Hello David, On 8/24/20 2:24 PM, David Howells wrote: > 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 +.\" manual page may be incorrect or out-of-date.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 Better: s/using that/in order/ > +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 s/descriptor/descriptor returned by fspick()/ > +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. What if there are no pending error messages to retrieve? What does read() do in that case? Please add an explanation here. > +.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 . MAXPATHLEN is not, I think, a constant known in user space. What is this? Should it be PATH_MAX? > +.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. Please note the necessary capability here. Also, there was no mention of capabilities/privileges in DESCRIPTION. Should there have been? > +.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) Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
