netdev - [PATCH net-next v2 4/8] net: netconsole: add docs for appending netconsole user data

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Message-ID: <20240126231348.281600-5-thepacketgeek@gmail.com>
Date: Fri, 26 Jan 2024 15:13:39 -0800
From: Matthew Wood <thepacketgeek@...il.com>
To: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Jonathan Corbet <corbet@....net>
Cc: leitao@...ian.org,
	netdev@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH net-next v2 4/8] net: netconsole: add docs for appending netconsole user data

Add a new User Data section to the netconsole docs to describe the
appending of user data capability (for netconsole dynamic configuration)
with usage and netconsole output examples.

Co-developed-by: Breno Leitao <leitao@...ian.org>
Signed-off-by: Breno Leitao <leitao@...ian.org>
Signed-off-by: Matthew Wood <thepacketgeek@...il.com>
---
 Documentation/networking/netconsole.rst | 68 +++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
index 390730a74332..54f072f47921 100644
--- a/Documentation/networking/netconsole.rst
+++ b/Documentation/networking/netconsole.rst
@@ -15,6 +15,8 @@ Extended console support by Tejun Heo <tj@...nel.org>, May 1 2015
 
 Release prepend support by Breno Leitao <leitao@...ian.org>, Jul 7 2023
 
+Userdata append support by Matthew Wood <thepacketgeek@...il.com>, Jan 22 2024
+
 Please send bug reports to Matt Mackall <mpm@...enic.com>
 Satyam Sharma <satyam.sharma@...il.com>, and Cong Wang <xiyou.wangcong@...il.com>
 
@@ -171,6 +173,72 @@ You can modify these targets in runtime by creating the following targets::
  cat cmdline1/remote_ip
  10.0.0.3
 
+Append User Data
+================
+
+Custom user data can be appended to the end of messages with netconsole
+dynamic configuration enabled. User data entries can be modified without
+changing the "enabled" attribute of a target.
+
+Directories (keys) under `userdata` are limited to 54 character length, and
+data in `userdata/<key>/value` are limited to 200 bytes::
+
+ cd /sys/kernel/config/netconsole && mkdir cmdline0
+ cd cmdline0
+ mkdir userdata/foo
+ echo bar > userdata/foo/value
+ mkdir userdata/qux
+ echo baz > userdata/qux/value
+
+Messages will now include this additional user data::
+
+ echo "This is a message" > /dev/kmsg
+
+Sends::
+
+ 12,607,22085407756,-;This is a message
+ foo=bar
+ qux=baz
+
+Preview the userdata that will be appended with::
+
+ cd /sys/kernel/config/netconsole/cmdline0/userdata
+ for f in `ls userdata`; do echo $f=$(cat userdata/$f/value); done
+
+If a `userdata` entry is created but no data is written to the `value` file,
+the entry will be omitted from netconsole messages::
+
+ cd /sys/kernel/config/netconsole && mkdir cmdline0
+ cd cmdline0
+ mkdir userdata/foo
+ echo bar > userdata/foo/value
+ mkdir userdata/qux
+
+The `qux` key is omitted since it has no value::
+
+ echo "This is a message" > /dev/kmsg
+ 12,607,22085407756,-;This is a message
+ foo=bar
+
+Delete `userdata` entries with `rmdir`::
+
+ rmdir /sys/kernel/config/netconsole/cmdline0/userdata/qux
+
+Beware of `userdata` entries with newlines since values are printed with
+newlines preserved. A userdatum value with a newline could cause the
+netconsole message receiver to interpret a value as a new userdata key::
+
+ cat userdata/foo/value
+ bar
+ bar2
+
+The `qux` key is omitted since it has no value::
+
+ echo "This is a message" > /dev/kmsg
+ 12,607,22085407756,-;This is a message
+ foo=bar
+ bar2
+
 Extended console:
 =================
 
-- 
2.43.0