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Date:   Thu, 21 Jul 2022 23:37:32 -0700
From:   Jeremy Bongio <bongiojp@...il.com>
To:     Ted Tso <tytso@....edu>, "Darrick J . Wong" <djwong@...nel.org>
Cc:     linux-ext4@...r.kernel.org, linux-man@...r.kernel.org,
        Jeremy Bongio <bongiojp@...il.com>
Subject: [PATCH v3] Add manpage for get/set fsuuid ioctl for ext4 filesystem.

Signed-off-by: Jeremy Bongio <bongiojp@...il.com>
---

Changes in v3:

Removed LIBRARY section since this ioctl won't be wrapped by a system library.

Code now uses 4-space indent.

Updated for semantic new lines. Please call out if I still break this rule.

Reworded based on Darrick's suggestions.

Unrecognized fsu_flags now results in EOPNOTSUPP error.

Added definitions of EXT4_IOC_GETFSUUID/EXT4_IOC_SETFSUUID which are not
contained in a library.

 man2/ioctl_fsuuid.2 | 113 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 113 insertions(+)
 create mode 100644 man2/ioctl_fsuuid.2

diff --git a/man2/ioctl_fsuuid.2 b/man2/ioctl_fsuuid.2
new file mode 100644
index 000000000..c9e2789b9
--- /dev/null
+++ b/man2/ioctl_fsuuid.2
@@ -0,0 +1,113 @@
+.\" Copyright (c) 2022 Google, Inc., written by Jeremy Bongio <bongiojp@...il.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.TH IOCTL_FSUUID 2 2022-07-20 "Linux" "Linux Programmer's Manual"
+.SH NAME
+ioctl_fsuuid \- get or set an ext4 filesystem uuid
+.SH SYNOPSIS
+.nf
+.B #include <sys/ioctl.h>
+.BR "#include <linux/fs.h>" "         /* Definition of " _IOR " and " _IOW " */"
+.PP
+.BI "#define EXT4_IOC_GETFSUUID      _IOR('f', 44, struct fsuuid)"
+.BI "#define EXT4_IOC_SETFSUUID      _IOW('f', 44, struct fsuuid)"
+.BI "int ioctl(int " fd ", EXT4_IOC_GETFSUUID, struct " fsuuid ");"
+.BI "int ioctl(int " fd ", EXT4_IOC_SETFSUUID, struct " fsuuid ");"
+.fi
+.SH DESCRIPTION
+If an ext4 filesystem supports uuid manipulation, these
+.BR ioctl (2)
+operations can be used to get or set the uuid for the ext4 filesystem on which
+.I fd
+resides.
+.PP
+The argument to these operations should be a pointer to a
+.IR "struct fsuuid" ":"
+.PP
+.in +4n
+.EX
+struct fsuuid {
+    __u32 fsu_len;      /* Number of bytes in a uuid */
+    __u32 fsu_flags;    /* Mapping flags */
+    __u8  fsu_uuid[];   /* Byte array for uuid */
+};
+.EE
+.PP
+The
+.I fsu_flags
+field must be set to 0.
+.PP
+If
+.BR EXT4_IOC_GETFSUUID
+is called with
+.I fsu_len
+set to 0,
+.I fsu_len
+will be set to the number of bytes in an ext4 filesystem uuid
+and the return code will be EINVAL.
+.PP
+If
+.BR EXT4_IOC_GETFSUUID
+is called with
+.I fsu_len
+matching the length of the ext4 filesystem uuid,
+then that uuid will be written to
+.I fsu_uuid[]
+and the return value will be zero.
+If
+.I fsu_len
+does not match, the return value will be
+.B EINVAL.
+.PP
+If
+.BR EXT4_IOC_SETFSUUID
+is called with
+.I fsu_len
+matching the length of the ext4 filesystem uuid,
+then the filesystem uuid will be set to the contents of
+.I fsu_uuid[]
+and  the return value will reflect the outcome of the update operation.
+If 
+.I fsu_len
+does not match, the return value will be
+.B EINVAL.
+.PP
+The
+.BR FS_IOC_SETFSUUID
+operation requires privilege
+.RB ( CAP_SYS_ADMIN ).
+If the filesystem is busy, an
+.BR EXT4_IOC_SETFSUUID
+operation will wait until it can apply the uuid changes.
+This may take a long time.
+.PP
+.SH RETURN VALUE
+On success zero is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+Possible errors include (but are not limited to) the following:
+.TP
+.B EFAULT
+Could not copy to/from userspace.
+.TP
+.B EINVAL
+.I fsu_len
+did not match the filesystem uuid length.
+.TP
+.B ENOTTY
+The filesystem does not support the ioctl.
+.TP
+.B EOPNOTSUPP
+The filesystem does not support changing the uuid through this ioctl.
+This may be due to incompatible filesystem feature flags or
+.I fsu_flags
+has bits set that are not recognized.
+.TP
+.B EPERM
+The calling process does not have sufficient permissions to set the uuid.
+.SH CONFORMING TO
+This API is Linux-specific.
+.SH SEE ALSO
+.BR ioctl (2)
-- 
2.37.1.359.gd136c6c3e2-goog

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