linux-kernel - Re: [PATCH printk-rework 01/14] printk: limit second loop of syslog_print

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Message-ID: <YC6SjvU0iCSeHVVI@alley>
Date:   Thu, 18 Feb 2021 17:15:10 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk-rework 01/14] printk: limit second loop of
 syslog_print_all

On Thu 2021-02-18 09:18:04, John Ogness wrote:
> The second loop of syslog_print_all() subtracts lengths that were
> added in the first loop. With commit b031a684bfd0 ("printk: remove
> logbuf_lock writer-protection of ringbuffer") it is possible that
> records are (over)written during syslog_print_all(). This allows the
> possibility of the second loop subtracting lengths that were never
> added in the first loop.
> 
> This situation can result in syslog_print_all() filling the buffer
> starting from a later record, even though there may have been room
> to fit the earlier record(s) as well.
> 
> Fixes: b031a684bfd0 ("printk: remove logbuf_lock writer-protection of ringbuffer")
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

It makes sense after all. We reach the limit only when many old
messages has got replaced. It means that there is a flood
of messages. And this limit looks like a reasonable point where
to start filling the provided buffer.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr