This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 729 insertions(+)
diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
new file mode 100644
index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
--- /dev/null
+++ b/man/man2/fsconfig.2
@@ -0,0 +1,729 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsconfig 2 (date) "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 name ,
+.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 name
+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 name
+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
+\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Configure whether the filesystem instance is read-only.
+.TP
+\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Make directory changes on this filesystem instance synchronous.
+.TP
+\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+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
+\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+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
+\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+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
+\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
+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.
+.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)
--
2.51.0Hi Aleksa,
On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
>
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
I've applied this patch, with a few amendments. Thanks!
<https://www.alejandro-colomar.es:80/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=d0b3cd8a5399f734037e87b8f93d5b536af20d7d>
(Use port :80/)
Have a lovely day!
Alex
> ---
> man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 729 insertions(+)
>
> diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> --- /dev/null
> +++ b/man/man2/fsconfig.2
> @@ -0,0 +1,729 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsconfig 2 (date) "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 name ,
> +.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 name
> +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 name
> +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
> +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance is read-only.
> +.TP
> +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Make directory changes on this filesystem instance synchronous.
> +.TP
> +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> +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.
> +.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)
>
> --
> 2.51.0
>
>
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
Hi Aleksa,
On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
>
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 729 insertions(+)
>
> diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> --- /dev/null
> +++ b/man/man2/fsconfig.2
> @@ -0,0 +1,729 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsconfig 2 (date) "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 name ,
> +.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 name
> +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 name
There's no such parameter. (I guess you meant 'key'?)
Cheers,
Alex
> +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
> +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance is read-only.
> +.TP
> +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Make directory changes on this filesystem instance synchronous.
> +.TP
> +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +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
> +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> +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.
> +.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)
>
> --
> 2.51.0
>
>
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
On 2025-09-25, Alejandro Colomar <alx@kernel.org> wrote:
> Hi Aleksa,
>
> On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> > This is loosely based on the original documentation written by David
> > Howells and later maintained by Christian Brauner, but has been
> > rewritten to be more from a user perspective (as well as fixing a few
> > critical mistakes).
> >
> > Co-authored-by: David Howells <dhowells@redhat.com>
> > Signed-off-by: David Howells <dhowells@redhat.com>
> > Co-authored-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > ---
> > man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 729 insertions(+)
> >
> > diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> > --- /dev/null
> > +++ b/man/man2/fsconfig.2
> > @@ -0,0 +1,729 @@
> > +.\" Copyright, the authors of the Linux man-pages project
> > +.\"
> > +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> > +.\"
> > +.TH fsconfig 2 (date) "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 name ,
> > +.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 name
> > +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 name
>
> There's no such parameter. (I guess you meant 'key'?)
Ah yes, I did mean "key". The same mistake was repeated for two EINVAL
cases above as well:
EINVAL One of the values of *name*, value, and/or aux were set to a
non-zero value when cmd required that they be zero (or NULL).
EINVAL The parameter named by *name* cannot be set using the type
specified with cmd.
Do you want me to send another version or would you able to fix it when
you apply?
Thanks.
> Cheers,
> Alex
>
> > +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
> > +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Configure whether the filesystem instance is read-only.
> > +.TP
> > +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Make directory changes on this filesystem instance synchronous.
> > +.TP
> > +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +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
> > +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +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
> > +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +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
> > +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> > +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.
> > +.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)
> >
> > --
> > 2.51.0
> >
> >
>
> --
> <https://www.alejandro-colomar.es>
> Use port 80 (that is, <...:80/>).
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/
On Fri, Sep 26, 2025 at 01:15:14AM +1000, Aleksa Sarai wrote: > > > +.TP > > > +.B ENOTBLK > > > +The parameter named by > > > +.I name > > > > There's no such parameter. (I guess you meant 'key'?) > > Ah yes, I did mean "key". The same mistake was repeated for two EINVAL > cases above as well: Thanks! > > EINVAL One of the values of *name*, value, and/or aux were set to a > non-zero value when cmd required that they be zero (or NULL). > > EINVAL The parameter named by *name* cannot be set using the type > specified with cmd. > > Do you want me to send another version or would you able to fix it when > you apply? I can fix it. Cheers, Alex -- <https://www.alejandro-colomar.es> Use port 80 (that is, <...:80/>).
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