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Date:   Tue, 16 Nov 2021 07:37:29 +0100
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Subject: Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter
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To:     Steven Rostedt <rostedt@goodmis.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc:     Alexander Popov <alex.popov@linux.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Paul McKenney <paulmck@kernel.org>,
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 <20211115110649.4f9cb390@gandalf.local.home>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
In-Reply-To: <20211115110649.4f9cb390@gandalf.local.home>
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Le 15/11/2021 à 17:06, Steven Rostedt a écrit :
> On Mon, 15 Nov 2021 14:59:57 +0100
> Lukas Bulwahn <lukas.bulwahn@gmail.com> 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.

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.

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.

Christophe