Re: [PATCH] Introduce the pkill_on_warn boot parameter

[Alexey Khoroshilov]

On 01.10.2021 22:59, Linus Torvalds wrote:
> On Thu, Sep 30, 2021 at 2:15 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>>
>> Honestly, I am not sure if panic_on_warn() or the new pkill_on_warn()
>> work as expected. I wonder who uses it in practice and what is
>> the experience.
> 
> Afaik, there are only two valid uses for panic-on-warn:
> 
>  (a) test boxes (particularly VM's) that are literally running things
> like syzbot and want to report any kernel warnings
> 
>  (b) the "interchangeable production machinery" fail-fast kind of situation
> 
> So in that (a) case, it's literally that you consider a warning to be
> a failure case, and just want to stop. Very useful as a way to get
> notified by syzbot that "oh, that assert can actually trigger".
> 
> And the (b) case is more of a "we have 150 million machines, we expect
> about a thousand of them to fail for any random reason any day
> _anyway_ - perhaps simply due to hardware failure, and we'd rather
> take a machine down quickly and then perhaps look at why only much
> later when we have some pattern to the failures".
> 
> You shouldn't expect panic-on-warn to ever be the case for any actual
> production machine that _matters_. If it is, that production
> maintainer only has themselves to blame if they set that flag.
> 
> But yes, the expectation is that warnings are for "this can't happen,
> but if it does, it's not necessarily fatal, I want to know about it so
> that I can think about it".
> 
> So it might be a case that you don't handle, but that isn't
> necessarily _wrong_ to not handle. You are ok returning an error like
> -ENOSYS for that case, for example, but at the same time you are "If
> somebody uses this, we should perhaps react to it".
> 
> In many cases, a "pr_warn()" is much better. But if you are unsure
> just _how_ the situation can happen, and want a call trace and
> information about what process did it, and it really is a "this
> shouldn't ever happen" situation, a WARN_ON() or a WARN_ON_ONCE() is
> certainly not wrong.
> 
> So think of WARN_ON() as basically an assert, but an assert with the
> intention to be able to continue so that the assert can actually be
> reported. BUG_ON() and friends easily result in a machine that is
> dead. That's unacceptable.

Hi Linus,

Coming back to the discussion of WARN_ON()/pr_warn("WARNING:") semantics.

We see a number of cases where WARNING is used to inform userspace that
it is doing something wrong, e.g.
https://elixir.bootlin.com/linux/v5.19-rc8/source/net/can/j1939/socket.c#L181
https://elixir.bootlin.com/linux/v5.19-rc8/source/drivers/video/fbdev/core/fbmem.c#L1023

It is definitely useful, but it does not make sense in case of fuzzing
when the userspace should do wrong things and check if kernel behaves
correctly.

As a result we have warnings with two different intentions:
- warn that something wrong happens in kernel, but we are able to continue;
- warn userspace that it is doing something wrong.

During fuzzing we would like to report the former and to ignore the
latter. Are any ideas how these intentions can be recognized automatically?

Best regards,
Alexey