From patchwork Fri Feb 7 17:42:36 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Petr Vorel X-Patchwork-Id: 11370855 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 4524D13A4 for ; Fri, 7 Feb 2020 17:42:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1070021775 for ; Fri, 7 Feb 2020 17:42:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727456AbgBGRmu (ORCPT ); Fri, 7 Feb 2020 12:42:50 -0500 Received: from mx2.suse.de ([195.135.220.15]:54488 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726974AbgBGRmu (ORCPT ); Fri, 7 Feb 2020 12:42:50 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id A4382AFF6; Fri, 7 Feb 2020 17:42:45 +0000 (UTC) From: Petr Vorel To: linux-man@vger.kernel.org Cc: David Howells , Michael Kerrisk , Zorro Lang , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Petr Vorel Subject: [PATCH v2 2/2] Add manpage for fsopen(2), fspick(2) and fsmount(2) Date: Fri, 7 Feb 2020 18:42:36 +0100 Message-Id: <20200207174236.18882-3-pvorel@suse.cz> X-Mailer: git-send-email 2.24.1 In-Reply-To: <20200207174236.18882-1-pvorel@suse.cz> References: <20200207174236.18882-1-pvorel@suse.cz> MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: David Howells Add a manual page to document the fsopen(), fspick() and fsmount() system calls. Reviewed-by: Petr Vorel Signed-off-by: David Howells Signed-off-by: Petr Vorel --- Hi, * man2/fsmount.2 maybe document these flags? #define MOUNT_ATTR_RDONLY 0x00000001 /* Mount read-only */ #define MOUNT_ATTR_NOSUID 0x00000002 /* Ignore suid and sgid bits */ #define MOUNT_ATTR_NODEV 0x00000004 /* Disallow access to device special files */ #define MOUNT_ATTR_NOEXEC 0x00000008 /* Disallow program execution */ #define MOUNT_ATTR__ATIME 0x00000070 /* Setting on how atime should be updated */ #define MOUNT_ATTR_RELATIME 0x00000000 /* - Update atime relative to mtime/ctime. */ #define MOUNT_ATTR_NOATIME 0x00000010 /* - Do not update access times. */ #define MOUNT_ATTR_STRICTATIME 0x00000020 /* - Always perform atime updates */ #define MOUNT_ATTR_NODIRATIME 0x00000080 /* Do not update directory access times */ * man2/fsopen.2 and these? enum fs_context_phase { FS_CONTEXT_CREATE_PARAMS, /* Loading params for sb creation */ FS_CONTEXT_CREATING, /* A superblock is being created */ FS_CONTEXT_AWAITING_MOUNT, /* Superblock created, awaiting fsmount() */ FS_CONTEXT_AWAITING_RECONF, /* Awaiting initialisation for reconfiguration */ FS_CONTEXT_RECONF_PARAMS, /* Loading params for reconfiguration */ FS_CONTEXT_RECONFIGURING, /* Reconfiguring the superblock */ FS_CONTEXT_FAILED, /* Failed to correctly transition a context */ }; Kind regards, Petr man2/fsconfig.2 | 282 ++++++++++++++++++++++++++++++++++++++++++++++++ man2/fsmount.2 | 1 + man2/fsopen.2 | 256 +++++++++++++++++++++++++++++++++++++++++++ man2/fspick.2 | 195 +++++++++++++++++++++++++++++++++ 4 files changed, 734 insertions(+) create mode 100644 man2/fsconfig.2 create mode 100644 man2/fsmount.2 create mode 100644 man2/fsopen.2 create mode 100644 man2/fspick.2 diff --git a/man2/fsconfig.2 b/man2/fsconfig.2 new file mode 100644 index 000000000..93b82997a --- /dev/null +++ b/man2/fsconfig.2 @@ -0,0 +1,282 @@ +'\" t +.\" Copyright (c) 2019 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 FSCONFIG 2 2019-10-10 "Linux" "Linux Programmer's Manual" +.SH NAME +fsconfig \- Filesystem parameterisation +.SH SYNOPSIS +.nf +.B #include +.br +.B #include +.br +.B #include +.br +.B #include +.PP +.BI "int fsconfig(int *" fd ", unsigned int " cmd ", const char *" key , +.br +.BI " const void __user *" value ", int " aux ");" +.br +.BI +.fi +.PP +.IR Note : +There is no glibc wrapper for this system call. +.SH DESCRIPTION +.PP +.BR fsconfig () +is used to supply parameters to and issue commands against a filesystem +configuration context as set up by +.BR fsopen (2) +or +.BR fspick (2). +The context is supplied attached to the file descriptor specified by +.I fd +argument. +.PP +The +.I cmd +argument indicates the command to be issued, where some of the commands simply +supply parameters to the context. The meaning of +.IR key ", " value " and " aux +are command-dependent; unless required for the command, these should be set to +NULL or 0. +.PP +The available commands are: +.TP +.B FSCONFIG_SET_FLAG +Set the parameter named by +.IR key +to true. This may incur error +.B EINVAL +if the parameter requires an argument. +.TP +.B FSCONFIG_SET_STRING +Set the parameter named by +.I key +to a string. This may incur error +.B EINVAL +if the parser doesn't want a parameter here, wants a non-string or the string +cannot be interpreted appropriately. +.I value +points to a NUL-terminated string. +.TP +.B FSCONFIG_SET_BINARY +Set the parameter named by +.I key +to be a binary blob argument. This may cause +.B EINVAL +to be returned if the filesystem parser isn't expecting a binary blob and it +can't be converted to something usable. +.I value +points to the data and +.I aux +indicates the size of the data. +.TP +.B FSCONFIG_SET_PATH +Set the parameter named by +.I key +to the object at the provided path. +.I value +should point to a NULL-terminated pathname string and aux may indicate +.B AT_FDCWD +or a file descriptor indicating a directory from which to begin a relative +pathwalk. This may return any errors incurred by the pathwalk and may return +.B EINVAL +if the parameter isn't expecting a path. +.IP +Note that FSCONFIG_SET_STRING can be used instead, implying AT_FDCWD. +.TP +.B FSCONFIG_SET_PATH_EMPTY +As FSCONFIG_SET_PATH, but with +.B AT_EMPTY_PATH +applied to the pathwalk. +.TP +.B FSCONFIG_SET_FD +Set the parameter named by +.I key +to the file descriptor specified by +.IR aux . +This will incur +.B EINVAL +if the parameter doesn't expect a file descriptor or +.B EBADF +if the file descriptor is invalid. +.IP +Note that FSCONFIG_SET_STRING can be used instead with the file descriptor +passed as a decimal string. +.TP +.B FSCONFIG_CMD_CREATE +This command causes the filesystem to take the parameters set in the context +and to try to create filesystem representation in the kernel. If it can share +an existing one, it may do that instead if the filesystem type and parameters +permit that. This is intended for use with +.BR fsopen (2). +.TP +.B FSCONFIG_CMD_RECONFIGURE +This command causes the filesystem to apply the parameters set in the context +to an already existing filesystem representation in memory and to alter it. +This is intended for use with +.BR fspick (2), +but may also by used against the context created by +.BR fsopen() +after +.BR fsmount (2) +has been called on it. + +.\"________________________________________________________ +.SH EXAMPLES +.PP +.in +4n +.nf +fsconfig(sfd, FSCONFIG_SET_FLAG, "ro", NULL, 0); + +fsconfig(sfd, FSCONFIG_SET_STRING, "user_xattr", "false", 0); + +fsconfig(sfd, FSCONFIG_SET_BINARY, "ms_pac", pac_buffer, pac_size); + +fsconfig(sfd, FSCONFIG_SET_PATH, "journal", "/dev/sdd4", AT_FDCWD); + +dirfd = open("/dev/", O_PATH); +fsconfig(sfd, FSCONFIG_SET_PATH, "journal", "sdd4", dirfd); + +fd = open("/overlays/mine/", O_PATH); +fsconfig(sfd, FSCONFIG_SET_PATH_EMPTY, "lower_dir", "", fd); + +pipe(pipefds); +fsconfig(sfd, FSCONFIG_SET_FD, "fd", NULL, pipefds[1]); +.fi +.in +.PP + +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.SH RETURN VALUE +On success, the function returns 0. On error, \-1 is returned, and +.I errno +is set appropriately. +.SH ERRORS +The error values given below result from filesystem type independent +errors. +Each filesystem type may have its own special errors and its +own special behavior. +See the Linux kernel source code for details. +.TP +.B EACCES +A component of a path was not searchable. +(See also +.BR path_resolution (7).) +.TP +.B EACCES +Mounting a read-only filesystem was attempted without specifying the +.RB ' ro ' +parameter. +.TP +.B EACCES +A specified block device is located on a filesystem mounted with the +.B MS_NODEV +option. +.\" mtk: Probably: write permission is required for MS_BIND, with +.\" the error EPERM if not present; CAP_DAC_OVERRIDE is required. +.TP +.B EBADF +The file descriptor given by +.I fd +or possibly by +.I aux +(depending on the command) is invalid. +.TP +.B EBUSY +The context attached to +.I fd +is in the wrong state for the given command. +.TP +.B EBUSY +The filesystem representation cannot be reconfigured read-only because it still +holds files open for writing. +.TP +.B EFAULT +One of the pointer arguments points outside the user address space. +.TP +.B EINVAL +.I fd +does not refer to a filesystem configuration context. +.TP +.B EINVAL +One of the source parameters referred to an invalid superblock. +.TP +.B ELOOP +Too many links encountered during pathname resolution. +.TP +.B ENAMETOOLONG +A path name was longer than +.BR MAXPATHLEN . +.TP +.B ENOENT +A pathname was