Message-ID: <CAKgNAkhbQSt-XyH1a6NkipnZeV_ZTenxcCpDx4JvSUEWL9HPSw@mail.gmail.com>
Date:   Fri, 13 Aug 2021 02:21:26 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian.brauner@...ntu.com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] Add manpages for move_mount(2)

Hello David,

As noted in another mail, I will ping on all of the mails, just to
raise all the patches to the top of the inbox.

Thanks,

Michael

On Thu, 27 Aug 2020 at 13:04, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
>
> Hello David,
>
> On 8/24/20 2:24 PM, David Howells wrote:
> > Add manual pages to document the move_mount() system call.
> >
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > ---
> >
> >  man2/move_mount.2 |  267 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 267 insertions(+)
> >  create mode 100644 man2/move_mount.2
> >
> > diff --git a/man2/move_mount.2 b/man2/move_mount.2
> > new file mode 100644
> > index 000000000..2ceb775d9
> > --- /dev/null
> > +++ b/man2/move_mount.2
> > @@ -0,0 +1,267 @@
> > +'\" t
> > +.\" Copyright (c) 2020 David Howells <dhowells@...hat.com>
> > +.\"
> > +.\" %%%LICENSE_START(VERBATIM)
> > +.\" 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 +.\" manual page may be incorrect or out-of-date.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.
> > +.\" %%%LICENSE_END
> > +.\"
> > +.TH MOVE_MOUNT 2 2020-08-24 "Linux" "Linux Programmer's Manual"
> > +.SH NAME
> > +move_mount \- Move mount objects around the filesystem topology
> > +.SH SYNOPSIS
> > +.nf
> > +.B #include <sys/types.h>
> > +.B #include <sys/mount.h>
> > +.B #include <unistd.h>
> > +.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
> > +.PP
> > +.BI "int move_mount(int " from_dirfd ", const char *" from_pathname ","
> > +.BI "               int " to_dirfd ", const char *" to_pathname ","
> > +.BI "               unsigned int " flags );
> > +.fi
> > +.PP
> > +.IR Note :
> > +There is no glibc wrapper for this system call.
> > +.SH DESCRIPTION
> > +The
> > +.BR move_mount ()
> > +call moves a mount from one place to another; it can also be used to attach an
> > +unattached mount that was created by
> > +.BR fsmount "() or " open_tree "() with " OPEN_TREE_CLONE .
> > +.PP
> > +If
> > +.BR move_mount ()
> > +is called repeatedly with a file descriptor that refers to a mount object,
> > +then the object will be attached/moved the first time and then moved
> > +repeatedly, detaching it from the previous mountpoint each time.
>
> s/mountpoint/mount point/
> (and all other instances below)
>
> > +.PP
> > +To access the source mount object or the destination mountpoint, no
> > +permissions are required on the object itself, but if either pathname is
> > +supplied, execute (search) permission is required on all of the directories
> > +specified in
> > +.IR from_pathname " or " to_pathname .
> > +.PP
> > +The caller does, however, require the appropriate privilege (Linux: the
>
> s/Linux: //
>
> > +.B CAP_SYS_ADMIN
> > +capability) to move or attach mounts.
> > +.PP
> > +.BR move_mount ()
> > +uses
> > +.IR from_pathname ", " from_dirfd " and part of " flags
> > +to locate the mount object to be moved and
> > +.IR to_pathname ", " to_dirfd " and another part of " flags
> > +to locate the destination mountpoint.  Each lookup can be done in one of a
> > +variety of ways:
> > +.TP
> > +[*] By absolute path.
> > +The pathname points to an absolute path and the dirfd is ignored.  The file is
> > +looked up by name, starting from the root of the filesystem as seen by the
> > +calling process.
> > +.TP
> > +[*] By cwd-relative path.
> > +The pathname points to a relative path and the dirfd is
> > +.IR AT_FDCWD .
> > +The file is looked up by name, starting from the current working directory.
> > +.TP
> > +[*] By dir-relative path.
> > +The pathname points to relative path and the dirfd indicates a file descriptor
> > +pointing to a directory.  The file is looked up by name, starting from the
> > +directory specified by
> > +.IR dirfd .
> > +.TP
> > +[*] By file descriptor.  The pathname is an empty string (""), the dirfd
>
> Formatting problem here... Add a newline before "The"
>
> > +points directly to the mount object to move or the destination mount point and
> > +the appropriate
> > +.B *_EMPTY_PATH
> > +flag is set.
> > +.PP
> > +.I flags
> > +can be used to influence a path-based lookup.  The value for
> > +.I flags
> > +is constructed by OR'ing together zero or more of the following constants:
> > +.TP
> > +.BR MOVE_MOUNT_F_EMPTY_PATH
> > +.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
> > +If
> > +.I from_pathname
> > +is an empty string, operate on the file referred to by
> > +.IR from_dirfd
> > +(which may have been obtained using the
> > +.BR open (2)
> > +.B O_PATH
> > +flag or
> > +.BR open_tree ())
> > +If
> > +.I from_dirfd
> > +is
> > +.BR AT_FDCWD ,
> > +the call operates on the current working directory.
> > +In this case,
> > +.I from_dirfd
> > +can refer to any type of file, not just a directory.
> > +This flag is Linux-specific; define
> > +.B _GNU_SOURCE
> > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
> > +to obtain its definition.
> > +.TP
> > +.B MOVE_MOUNT_T_EMPTY_PATH
> > +As above, but operating on
>
> s/As above/As for MOVE_MOUNT_F_EMPTY_PATH/
>
> > +.IR to_pathname " and " to_dirfd .
> > +.TP
> > +.B MOVE_MOUNT_F_AUTOMOUNTS
> > +Don't automount the terminal ("basename") component of
> > +.I from_pathname
> > +if it is a directory that is an automount point.  This allows a mount object
> > +that has an automount point at its root to be moved and prevents unintended
> > +triggering of an automount point.
> > +The
> > +.B MOVE_MOUNT_F_AUTOMOUNTS
> > +flag has no effect if the automount point has already been mounted over.  This
> > +flag is Linux-specific; define
> > +.B _GNU_SOURCE
> > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
> > +to obtain its definition.
> > +.TP
> > +.B MOVE_MOUNT_T_AUTOMOUNTS
> > +As above, but operating on
> > +.IR to_pathname " and " to_dirfd .
> > +This allows an automount point to be manually mounted over.
> > +.TP
> > +.B MOVE_MOUNT_F_SYMLINKS
> > +If
> > +.I from_pathname
> > +is a symbolic link, then dereference it.  The default for
> > +.BR move_mount ()
> > +is to not follow symlinks.
> > +.TP
> > +.B MOVE_MOUNT_T_SYMLINKS
> > +As above, but operating on
> > +.IR to_pathname " and " to_dirfd .
> > +.SH RETURN VALUE
> > +On success, 0 is returned.  On error, \-1 is returned, and
> > +.I errno
> > +is set appropriately.
> > +.SH ERRORS
>
> Should EPERM be in the following list?
>
> > +.TP
> > +.B EACCES
> > +Search permission is denied for one of the directories
> > +in the path prefix of
> > +.IR pathname .
> > +(See also
> > +.BR path_resolution (7).)
> > +.TP
> > +.B EBADF
> > +.IR from_dirfd " or " to_dirfd
> > +is not a valid open file descriptor.
> > +.TP
> > +.B EFAULT
> > +.IR from_pathname " or " to_pathname
> > +is NULL or either one point to a location outside the process's accessible
> > +address space.
> > +.TP
> > +.B EINVAL
> > +Reserved flag specified in
>
> Should this rather be, "Invalid flag specified in..." ?
>
> > +.IR flags .
> > +.TP
> > +.B ELOOP
> > +Too many symbolic links encountered while traversing the pathname.
> > +.TP
> > +.B ENAMETOOLONG
> > +.IR from_pathname " or " to_pathname
> > +is too long.
> > +.TP
> > +.B ENOENT
> > +A component of
> > +.IR from_pathname " or " to_pathname
> > +does not exist, or one is an empty string and the appropriate
> > +.B *_EMPTY_PATH
> > +was not specified in
> > +.IR flags .
> > +.TP
> > +.B ENOMEM
> > +Out of memory (i.e., kernel memory).
> > +.TP
> > +.B ENOTDIR
> > +A component of the path prefix of
> > +.IR from_pathname " or " to_pathname
> > +is not a directory or one or the other is relative and the appropriate
> > +.I *_dirfd
> > +is a file descriptor referring to a file other than a directory.
> > +.SH VERSIONS
> > +.BR move_mount ()
> > +was added to Linux in kernel 5.2.
> > +.SH CONFORMING TO
> > +.BR move_mount ()
> > +is Linux-specific.
> > +.SH NOTES
> > +Glibc does not (yet) provide a wrapper for the
> > +.BR move_mount ()
> > +system call; call it using
> > +.BR syscall (2).
> > +.SH EXAMPLES
> > +The
> > +.BR move_mount ()
> > +function can be used like the following:
> > +.PP
> > +.RS
> > +.nf
> > +move_mount(AT_FDCWD, "/a", AT_FDCWD, "/b", 0);
> > +.fi
> > +.RE
> > +.PP
> > +This would move the object mounted on "/a" to "/b".  It can also be used in
>
> s/It/move_mount()/
>
> > +conjunction with
> > +.BR open_tree "(2) or " open "(2) with " O_PATH :
> > +.PP
> > +.RS
> > +.nf
> > +fd = open_tree(AT_FDCWD, "/mnt", 0);
> > +move_mount(fd, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH);
> > +move_mount(fd, "", AT_FDCWD, "/mnt3", MOVE_MOUNT_F_EMPTY_PATH);
> > +move_mount(fd, "", AT_FDCWD, "/mnt4", MOVE_MOUNT_F_EMPTY_PATH);
> > +.fi
> > +.RE
> > +.PP
> > +This would attach the path point for "/mnt" to fd, then it would move the
> > +mount to "/mnt2", then move it to "/mnt3" and finally to "/mnt4".
> > +.PP
> > +It can also be used to attach new mounts:
>
> s/It/move_mount()/
>
> > +.PP
> > +.RS
> > +.nf
> > +sfd = fsopen("ext4", FSOPEN_CLOEXEC);
> > +fsconfig(sfd, FSCONFIG_SET_STRING, "source", "/dev/sda1", 0);
> > +fsconfig(sfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> > +fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> > +mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NODEV);
> > +move_mount(mfd, "", AT_FDCWD, "/home", MOVE_MOUNT_F_EMPTY_PATH);
> > +.fi
> > +.RE
> > +.PP
> > +Which would open the Ext4 filesystem mounted on "/dev/sda1", turn on user
> > +extended attribute support and create a mount object for it.  Finally, the new
>
> Please replace "it" with a noun (phrase).
>
> > +mount object would be attached with
> > +.BR move_mount ()
> > +to "/home".
> > +.SH SEE ALSO
> > +.BR fsmount (2),
> > +.BR fsopen (2),
> > +.BR open_tree (2)
>
> Thanks,
>
> Michael
>
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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