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Date:   Tue, 3 Jan 2023 14:29:30 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH printk v3 6/6] printk: introduce
 console_prepend_dropped() for dropped messages
Message-ID: <Y7QtusGlIX3AU+TN@alley>
References: <20221221202704.857925-1-john.ogness@linutronix.de>
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 <Y7MEDmP1zqWblj0N@alley>
 <87y1qjdimw.fsf@jogness.linutronix.de>
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On Tue 2023-01-03 11:26:23, John Ogness wrote:
> On 2023-01-02, Petr Mladek <pmladek@suse.com> wrote:
> >> +static void console_prepend_dropped(struct console_message *cmsg, unsigned long dropped)
> >> +{
> >> +	struct console_buffers *cbufs = cmsg->cbufs;
> >> +	const size_t scratchbuf_sz = sizeof(cbufs->scratchbuf);
> >> +	const size_t outbuf_sz = sizeof(cbufs->outbuf);
> >> +	char *scratchbuf = &cbufs->scratchbuf[0];
> >> +	char *outbuf = &cbufs->outbuf[0];
> >> +	size_t len;
> >> +
> >> +	len = snprintf(scratchbuf, scratchbuf_sz,
> >> +		       "** %lu printk messages dropped **\n", dropped);
> >> +
> >> +	/*
> >> +	 * Make sure outbuf is sufficiently large before prepending. Space
> >> +	 * for a terminator is also counted in case truncation occurs.
> >> +	 */
> >> +	if (WARN_ON_ONCE(len + 1 >= outbuf_sz))
> >> +		return;
> >
> > Strictly speaking, this should be:
> >
> > 	if (WARN_ON_ONCE(len >= outbuf_sz))
> > 		return;
> >
> > The trailing '\0' will fit into the buffer when len < outbuf_sz.
> 
> You are correct.
> 
> My concern was that if "@len = @outbuf_sz - 1", the entire message will
> be truncated and you will only see the dropped messages text. I wanted
> at least 1 character from the message to survive. ;-)

I see.

> How about the commments:
> 
> 	/*
> 	 * Make sure outbuf is sufficiently large before prepending. Space
> 	 * for a terminator and at least 1 byte of the message is also
> 	 * checked in case truncation is needed.
> 	 */

It is better than the original.

Well, what about making sure that something more useful is always
printed. For example:

	/*
	 * Make sure outbuf is sufficiently large before prepending.
	 * Keep at least the prefix when the message has to be truncated.
	 * It is a rather theoretical problem when someone tries to
	 * use a minimalist buffer.
	 */
	if (WARN_ON_ONCE(len + PREFIX_MAX + 1 >= outbuf_sz))
		return;

If we want to use this way. It would probably make sense to
rename PREFIX_MAX to CONSOLE_PREFIX_MAX.

Best Regards,
Petr