printk: ringbuffer: fix line counting

printk: ringbuffer: fix line counting

[ Upstream commit 668af87f995b6d6d09595c088ad1fb5dd9ff25d2 ]

Counting text lines in a record simply involves counting the number
of newline characters (+1). However, it is searching the full data
block for newline characters, even though the text data can be (and
often is) a subset of that area. Since the extra area in the data
block was never initialized, the result is that extra newlines may
be seen and counted.

Restrict newline searching to the text data length.

Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210113144234.6545-1-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>