configuring path parameters with
.B \%FSCONFIG_SET_PATH
(despite the name),
which can lead to somewhat confusing
.B EINVAL
errors.
(For example, the generic
.I source
parameter\[em]\c
which is usually a path\[em]\c
can only be configured
with
.BR \%FSCONFIG_SET_STRING .)
.P
When writing programs that use
.BR fsconfig ()
to configure parameters
with commands other than
.BR \%FSCONFIG_SET_STRING ,
users should verify
that the
.BI \%FSCONFIG_SET_ *
commands used to configure each parameter
are supported by the corresponding filesystem driver.
.\" Aleksa Sarai
.\"   While this (quite confusing) inconsistency in behaviour is true today
.\"   (and has been true since this was merged), this appears to mostly be an
.\"   unintended consequence of filesystem drivers hand-coding fsparam parsing.
.\"   Path parameters are the most eggregious causes of confusion.
.\"   Hopefully we can make this no longer the case in a future kernel.
.SH EXAMPLES
To illustrate the different kinds of flags that can be configured with
.BR fsconfig (),
here are a few examples of some different filesystems being created:
.P
.in +4n
.EX
int fsfd, mntfd;
\&
fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
\&
fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
.EE
.in
.P
Usually,
specifying the same parameter named by
.I key
multiple times with
.BR fsconfig ()
causes the parameter value to be replaced.
However, some filesystems may have unique behaviour:
.P
.in +4n
.EX
\&
int fsfd, mntfd;
int lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
\&
fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
/* "lowerdir+" appends to the lower dir stack each time */
fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower3", 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower4", 0);
.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower5", AT_FDCWD);
.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
.\" Aleksa Sarai: Hopefully these will also be supported in the future.
fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
.EE
.in
.P
And here is an example of how
.BR fspick (2)
can be used with
.BR fsconfig ()
to reconfigure the parameters
of an extant filesystem instance
attached to
.IR /proc :
.P
.in +4n
.EX
int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
.EE
.in
.SH SEE ALSO
.BR fsmount (2),
.BR fsopen (2),
.BR fspick (2),
.BR mount (2),
.BR mount_setattr (2),
.BR move_mount (2),
.BR open_tree (2),
.BR mount_namespaces (7)