empty or had a nonexistent component. +.TP +.B ENOMEM +The kernel could not allocate sufficient memory to complete the call. +.TP +.B ENOTBLK +Once of the parameters does not refer to a block device (and a device was +required). +.TP +.B ENOTDIR +.IR pathname , +or a prefix of +.IR source , +is not a directory. +.TP +.B EOPNOTSUPP +The command given by +.I cmd +was not valid. +.TP +.B ENXIO +The major number of a block device parameter is out of range. +.TP +.B EPERM +The caller does not have the required privileges. +.SH CONFORMING TO +These functions are Linux-specific and should not be used in programs intended +to be portable. +.SH VERSIONS +.BR fsconfig () +was added to Linux in kernel 5.2. +.SH NOTES +Glibc does not (yet) provide a wrapper for the +.BR fspick () +system call; call it using +.BR syscall (2). +.SH SEE ALSO +.BR mountpoint (1), +.BR fsmount (2), +.BR fsopen (2), +.BR fspick (2), +.BR mount_namespaces (7), +.BR path_resolution (7) diff --git a/man2/fsmount.2 b/man2/fsmount.2 new file mode 100644 index 000000000..2bf59fc3e --- /dev/null +++ b/man2/fsmount.2 @@ -0,0 +1 @@ +.so man2/fsopen.2 diff --git a/man2/fsopen.2 b/man2/fsopen.2 new file mode 100644 index 000000000..6bed60cce --- /dev/null +++ b/man2/fsopen.2 @@ -0,0 +1,256 @@ +'\" t +.\" Copyright (c) 2019 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 FSOPEN 2 2019-10-10 "Linux" "Linux Programmer's Manual" +.SH NAME +fsopen, fsmount \- Filesystem parameterisation and mount creation +.SH SYNOPSIS +.nf +.B #include +.br +.B #include +.br +.B #include +.br +.BR "#include " "/* Definition of AT_* constants */" +.br +.BR "#include " +.PP +.BI "int fsopen(const char *" fsname ", unsigned int " flags ); +.PP +.BI "int fsmount(int " fd ", unsigned int " flags ", unsigned int " mount_attrs ); +.fi +.PP +.IR Note : +There are no glibc wrappers for these system calls. +.SH DESCRIPTION +.PP +.BR fsopen () +creates a blank filesystem configuration context within the kernel for the +filesystem named in the +.I fsname +parameter, puts it into creation mode and attaches it to a file descriptor, +which it then returns. The file descriptor can be marked close-on-exec by +setting +.B FSOPEN_CLOEXEC +in +.IR flags . +.PP +After calling fsopen(), the file descriptor should be passed to the +.BR fsconfig (2) +system call, using that to specify the desired filesystem and security +parameters. +.PP +When the parameters are all set, the +.BR fsconfig () +system call should then be called again with +.B FSCONFIG_CMD_CREATE +as the command argument to effect the creation. +.RS +.PP +.BR "[!]\ NOTE" : +Depending on the filesystem type and parameters, this may rather share an +existing in-kernel filesystem representation instead of creating a new one. +In such a case, the parameters specified may be discarded or may overwrite the +parameters set by a previous mount - at the filesystem's discretion. +.RE +.PP +The file descriptor also serves as a channel by which more comprehensive error, +warning and information messages may be retrieved from the kernel using +.BR read (2). + +.PP +Once the creation command has been successfully run on a context, the context +is switched into need-mount mode which prevents further configuration. At +this point, +.BR fsmount () +should be called to create a mount object. +.PP +.BR fsmount () +takes the file descriptor returned by +.BR fsopen () +and creates a mount object for the filesystem root specified there. The +attributes of the mount object are set from the +.I mount_attrs +parameter. The attributes specify the propagation and mount restrictions to +be applied to accesses through this mount. +.PP +The mount object is then attached to a new file descriptor that looks like one +created by +.BR open "(2) with " O_PATH " or " open_tree (2). +This can be passed to +.BR move_mount (2) +to attach the mount object to a mountpoint, thereby completing the process. +.PP +The file descriptor returned by fsmount() is marked close-on-exec if +FSMOUNT_CLOEXEC is specified in +.IR flags . +.PP +After fsmount() has completed, the context created by fsopen() is reset and +moved to reconfiguration state, allowing the new superblock to be +reconfigured. See +.BR fspick (2) +for details. +.PP + +.