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([2001:a61:253c:4c01:2cf1:7133:9da2:66a9]) by smtp.gmail.com with ESMTPSA id k25sm1275810edx.96.2020.08.27.04.04.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Aug 2020 04:04:51 -0700 (PDT) Cc: mtk.manpages@gmail.com, linux-fsdevel@vger.kernel.org, linux-man@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/5] Add manpages for move_mount(2) To: David Howells , viro@zeniv.linux.org.uk References: <159827188271.306468.16962617119460123110.stgit@warthog.procyon.org.uk> <159827189025.306468.4916341547843731338.stgit@warthog.procyon.org.uk> From: "Michael Kerrisk (man-pages)" Message-ID: <77b67542-f094-3395-80bd-18f82ea060b7@gmail.com> Date: Thu, 27 Aug 2020 13:04:48 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <159827189025.306468.4916341547843731338.stgit@warthog.procyon.org.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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 > --- > > 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 > +.\" > +.\" %%%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 +.\" manual page may be incorrect or out-of-date.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 > +.B #include > +.B #include > +.BR "#include " "/* 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/