From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Howells Subject: [MANPAGE PATCH] Add manpages for move_mount(2) and open_tree(2) Date: Tue, 10 Jul 2018 23:52:42 +0100 Message-ID: <15449.1531263162@warthog.procyon.org.uk> References: <153126249579.14533.18370053915999978123.stgit@warthog.procyon.org.uk> <153126248868.14533.9751473662727327569.stgit@warthog.procyon.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: <153126249579.14533.18370053915999978123.stgit@warthog.procyon.org.uk> Content-ID: <15448.1531263162.1@warthog.procyon.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: Michael Kerrisk Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, linux-man@vger.kernel.org List-Id: linux-man@vger.kernel.org Add manual pages to document the move_mount and open_tree() system calls. Signed-off-by: David Howells --- man2/move_mount.2 | 274 +++++++++++++++++++++++++++++++++++++++++++++++++++++ man2/open_tree.2 | 260 ++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 534 insertions(+) create mode 100644 man2/move_mount.2 create mode 100644 man2/open_tree.2 diff --git a/man2/move_mount.2 b/man2/move_mount.2 new file mode 100644 index 000000000..3a819fb84 --- /dev/null +++ b/man2/move_mount.2 @@ -0,0 +1,274 @@ +'\" t +.\" Copyright (c) 2018 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 +.\" +.\" Since the Linux kernel and libraries are constantly changing, this +.\" manual page may be incorrect or out-of-date.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 2018-06-08 "Linux" "Linux Programmer's Manual" +.SH NAME +move_mount \- Move mount objects around the filesystem topology +.SH SYNOPSIS +.nf +.B #include +.br +.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 are no glibc wrappers for these system calls. +.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 +.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. +.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. +.PP +.BR move_mount () +uses +.IR from_pathname ", " from_dirfd " and some " flags +to locate the mount object to be moved and +.IR to_pathname ", " to_dirfd " and some other " 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 +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 +.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 +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 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); +write(sfd, "s /dev/sda1"); +write(sfd, "o user_xattr"); +mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MS_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". + + +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.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. +.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 SEE ALSO +.BR fsmount (2), +.BR fsopen (2), +.BR open_tree (2) diff --git a/man2/open_tree.2 b/man2/open_tree.2 new file mode 100644 index 000000000..7e9c86fe3 --- /dev/null +++ b/man2/open_tree.2 @@ -0,0 +1,260 @@ +'\" t +.\" Copyright (c) 2018 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 OPEN_TREE 2 2018-06-08 "Linux" "Linux Programmer's Manual" +.SH NAME +open_tree \- Pick or clone mount object and attach to fd +.SH SYNOPSIS +.nf +.B #include +.br +.B #include +.br +.B #include +.br +.BR "#include " "/* Definition of AT_* constants */" +.PP +.BI "int open_tree(int " dirfd ", const char *" pathname ", unsigned int " flags ); +.fi +.PP +.IR Note : +There are no glibc wrappers for these system calls. +.SH DESCRIPTION +.BR open_tree () +picks the mount object specified by the pathname and attaches it to a new file +descriptor or clones it and attaches the clone to the file descriptor. The +resultant file descriptor is indistinguishable from one produced by +.BR open "(2) with " O_PATH . +.PP +In the case that the mount object is cloned, the clone will be "unmounted" and +destroyed when the file descriptor is closed if it is not otherwise mounted +somewhere by calling +.BR move_mount (2). +.PP +To select a mount object, no permissions are required on the object referred +to by the path, but execute (search) permission is required on all of the +directories in +.I pathname +that lead to the object. +.PP +To clone an object, however, the caller must have mount capabilities and +permissions. +.PP +.BR open_tree () +uses +.IR pathname ", " dirfd " and " flags +to locate the target object in one of a variety of ways: +.TP +[*] By absolute path. +.I pathname +points to an absolute path and +.I dirfd +is ignored. The object is looked up by name, starting from the root of the +filesystem as seen by the calling process. +.TP +[*] By cwd-relative path. +.I pathname +points to a relative path and +.IR dirfd " is " AT_FDCWD . +The object is looked up by name, starting from the current working directory. +.TP +[*] By dir-relative path. +.I pathname +points to relative path and +.I dirfd +indicates a file descriptor pointing to a directory. The object is looked up +by name, starting from the directory specified by +.IR dirfd . +.TP +[*] By file descriptor. +.I pathname +is "", +.I dirfd +indicates a file descriptor and +.B AT_EMPTY_PATH +is set in +.IR flags . +The mount attached to the file descriptor is queried directly. The file +descriptor may point to any type of file, not just a directory. + +.\"______________________________________________________________ +.PP +.I flags +can be used to control the operation of the function and to influence a +path-based lookup. A value for +.I flags +is constructed by OR'ing together zero or more of the following constants: +.TP +.BR AT_EMPTY_PATH +.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d +If +.I pathname +is an empty string, operate on the file referred to by +.IR dirfd +(which may have been obtained from +.BR open "(2) with" +.BR O_PATH ", from " fsmount (2) +or from another +.BR open_tree ()). +If +.I dirfd +is +.BR AT_FDCWD , +the call operates on the current working directory. +In this case, +.I 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 +.BR AT_NO_AUTOMOUNT +Don't automount the terminal ("basename") component of +.I pathname +if it is a directory that is an automount point. This flag allows the +automount point itself to be picked up or a mount cloned that is rooted on the +automount point. The +.B AT_NO_AUTOMOUNT +flag has no effect if the mount 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 AT_SYMLINK_NOFOLLOW +If +.I pathname +is a symbolic link, do not dereference it: instead pick up or clone a mount +rooted on the link itself. +.TP +.B OPEN_TREE_CLOEXEC +Set the close-on-exec flag for the new file descriptor. This will cause the +file descriptor to be closed automatically when a process exec's. +.TP +.B OPEN_TREE_CLONE +Rather than directly attaching the selected object to the file descriptor, +clone the object, set the root of the new mount object to that point and +attach the clone to the file descriptor. +.TP +.B AT_RECURSIVE +This is only permitted in conjunction with OPEN_TREE_CLONE. It causes the +entire mount subtree rooted at the selected spot to be cloned rather than just +that one mount object. + + +.SH EXAMPLE +The +.BR open_tree () +function can be used like the following: +.PP +.RS +.nf +fd1 = open_tree(AT_FDCWD, "/mnt", 0); +fd2 = open_tree(fd1, "", + AT_EMPTY_PATH | OPEN_TREE_CLONE | AT_RECURSIVE); +move_mount(fd2, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH); +.fi +.RE +.PP +This would attach the path point for "/mnt" to fd1, then it would copy the +entire subtree at the point referred to by fd1 and attach that to fd2; lastly, +it would attach the clone to "/mnt2". + + +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.SH RETURN VALUE +On success, the new file descriptor 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 +.I dirfd +is not a valid open file descriptor. +.TP +.B EFAULT +.I pathname +is NULL or +.IR pathname +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 +.I pathname +is too long. +.TP +.B ENOENT +A component of +.I pathname +does not exist, or +.I pathname +is an empty string and +.B AT_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 +.I pathname +is not a directory or +.I pathname +is relative and +.I dirfd +is a file descriptor referring to a file other than a directory. +.SH VERSIONS +.BR open_tree () +was added to Linux in kernel 4.18. +.SH CONFORMING TO +.BR open_tree () +is Linux-specific. +.SH NOTES +Glibc does not (yet) provide a wrapper for the +.BR open_tree () +system call; call it using +.BR syscall (2). +.SH SEE ALSO +.BR fsmount (2), +.BR move_mount (2), +.BR open (2)