\"________________________________________________________ +.SS Message Retrieval Interface +The context file descriptor may be queried for message strings at any time by +calling +.BR read (2) +on the file descriptor. This will return formatted messages that are prefixed +to indicate their class: +.TP +\fB"e "\fP +An error message string was logged. +.TP +\fB"i "\fP +An informational message string was logged. +.TP +\fB"w "\fP +An warning message string was logged. +.PP +Messages are removed from the queue as they're read. + +.\"________________________________________________________ +.SH EXAMPLES +To illustrate the process, here's an example whereby this can be used to mount +an ext4 filesystem on /dev/sdb1 onto /mnt. +.PP +.in +4n +.nf +sfd = fsopen("ext4", FSOPEN_CLOEXEC); +fsconfig(sfd, FSCONFIG_SET_FLAG, "ro", NULL, 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "source", "/dev/sdb1", 0); +fsconfig(sfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0); +fsconfig(sfd, FSCONFIG_SET_FLAG, "acl", NULL, 0); +fsconfig(sfd, FSCONFIG_SET_FLAG, "user_attr", NULL, 0); +fsconfig(sfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0); +fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0); +mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MS_RELATIME); +move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH); +.fi +.in +.PP +Here, an ext4 context is created first and attached to sfd. This is then told +where its source will be, given a bunch of options and created. Then +fsmount() is called to create a mount object and +.BR move_mount (2) +is called to attach it to its intended mountpoint. +.PP +And here's an example of mounting from an NFS server and setting a Smack +security module label on it too: +.PP +.in +4n +.nf +sfd = fsopen("nfs", 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "source", "example.com/pub/linux", 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "rsize", "65536", 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "wsize", "65536", 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0); +fsconfig(sfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0); +fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0); +mfd = fsmount(sfd, 0, MS_NODEV); +move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH); +.fi +.in +.PP + + +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.SH RETURN VALUE +On success, both functions return a file descriptor. On error, \-1 is +returned, and +.I errno +is set appropriately. +.SH ERRORS +The error values given below result from filesystem type independent +errors. +Each filesystem type may have its own special errors and its +own special behavior. +See the Linux kernel source code for details. +.TP +.B EBUSY +The context referred to by +.I fd +is not in the right state to be used by +.BR fsmount (). +.TP +.B EFAULT +One of the pointer arguments points outside the user address space. +.TP +.B EINVAL +.I flags +had an invalid flag set. +.TP +.B EINVAL +.I mount_attrs, +includes invalid +.BR MOUNT_ATTR_* +flags. +.TP +.B EMFILE +The system has too many open files to create more. +.TP +.B ENFILE +The process has too many open files to create more. +.TP +.B ENODEV +Filesystem +.I fsname +not configured in the kernel. +.TP +.B ENOMEM +The kernel could not allocate sufficient memory to complete the call. +.TP +.B EPERM +The caller does not have the required privileges. +.SH CONFORMING TO +These functions are Linux-specific and should not be used in programs intended +to be portable. +.SH VERSIONS +.BR fsopen "(), and " fsmount () +were added to Linux in kernel 5.2. +.SH NOTES +Glibc does not (yet) provide a wrapper for the +.BR fsopen "() or " fsmount "()" +system calls; call them using +.BR syscall (2). +.SH SEE ALSO +.BR mountpoint (1), +.BR fsconfig (2), +.BR fspick (2), +.BR move_mount (2), +.BR open_tree (2), +.BR umount (2), +.BR mount_namespaces (7), +.BR path_resolution (7), +.BR findmnt (8), +.BR lsblk (8), +.BR mount (8), +.BR umount (8) diff --git a/man2/fspick.2 b/man2/fspick.2 new file mode 100644 index 000000000..02f0ccf48 --- /dev/null +++ b/man2/fspick.2 @@ -0,0 +1,195 @@ +'\" t +.