Message-ID: <2025-09-25-azure-rubber-flair-menus-42bRw8@cyphar.com>
Date: Fri, 26 Sep 2025 01:15:14 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: "Michael T. Kerrisk" <mtk.manpages@...il.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, Askar Safin <safinaskar@...omail.com>, 
	"G. Branden Robinson" <g.branden.robinson@...il.com>, linux-man@...r.kernel.org, linux-api@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	David Howells <dhowells@...hat.com>, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

On 2025-09-25, Alejandro Colomar <alx@...nel.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@...hat.com>
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > Co-authored-by: Christian Brauner <brauner@...nel.org>
> > Signed-off-by: Christian Brauner <brauner@...nel.org>
> > Signed-off-by: Aleksa Sarai <cyphar@...har.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/

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