Re: [PATCH printk v3 6/6] printk: introduce console_prepend_dropped() for dropped messages

[John Ogness <john.ogness@linutronix.de>]

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. ;-)

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.
	 */

John