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

From: John Ogness
Date: Thu Jan 05 2023 - 08:58:00 EST


On 2023-01-05, kernel test robot <lkp@xxxxxxxxx> 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