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Date:   Sat, 22 Aug 2020 22:18:04 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] Add manpage for fsconfig(2)

Hello David,

On Fri, 7 Aug 2020 at 16:03, David Howells <dhowells@...hat.com> wrote:
>
> Add a manual page to document the fsconfig() system call.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
>
>  man2/fsconfig.2 |  282 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 282 insertions(+)
>  create mode 100644 man2/fsconfig.2
>
> diff --git a/man2/fsconfig.2 b/man2/fsconfig.2
> new file mode 100644
> index 000000000..32f0bce13
> --- /dev/null
> +++ b/man2/fsconfig.2
> @@ -0,0 +1,282 @@
> +'\" 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 +.\" manual page may be incorrect or out-of-date.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 FSCONFIG 2 2020-08-07 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +fsconfig \- Filesystem parameterisation
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/types.h>
> +.br

Remove all instances of ".br" here in the SYNOPSIS. They aren't needed.

> +.B #include <sys/mount.h>
> +.br
> +.B #include <unistd.h>
> +.br
> +.B #include <sys/mount.h>
> +.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

s/incur error/fail with the error/
(and below as well please)

> +.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

"pathwalk" is not a standard term (at least not for user-space
programmers). What do you mean with this term, and can you replace all
instances please.

> +.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.

Too many pronouns ("It", "one", "it", "that"). Could you replace at
least some of them with nouns please.

> 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.
> +
> +.\"________________________________________________________

Please remove above two lines

> +.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
> +
> +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
> +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
> +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

Please remove above 5 lines.

> +.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

s/Once/One/

> +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 5.1.

5.2!

> +.SH NOTES
> +Glibc does not (yet) provide a wrapper for the
> +.BR fsconfig ()
> +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)

David, this is just my first round of comments. Could you please also
carefully check the pages to see what is out of date. I've already
pointed out a few obvious that I would have hope you might have caught
already before submitting, and perhaps there are others also.

Thanks,

Michael

--
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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