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Date: Tue, 16 Nov 2021 11:34:17 +0300
From: Alexander Popov <>
To: Christophe Leroy <>,
 Steven Rostedt <>, Lukas Bulwahn <>
Cc: Linus Torvalds <>,
 Jonathan Corbet <>, Paul McKenney <>,
 Andrew Morton <>,
 Thomas Gleixner <>, Peter Zijlstra <>,
 Joerg Roedel <>, Maciej Rozycki <>,
 Muchun Song <>,
 Viresh Kumar <>, Robin Murphy <>,
 Randy Dunlap <>, Lu Baolu <>,
 Petr Mladek <>, Kees Cook <>,
 Luis Chamberlain <>, Wei Liu <>,
 John Ogness <>,
 Andy Shevchenko <>,
 Alexey Kardashevskiy <>, Jann Horn <>,
 Greg Kroah-Hartman <>,
 Mark Rutland <>, Andy Lutomirski <>,
 Dave Hansen <>, Will Deacon <>,
 Ard Biesheuvel <>, Laura Abbott <>,
 David S Miller <>, Borislav Petkov <>,
 Arnd Bergmann <>, Andrew Scull <>,
 Marc Zyngier <>, Jessica Yu <>,
 Iurii Zaikin <>,
 Rasmus Villemoes <>, Wang Qing <>,
 Mel Gorman <>,
 Mauro Carvalho Chehab <>,
 Andrew Klychkov <>,
 Mathieu Chouquet-Stringer <>,
 Daniel Borkmann <>, Stephen Kitt <>,
 Stephen Boyd <>,
 Thomas Bogendoerfer <>,
 Mike Rapoport <>, Bjorn Andersson
 Kernel Hardening <>,,
 "open list:DOCUMENTATION" <>,
 linux-arch <>,
 Linux Kernel Mailing List <>,
 linux-fsdevel <>,,,,, Shuah Khan <>,
 Gabriele Paoloni <>, Robert Krutsch <>
Subject: 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 <> 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
> 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:

Best regards,

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