Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter

On 16.11.2021 09:37, Christophe Leroy wrote:
> Le 15/11/2021 à 17:06, Steven Rostedt a écrit :
>> On Mon, 15 Nov 2021 14:59:57 +0100
>> Lukas Bulwahn <[email protected]> wrote:
>>
>>> 1. Allow a reasonably configured kernel to boot and run with
>>> panic_on_warn set. Warnings should only be raised when something is
>>> not configured as the developers expect it or the kernel is put into a
>>> state that generally is _unexpected_ and has been exposed little to
>>> the critical thought of the developer, to testing efforts and use in
>>> other systems in the wild. Warnings should not be used for something
>>> informative, which still allows the kernel to continue running in a
>>> proper way in a generally expected environment. Up to my knowledge,
>>> there are some kernels in production that run with panic_on_warn; so,
>>> IMHO, this requirement is generally accepted (we might of course
>>
>> To me, WARN*() is the same as BUG*(). If it gets hit, it's a bug in the
>> kernel and needs to be fixed. I have several WARN*() calls in my code, and
>> it's all because the algorithms used is expected to prevent the condition
>> in the warning from happening. If the warning triggers, it means either that
>> the algorithm is wrong or my assumption about the algorithm is wrong. In
>> either case, the kernel needs to be updated. All my tests fail if a WARN*()
>> gets hit (anywhere in the kernel, not just my own).
>>
>> After reading all the replies and thinking about this more, I find the
>> pkill_on_warning actually worse than not doing anything. If you are
>> concerned about exploits from warnings, the only real solution is a
>> panic_on_warning. Yes, it brings down the system, but really, it has to be
>> brought down anyway, because it is in need of a kernel update.
>>
> 
> We also have LIVEPATCH to avoid bringing down the system for a kernel
> update, don't we ? So I wouldn't expect bringing down a vital system
> just for a WARN.

Hello Christophe,

I would say that different systems have different requirements.
Not every Linux-based system needs live patching (it also has own limitations).

That's why I proposed a sysctl and didn't change the default kernel behavior.

> As far as I understand from
> https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on,
> WARN() and WARN_ON() are meant to deal with those situations as
> gracefull as possible, allowing the system to continue running the best
> it can until a human controled action is taken.

I can't agree here. There is a very strong push against adding BUG*() to the 
kernel source code. So there are a lot of cases when WARN*() is used for severe 
problems because kernel developers just don't have other options.

Currently, it looks like there is no consistent error handling policy in the kernel.

> So I'd expect the WARN/WARN_ON to be handled and I agree that that
> pkill_on_warning seems dangerous and unrelevant, probably more dangerous
> than doing nothing, especially as the WARN may trigger for a reason
> which has nothing to do with the running thread.

Sorry, I see a contradiction.
If killing a process hitting a kernel warning is "dangerous and unrelevant",
why killing a process on a kernel oops is fine? That's strange.

Linus calls that behavior "fairly benign" here: 
http://lkml.iu.edu/hypermail/linux/kernel/1610.0/01217.html

Best regards,
Alexander