Message-Id: <20250807-new-mount-api-v2-3-558a27b8068c@cyphar.com>
Date: Thu, 07 Aug 2025 03:44:37 +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>, 
 Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH v2 03/11] fsopen.2: document 'new' mount api

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-developed-by: David Howells <dhowells@...hat.com>
Co-developed-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 man/man2/fsopen.2 | 319 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 319 insertions(+)

diff --git a/man/man2/fsopen.2 b/man/man2/fsopen.2
new file mode 100644
index 000000000000..ad38ef0782be
--- /dev/null
+++ b/man/man2/fsopen.2
@@ -0,0 +1,319 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsopen 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsopen \- create a new filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fsopen(const char *" fsname ", unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fsopen ()
+system call is part of the suite of file descriptor based mount facilities in
+Linux.
+.P
+.BR fsopen ()
+creates a blank filesystem configuration context within the kernel
+for the filesystem named by
+.IR fsname ,
+puts the context into creation mode and attaches it to a file descriptor,
+which is then returned.
+The calling process must have the
+.B \%CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+A filesystem configuration context is an in-kernel representation of a pending
+transaction,
+containing a set of configuration parameters that are to be applied
+when creating a new instance of a filesystem
+(or modifying the configuration of an existing filesystem instance,
+such as when using
+.BR fspick (2)).
+.P
+After obtaining a filesystem configuration context with
+.BR fsopen (),
+the general workflow for operating on the context looks like the following:
+.IP (1) 5
+Pass the filesystem context file descriptor to
+.BR fsconfig (2)
+to specify any desired filesystem parameters.
+This may be done as many times as necessary.
+.IP (2)
+Pass the same filesystem context file descriptor to
+.BR fsconfig (2)
+with
+.B \%FSCONFIG_CMD_CREATE
+to create an instance of the configured filesystem.
+.IP (3)
+Pass the same filesystem context file descriptor to
+.BR fsmount (2)
+to create a new mount object for the root of the filesystem,
+which is then attached to a new file descriptor.
+This also places the filesystem context file descriptor into reconfiguration
+mode,
+similar to the mode produced by
+.BR fspick (2).
+.IP (4)
+Use the mount object file descriptor as a
+.I dirfd
+argument to "*at()" system calls;
+or attach the mount object to a mount point
+by passing the mount object file descriptor to
+.BR move_mount (2).
+.P
+A filesystem context will move between different modes throughout its
+lifecycle
+(such as the creation phase when created with
+.BR fsopen (),
+the reconfiguration phase when an existing filesystem instance is selected by
+.BR fspick (2),
+and the intermediate "needs-mount" phase between
+.\" FS_CONTEXT_NEEDS_MOUNT is the term the kernel uses for this.
+.BR \%FSCONFIG_CMD_CREATE
+and
+.BR fsmount (2)),
+which has an impact on what operations are permitted on the filesystem context.
+.P
+The file descriptor returned by
+.BR fsopen ()
+also acts as a channel for filesystem drivers to provide more comprehensive
+error, warning, and information messages
+than are normally provided through the standard
+.BR errno (3)
+interface for system calls.
+If an error occurs at any time during the workflow mentioned above,
+calling
+.BR read (2)
+on the filesystem context file descriptor will retrieve any ancillary
+information about the encountered errors.
+(See the "Message retrieval interface" section for more details on the message
+format.)
+.P
+.I flags
+can be used to control aspects of the creation of the filesystem configuration
+context file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSOPEN_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.RE
+.P
+A list of filesystems supported by the running kernel
+(and thus a list of valid values for
+.IR fsname )
+can be obtained from
+.IR /proc/filesystems .
+(See also
+.BR proc_filesystems (5).)
+.SS Message retrieval interface
+When doing operations on a filesystem configuration context,
+the filesystem driver may choose to provide ancillary information to userspace
+in the form of message strings.
+.P
+The filesystem context file descriptors returned by
+.BR fsopen ()
+and
+.BR fspick (2)
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+Each call to
+.BR read (2)
+will return a single message,
+prefixed to indicate its class:
+.RS
+.TP
+.B "e <message>"
+An error message was logged.
+This is usually associated with an error being returned from the corresponding
+system call which triggered this message.
+.TP
+.B "w <message>"
+A warning message was logged.
+.TP
+.B "i <message>"
+An informational message was logged.
+.RE
+.P
+Messages are removed from the queue as they are read.
+Note that the message queue has limited depth,
+so it is possible for messages to get lost.
+If there are no messages in the message queue,
+.B read(2)
+will return no data and
+.I errno
+will be set to
+.BR \%ENODATA .
+If the
+.I buf
+argument to
+.BR read (2)
+is not large enough to contain the message,
+.BR read (2)
+will return no data and
+.I errno
+will be set to
+.BR \%EMSGSIZE .
+.P
+If there are multiple filesystem context file descriptors referencing the same
+filesystem instance
+(such as if you call
+.BR fspick (2)
+multiple times for the same mount),
+each one gets its own independent message queue.
+This does not apply to file descriptors that were duplicated with
+.BR dup (2).
+.P
+Messages strings will usually be prefixed by the filesystem driver that logged
+the message, though this may not always be the case.
+See the Linux kernel source code for details.
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EFAULT
+.I fsname
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+.I flags
+had an invalid flag set.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENODEV
+The filesystem named by
+.I fsname
+is not supported by the kernel.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 24dcb3d90a1f67fe08c68a004af37df059d74005
+glibc 2.36.
+.SH EXAMPLES
+To illustrate the workflow for creating a new mount,
+the following is an example of how to mount an
+.BR ext4 (5)
+filesystem stored on
+.I /dev/sdb1
+onto
+.IR /mnt .
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_PATH, "source", "/dev/sdb1", AT_FDCWD);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0)
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_RELATIME);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+First, an ext4 configuration context is created and attached to the file
+descriptor
+.IR fsfd .
+Then, a series of parameters
+(such as the source of the filesystem)
+are provided using
+.BR fsconfig (2),
+followed by the filesystem instance being created with
+.BR \%FSCONFIG_CMD_CREATE .
+.BR fsmount (2)
+is then used to create a new mount object attached to the file descriptor
+.IR mntfd ,
+which is then attached to the intended mount point using
+.BR move_mount (2).
+.P
+The above procedure is functionally equivalent to the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/dev/sdb1", "/mnt", "ext4", MS_RELATIME,
+      "ro,noatime,acl,user_xattr,iversion");
+.EE
+.in
+.P
+And here's an example of creating a mount object
+of an NFS server share
+and setting a Smack security module label.
+However, instead of attaching it to a mount point,
+the program uses the mount object directly
+to open a file from the NFS share.
+.P
+.in +4n
+.EX
+int fsfd, mntfd, fd;
+\&
+fsfd = fsopen("nfs", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "example.com/pub/linux", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "rsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "wsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, 0, MOUNT_ATTR_NODEV);
+fd = openat(mntfd, "src/linux-5.2.tar.xz", O_RDONLY);
+.EE
+.in
+.P
+Unlike the previous example,
+this operation has no trivial equivalent with
+.BR mount (2),
+as it was not previously possible to create a mount object
+that is not attached to any mount point.
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.50.1

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