[PATCH 4/5] Add manpage for fsopen(2) and fsmount(2)

From: David Howells
Date: Fri Aug 07 2020 - 10:08:11 EST

Add a manual page to document the fsopen() and fsmount() system calls.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

man2/fsmount.2 | 1
man2/fsopen.2 | 254 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 255 insertions(+)
create mode 100644 man2/fsmount.2
create mode 100644 man2/fsopen.2

diff --git a/man2/fsmount.2 b/man2/fsmount.2
new file mode 100644
index 000000000..2bf59fc3e
--- /dev/null
+++ b/man2/fsmount.2
@@ -0,0 +1 @@
+.so man2/fsopen.2
diff --git a/man2/fsopen.2 b/man2/fsopen.2
new file mode 100644
index 000000000..b36ea1609
--- /dev/null
+++ b/man2/fsopen.2
@@ -0,0 +1,254 @@
+'\" t
+.\" Copyright (c) 2020 David Howells <dhowells@xxxxxxxxxx>
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date. The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein. The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.TH FSOPEN 2 2020-08-07 "Linux" "Linux Programmer's Manual"
+fsopen, fsmount \- Filesystem parameterisation and mount creation
+.B #include <sys/types.h>
+.B #include <sys/mount.h>
+.B #include <unistd.h>
+.BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
+.BI "int fsopen(const char *" fsname ", unsigned int " flags );
+.BI "int fsmount(int " fd ", unsigned int " flags ", unsigned int " mount_attrs );
+.IR Note :
+There are no glibc wrappers for these system calls.
+.BR fsopen ()
+creates a blank filesystem configuration context within the kernel for the
+filesystem named in the
+.I fsname
+parameter, puts it into creation mode and attaches it to a file descriptor,
+which it then returns. The file descriptor can be marked close-on-exec by
+.IR flags .
+After calling fsopen(), the file descriptor should be passed to the
+.BR fsconfig (2)
+system call, using that to specify the desired filesystem and security
+When the parameters are all set, the
+.BR fsconfig ()
+system call should then be called again with
+as the command argument to effect the creation.
+.BR "[!]\ NOTE" :
+Depending on the filesystem type and parameters, this may rather share an
+existing in-kernel filesystem representation instead of creating a new one.
+In such a case, the parameters specified may be discarded or may overwrite the
+parameters set by a previous mount - at the filesystem's discretion.
+The file descriptor also serves as a channel by which more comprehensive error,
+warning and information messages may be retrieved from the kernel using
+.BR read (2).
+Once the creation command has been successfully run on a context, the context
+is switched into need-mount mode which prevents further configuration. At
+this point,
+.BR fsmount ()
+should be called to create a mount object.
+.BR fsmount ()
+takes the file descriptor returned by
+.BR fsopen ()
+and creates a mount object for the filesystem root specified there. The
+attributes of the mount object are set from the
+.I mount_attrs
+parameter. The attributes specify the propagation and mount restrictions to
+be applied to accesses through this mount.
+The mount object is then attached to a new file descriptor that looks like one
+created by
+.BR open "(2) with " O_PATH " or " open_tree (2).
+This can be passed to
+.BR move_mount (2)
+to attach the mount object to a mountpoint, thereby completing the process.
+The file descriptor returned by fsmount() is marked close-on-exec if
+FSMOUNT_CLOEXEC is specified in
+.IR flags .
+After fsmount() has completed, the context created by fsopen() is reset and
+moved to reconfiguration state, allowing the new superblock to be
+reconfigured. See
+.BR fspick (2)
+for details.
+.SS Message Retrieval Interface
+The context file descriptor may be queried for message strings at any time by
+.BR read (2)
+on the file descriptor. This will return formatted messages that are prefixed
+to indicate their class:
+\fB"e <message>"\fP
+An error message string was logged.
+\fB"i <message>"\fP
+An informational message string was logged.
+\fB"w <message>"\fP
+An warning message string was logged.
+Messages are removed from the queue as they're read.
+To illustrate the process, here's an example whereby this can be used to mount
+an ext4 filesystem on /dev/sdb1 onto /mnt.
+.in +4n
+sfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "source", "/dev/sdb1", 0);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "user_attr", NULL, 0);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0);
+fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MS_RELATIME);
+move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+Here, an ext4 context is created first and attached to sfd. This is then told
+where its source will be, given a bunch of options and created. Then
+fsmount() is called to create a mount object and
+.BR move_mount (2)
+is called to attach it to its intended mountpoint.
+And here's an example of mounting from an NFS server and setting a Smack
+security module label on it too:
+.in +4n
+sfd = fsopen("nfs", 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "source", "example.com:/pub", 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "rsize", "65536", 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "wsize", "65536", 0);
+fsconfig(sfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0);
+fsconfig(sfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0);
+fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mfd = fsmount(sfd, 0, MS_NODEV);
+move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+On success, both functions return a file descriptor. On error, \-1 is
+returned, and
+.I errno
+is set appropriately.
+The error values given below result from filesystem type independent
+Each filesystem type may have its own special errors and its
+own special behavior.
+See the Linux kernel source code for details.
+The context referred to by
+.I fd
+is not in the right state to be used by
+.BR fsmount ().
+One of the pointer arguments points outside the user address space.
+.I flags
+had an invalid flag set.
+.I mount_attrs,
+includes invalid
+The system has too many open files to create more.
+The process has too many open files to create more.
+.I fsname
+not configured in the kernel.
+The kernel could not allocate sufficient memory to complete the call.
+The caller does not have the required privileges.
+These functions are Linux-specific and should not be used in programs intended
+to be portable.
+.BR fsopen "(), and " fsmount ()
+were added to Linux in kernel 5.1.
+Glibc does not (yet) provide a wrapper for the
+.BR fsopen "() or " fsmount "()"
+system calls; call them using
+.BR syscall (2).
+.BR mountpoint (1),
+.BR fsconfig (2),
+.BR fspick (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR umount (2),
+.BR mount_namespaces (7),
+.BR path_resolution (7),
+.BR findmnt (8),
+.BR lsblk (8),
+.BR mount (8),
+.BR umount (8)