Add a manual page to document the fspick() system call. Signed-off-by: David Howells <dhowells@xxxxxxxxxx> --- man2/fspick.2 | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 180 insertions(+) create mode 100644 man2/fspick.2 diff --git a/man2/fspick.2 b/man2/fspick.2 new file mode 100644 index 000000000..72bf645dd --- /dev/null +++ b/man2/fspick.2 @@ -0,0 +1,180 @@ +'\" t +.\" Copyright (c) 2020 David Howells <dhowells@xxxxxxxxxx> +.\" +.\" %%%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 +.\" +.\" Since the Linux kernel and libraries are constantly changing, this +.\" manual page may be incorrect or out-of-date.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 2020-08-24 "Linux" "Linux Programmer's Manual" +.SH NAME +fspick \- Select filesystem for reconfiguration +.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 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. The file descriptor 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 final component of the path. +.TP +.B FSPICK_NO_AUTOMOUNT +Don't follow automounts in the final 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 the target mount 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 <message>"\fP +An error message string was logged. +.TP +\fB"i <message>"\fP +An informational message string was logged. +.TP +\fB"w <message>"\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 of 8 messages, so it's possible for some to get lost. +.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. +Additionally, 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 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 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)
