Date:   Fri, 13 Aug 2021 02:22:01 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH 3/5] Add manpage for fspick(2)

Hello David,

As noted in another mail, I will ping on all of the mails, just to
raise all the patches to the top of the inbox.

Thanks,

Michael

On Thu, 27 Aug 2020 at 13:05, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
>
> 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@...hat.com>
> > ---
> >
> >  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@...hat.com>
> > +.\"
> > +.\" %%%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 +.\" manual page may be incorrect or out-of-date.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/



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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