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

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

On 2023-01-05, kernel test robot <lkp@intel.com> wrote:
> smatch warnings:
> kernel/printk/printk.c:2735 console_prepend_dropped() warn: always true condition '(cmsg->outbuf_len + len >= outbuf_sz) => (0-u64max >= 0)'

Thank you kernel test robot, but actually this code will never be hit
when outbuf_sz is 0. Explanation below.

> vim +2735 kernel/printk/printk.c
>
>   2701	
>   2702	/*
>   2703	 * Prepend the message in @cmsg->cbufs->outbuf with a "dropped message". This
>   2704	 * is achieved by shifting the existing message over and inserting the dropped
>   2705	 * message.
>   2706	 *
>   2707	 * @cmsg is the console message to prepend.
>   2708	 *
>   2709	 * @dropped is the dropped count to report in the dropped message.
>   2710	 *
>   2711	 * If the message text in @cmsg->cbufs->outbuf does not have enough space for
>   2712	 * the dropped message, the message text will be sufficiently truncated.
>   2713	 *
>   2714	 * If @cmsg->cbufs->outbuf is modified, @cmsg->outbuf_len is updated.
>   2715	 */
>   2716	static void console_prepend_dropped(struct console_message *cmsg, unsigned long dropped)
>   2717	{
>   2718		struct console_buffers *cbufs = cmsg->cbufs;
>   2719		const size_t scratchbuf_sz = sizeof(cbufs->scratchbuf);
>   2720		const size_t outbuf_sz = sizeof(cbufs->outbuf);
>   2721		char *scratchbuf = &cbufs->scratchbuf[0];
>   2722		char *outbuf = &cbufs->outbuf[0];
>   2723		size_t len;
>   2724	
>   2725		len = snprintf(scratchbuf, scratchbuf_sz,
>   2726			       "** %lu printk messages dropped **\n", dropped);
>   2727	
>   2728		/*
>   2729		 * Make sure outbuf is sufficiently large before prepending. Space
>   2730		 * for a terminator is also counted in case truncation occurs.
>   2731		 */
>   2732		if (WARN_ON_ONCE(len + 1 >= outbuf_sz))
>   2733			return;

If outbuf_sz is 0, the above check will return. It is interesting that
smatch did not complain about this line instead. I suppose the
WARN_ON_ONCE confused it.

Note that if outbuf_sz is 0, then CONFIG_PRINTK is undefined. In that
case this function should not be called anyway. So the check and warning
are appropriate here.

>   2734	
>> 2735		if (cmsg->outbuf_len + len >= outbuf_sz) {

John Ogness