\" Copyright (c) 2019 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 FSPICK 2 2019-10-10 "Linux" "Linux Programmer's Manual" +.SH NAME +fspick \- Select filesystem for reconfiguration +.SH SYNOPSIS +.nf +.B #include +.br +.B #include +.br +.B #include +.br +.BR "#include " "/* Definition of AT_* constants */" +.PP +.BI "int fspick(int " dirfd ", const char *" pathname ", unsigned int " flags ); +.fi +.PP +.IR Note : +There is no glibc wrapper for this system call. +.SH DESCRIPTION +.PP +.BR fspick () +creates a new filesystem configuration context within the kernel and attaches a +pre-existing superblock to it so that it can be reconfigured (similar to +.BR mount (8) +with the "-o remount" option). The configuration context is marked as being in +reconfiguration mode and attached to a file descriptor, which is returned to +the caller. This can be marked close-on-exec by setting +.B FSPICK_CLOEXEC +in +.IR flags . +.PP +The target is whichever superblock backs the object determined by +.IR dfd ", " pathname " and " flags . +The following can be set in +.I flags +to control the pathwalk to that object: +.TP +.B FSPICK_SYMLINK_NOFOLLOW +Don't follow symbolic links in the terminal component of the path. +.TP +.B FSPICK_NO_AUTOMOUNT +Don't follow automounts in the terminal component of the path. +.TP +.B FSPICK_EMPTY_PATH +Allow an empty string to be specified as the pathname. This allows +.I dirfd +to specify a path exactly. +.PP +After calling fspick(), the file descriptor should be passed to the +.BR fsconfig (2) +system call, using that to specify the desired changes to filesystem and +security parameters. +.PP +When the parameters are all set, the +.BR fsconfig () +system call should then be called again with +.B FSCONFIG_CMD_RECONFIGURE +as the command argument to effect the reconfiguration. +.PP +After the reconfiguration has taken place, the context is wiped clean (apart +from the superblock attachment, which remains) and can be reused to make +another reconfiguration. +.PP +The file descriptor also serves as a channel by which more comprehensive error, +warning and information messages may be retrieved from the kernel using +.BR read (2). + + +.\"________________________________________________________ +.SS Message Retrieval Interface +The context file descriptor may be queried for message strings at any time by +calling +.BR read (2) +on the file descriptor. This will return formatted messages that are prefixed +to indicate their class: +.TP +\fB"e "\fP +An error message string was logged. +.TP +\fB"i "\fP +An informational message string was logged. +.TP +\fB"w "\fP +An warning message string was logged. +.PP +Messages are removed from the queue as they're read and the queue has a limited +depth, so it's possible for some to get lost. + +.\"________________________________________________________ +.SH EXAMPLES +To illustrate the process, here's an example whereby this can be used to +reconfigure a filesystem: +.PP +.in +4n +.nf +sfd = fspick(AT_FDCWD, "/mnt", FSPICK_NO_AUTOMOUNT | FSPICK_CLOEXEC); +fsconfig(sfd, FSCONFIG_SET_FLAG, "ro", NULL, 0); +fsconfig(sfd, FSCONFIG_SET_STRING, "user_xattr", "false", 0); +fsconfig(sfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0); +.fi +.in +.PP + + +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.SH RETURN VALUE +On success, the function returns a file descriptor. On error, \-1 is returned, +and +.I errno +is set appropriately. +.SH ERRORS +The error values given below result from filesystem type independent +errors. +Each filesystem type may have its own special errors and its +own special behavior. +See the Linux kernel source code for details. +.TP +.B EACCES +A component of a path was not searchable. +(See also +.BR path_resolution (7).) +.TP +.B EFAULT +.I pathname +points outside the user address space. +.TP +.B EINVAL +.I flags +includes an undefined value. +.TP +.B ELOOP +Too many links encountered during pathname resolution. +.TP +.B EMFILE +The system has too many open files to create more. +.TP +.B ENFILE +The process has too many open files to create more. +.TP +.B ENAMETOOLONG +A pathname was longer than +.BR MAXPATHLEN . +.TP +.B ENOENT +A pathname was empty or had a nonexistent component. +.TP +.B ENOMEM +The kernel could not allocate sufficient memory to complete the call. +.TP +.B EPERM +The caller does not have the required privileges. +.SH CONFORMING TO +These functions are Linux-specific and should not be used in programs intended +to be portable. +.SH VERSIONS +.BR fsopen "(), " fsmount "() and " fspick () +were added to Linux in kernel 5.2. +.SH NOTES +Glibc does not (yet) provide a wrapper for the +.BR fspick "()" +system call; call it using +.BR syscall (2). +.SH SEE ALSO +.BR mountpoint (1), +.BR fsconfig (2), +.BR fsopen (2), +.BR path_resolution (7), +.BR mount (8)