Re: [PATCH 3/5] Add manpage for fspick(2)

[Michael Kerrisk (man-pages)]

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 +.\" manual page may be incorrect or out-of-date.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/