From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,WEIRD_QUOTING autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7DD3AC433DF for ; Sat, 22 Aug 2020 20:04:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 545CE2086A for ; Sat, 22 Aug 2020 20:04:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Jp7brEmK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726391AbgHVUEQ (ORCPT ); Sat, 22 Aug 2020 16:04:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36364 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725932AbgHVUEN (ORCPT ); Sat, 22 Aug 2020 16:04:13 -0400 Received: from mail-ot1-x344.google.com (mail-ot1-x344.google.com [IPv6:2607:f8b0:4864:20::344]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7DBAAC061573; Sat, 22 Aug 2020 13:04:13 -0700 (PDT) Received: by mail-ot1-x344.google.com with SMTP id q9so4231017oth.5; Sat, 22 Aug 2020 13:04:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:reply-to:from:date:message-id :subject:to:cc; bh=MmBUbsV40+0aUblDPNmZSek7KpIhqCuoyTrbZcoCKns=; b=Jp7brEmKSRvUqf18BGroFxaeRvLp9ugF80QHHWpEHM6GXCyA7giXzXo9UzgEZrPnqf XqiH6AZoes35UcrMUY2bgG6SKbVVH2BNddYSNbxePZ0WSaN7Naf+gu3wewAOWqxvuadN VrvoJUEH7DGFNfHNWur7BMeHQ7dzVHvtE9Y0jNr1LoZfMsFZ3fbqCUOYHjujD7Vx4jMy N5TWThgw1EpE0hYWhD3C3XaKmQ4slpgus4/4kpGINZP1NY1JULfbns7eOf6n1968Zm+F HYrQ666LRI9u457UTZOljkHFOlxIl/xdc2cIQ7kf+xL3hFHddl0ZgUtbMMRZQQJXoyXJ PESA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:reply-to :from:date:message-id:subject:to:cc; bh=MmBUbsV40+0aUblDPNmZSek7KpIhqCuoyTrbZcoCKns=; b=RaiLWepYGa4v+SQbQ67SPZ1yh3YIjfoS925CXEmmm/hX7dKOdmskTZDXSQsfPk3mxI l3rfy0cu4OIdNK3ENoFqdnKIk8aP4sO/1tkL9qb+Ou55rN/kBL6Pauoese+jyqOuK7Ei kp5BCIY/nJu+L/cd/Re67QUpsN5zr0F20Y8KbPupv3dL1zWp35GSa28e2QzpX8PlVM98 4SqHBdZCb3NXnVR+qrBsmWFKP2246fU67rddY26n1GYKtKm7M6Kk9iar2ViHV5pzy+3+ 4obTS5SNuyTkAYNFQ36oLKk0Eqv3yRTcY/v+gfDnPHyo/da3WJ2gkFCFValJms3vYklY 3yBw== X-Gm-Message-State: AOAM530+wSdlj/GdbOvzkT1winae9R5706qxxuqp35VW1CDsggLTAA3H Zls8tib5j87zefEU/9eDFEdshhFZAfNVwkOpcmE= X-Google-Smtp-Source: ABdhPJyQAQcxoRATY5x7sJAiXwg4qyYT4pt2sUCd/k5d3A5CK4uBxevymlNMEgT9w1s+W5SYLzZ/bBVVUwTXbYTacMk= X-Received: by 2002:a05:6830:148f:: with SMTP id s15mr6185648otq.323.1598126652663; Sat, 22 Aug 2020 13:04:12 -0700 (PDT) MIME-Version: 1.0 References: <159680892602.29015.6551860260436544999.stgit@warthog.procyon.org.uk> <159680894741.29015.5588747939240667925.stgit@warthog.procyon.org.uk> In-Reply-To: <159680894741.29015.5588747939240667925.stgit@warthog.procyon.org.uk> Reply-To: mtk.manpages@gmail.com From: "Michael Kerrisk (man-pages)" Date: Sat, 22 Aug 2020 22:04:01 +0200 Message-ID: Subject: Re: [PATCH 2/5] Add manpages for move_mount(2) and open_tree(2) To: David Howells Cc: Alexander Viro , "linux-fsdevel@vger.kernel.org" , Christian Brauner , linux-man , Linux API , lkml , Karel Zak Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hello David, On Fri, 7 Aug 2020 at 16:02, David Howells wrote: > > Add manual pages to document the move_mount and open_tree() system calls. Notwithstanding the title and the commit message, this patch documents just move_mount(). Could you revise the title and commit message for the next iteration please. > Signed-off-by: David Howells > --- > > man2/move_mount.2 | 275 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 275 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..aae9013fa > --- /dev/null > +++ b/man2/move_mount.2 > @@ -0,0 +1,275 @@ > +'\" 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 2020-08-07 "Linux" "Linux Programmer's Manual" > +.SH NAME > +move_mount \- Move mount objects around the filesystem topology > +.SH SYNOPSIS > +.nf > +.B #include > +.br Delete the previous line (".br), and the same below (twice). > +.B #include > +.br > +.B #include > +.br > +.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 created by s/created/previously created/ ? > +.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 again and > +again and again, detaching it from the previous mountpoint each time. s/again and again and again/repeatedly/ s/mountpoint/mount point/ > +.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 capabilities or permission > +to effect a mount. Maybe better: s/effect/create/ > +.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 points to "", the dirfd points directly to the mount object to Better: The pathname is an empty string ("") > +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. A value for s/A value/The value/ > +.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 > +.IR to_pathname " and " to_dirfd . > +.TP > +.B MOVE_MOUNT_F_NO_AUTOMOUNT As noted by Karel in https://lore.kernel.org/lkml/20191202115540.gx2dcimzns7osbra@10.255.255.10/ there is no MOVE_MOUNT_F_NO_AUTOMOUNT, but rather MOVE_MOUNT_F_AUTOMOUNTS. Could you revise accordingly please. > +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_NO_AUTOMOUNT > +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_NO_AUTOMOUNT As noted by Karel in https://lore.kernel.org/lkml/20191202115540.gx2dcimzns7osbra@10.255.255.10/ there is no MOVE_MOUNT_T_NO_AUTOMOUNT, but rather MOVE_MOUNT_T_AUTOMOUNTS. Could you revise accordingly please. > +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 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 > +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: > +.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 > +mount object would be attached with > +.BR move_mount () > +to "/home". > + > + Manual page source should in general never contain blank lines; these can cause odd renderings in various formats (e.g., PDF). Could you please fix this in all pages. > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" Lines like the above are rather clutter in the page source. I prefer not to have them. Could you remove in all pages please. > +.SH RETURN VALUE > +On success, 0 is returned. On error, \-1 is returned, and > +.I errno > +is set appropriately. > +.SH ERRORS > +.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 > +.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 4.18. The version number here is wrong. It's 5.2. I presume this needs to be fixed in all of the other pages. > +.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). By the way, do you know if anything is happening for the system calls on the glibc side? > +.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/