Message-Id: <20250807-new-mount-api-v2-4-558a27b8068c@cyphar.com>
Date: Thu, 07 Aug 2025 03:44:38 +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 04/11] fspick.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/fspick.2 | 305 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 305 insertions(+)

diff --git a/man/man2/fspick.2 b/man/man2/fspick.2
new file mode 100644
index 000000000000..5215d706428b
--- /dev/null
+++ b/man/man2/fspick.2
@@ -0,0 +1,305 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fspick 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fspick \- select filesystem for reconfiguration
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" \
+"          /* Definition of " AT_* " constants */"
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fspick(int " dirfd ", const char *" path ", unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fspick ()
+system call is part of
+the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR fspick()
+creates a new filesystem configuration context
+for the filesystem instance
+associated with the path described by
+.IR dirfd
+and
+.IR path ,
+places it into reconfiguration mode
+(similar to
+.BR mount (8)
+with the
+.I -o remount
+option),
+and attaches it to a new file descriptor,
+which is then returned.
+The calling process must have the
+.BR CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+The resultant file descriptor can be used with
+.BR fsconfig (2)
+to specify the desired set of changes
+to mount parameters
+of the filesystem instance.
+Once the desired set of changes have been configured,
+the changes can be effectuated by calling
+.BR fsconfig (2)
+with the
+.B \%FSCONFIG_CMD_RECONFIGURE
+command.
+.P
+As with "*at()" system calls,
+.BR fspick ()
+uses the
+.I dirfd
+argument in conjunction with the
+.I path
+argument to determine the path to operate on, as follows:
+.IP \[bu] 3
+If the pathname given in
+.I path
+is absolute, then
+.I dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I path
+is relative and
+.I dirfd
+is the special value
+.BR AT_FDCWD ,
+then
+.I path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I path
+is relative,
+then it is interpreted relative to
+the directory referred to by the file descriptor
+.I dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+.I dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I path
+is an empty string,
+and
+.I flags
+contains
+.BR \%FSPICK_EMPTY_PATH ,
+then the file descriptor referenced by
+.I dirfd
+is operated on directly.
+In this case,
+.I dirfd
+can refer to any type of file,
+not just a directory.
+.P
+.I flags
+can be used to control aspects of how
+.I path
+is resolved
+and properties of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSPICK_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.
+.TP
+.B FSPICK_EMPTY_PATH
+If
+.I path
+is an empty string,
+operate on the file referred to by
+.I dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or
+.BR open_tree (2)).
+In this case,
+.I dirfd
+can refer to any type of file, not just a directory.
+If
+.I dirfd
+is
+.BR \%AT_FDCWD ,
+the call operates on the current working directory
+of the calling process.
+.TP
+.B FSPICK_SYMLINK_NOFOLLOW
+Do not follow symbolic links
+in the terminal component of
+.IR path .
+If
+.I path
+references a symbolic link,
+the returned filesystem context will reference
+the filesystem that the symbolic link itself resides on.
+.TP
+.B FSPICK_NO_AUTOMOUNT
+Do not follow automounts in the terminal component of
+.IR path .
+This allows you to configure the underlying automount point
+without triggering the automount.
+This flag has no effect if the automount point has already been mounted over.
+.RE
+.P
+As with filesystem contexts created with
+.BR fsopen (2),
+the file descriptor returned by
+.BR fspick ()
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+(See the "Message retrieval interface" subsection in
+.BR fsopen (2)
+for more details on the message format.)
+.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 EACCES
+Search permission is denied
+for one of the directories
+in the path prefix of
+.IR path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+.I path
+is relative but
+.I dirfd
+is neither
+.B AT_FDCWD
+nor a valid file descriptor.
+.TP
+.B EFAULT
+.I path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B ELOOP
+Too many symbolic links encountered when resolving
+.IR path .
+.TP
+.B ENAMETOOLONG
+.I path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A component of
+.I path
+does not exist,
+or is a dangling symbolic link.
+.TP
+.B ENOENT
+.I path
+is an empty string, but
+.B \%FSPICK_EMPTY_PATH
+is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of
+.I path
+is not a directory, or
+.I path
+is relative and
+.I dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.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 EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit cf3cba4a429be43e5527a3f78859b1bfd9ebc5fb
+glibc 2.36.
+.SH EXAMPLES
+The following example sets the read-only flag
+on the filesystem instance referenced by
+the mount object attached at
+.IR /tmp .
+.P
+.in +4n
+.EX
+int fsfd = fspick(AT_FDCWD, "/tmp", FSPICK_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
+.EE
+.in
+.P
+The above procedure is functionally equivalent to the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount(NULL, "/tmp", NULL, MS_REMOUNT | MS_RDONLY, NULL);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (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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