netconsole: implement extended console support

netconsole: implement extended console support

printk logbuf keeps various metadata and optional key=value dictionary for
structured messages, both of which are stripped when messages are handed
to regular console drivers.

It can be useful to have this metadata and dictionary available to
netconsole consumers.  This obviously makes logging via netconsole more
complete and the sequence number in particular is useful in environments
where messages may be lost or reordered in transit - e.g.  when netconsole
is used to collect messages in a large cluster where packets may have to
travel congested hops to reach the aggregator.  The lost and reordered
messages can easily be identified and handled accordingly using the
sequence numbers.

printk recently added extended console support which can be selected by
setting CON_EXTENDED flag.  From console driver side, not much changes.
The only difference is that the text passed to the write callback is
formatted the same way as /dev/kmsg.

This patch implements extended console support for netconsole which can be
enabled by either prepending "+" to a netconsole boot param entry or
echoing 1 to "extended" file in configfs.  When enabled, netconsole
transmits extended log messages with headers identical to /dev/kmsg
output.

There's one complication due to message fragments.  netconsole limits the
maximum message size to 1k and messages longer than that are split into
multiple fragments.  As all extended console messages should carry
matching headers and be uniquely identifiable, each extended message
fragment carries full copy of the metadata and an extra header field to
identify the specific fragment.  The optional header is of the form
"ncfrag=OFF/LEN" where OFF is the byte offset into the message body and
LEN is the total length.

To avoid unnecessarily making printk format extended messages, Extended
netconsole is registered with printk when the first extended netconsole is
configured.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Petr Mladek <pmladek@suse.cz>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>