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Date: Thu, 30 Sep 2021 11:28:20 -0700
From: Kees Cook <>
To: Petr Mladek <>
Cc: "Paul E. McKenney" <>,
	Alexander Popov <>,
	Jonathan Corbet <>,
	Andrew Morton <>,
	Thomas Gleixner <>,
	Peter Zijlstra <>,
	Joerg Roedel <>, Maciej Rozycki <>,
	Muchun Song <>,
	Viresh Kumar <>,
	Robin Murphy <>,
	Randy Dunlap <>,
	Lu Baolu <>,
	Luis Chamberlain <>, Wei Liu <>,
	John Ogness <>,
	Andy Shevchenko <>,
	Alexey Kardashevskiy <>,
	Christophe Leroy <>,
	Jann Horn <>,
	Greg Kroah-Hartman <>,
	Mark Rutland <>,
	Andy Lutomirski <>,
	Dave Hansen <>,
	Steven Rostedt <>,
	Thomas Garnier <>,
	Will Deacon <>,
	Ard Biesheuvel <>,
	Laura Abbott <>,
	David S Miller <>,
	Borislav Petkov <>,,,,,,
	Linus Torvalds <>
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter

On Thu, Sep 30, 2021 at 11:15:41AM +0200, Petr Mladek wrote:
> On Wed 2021-09-29 12:49:24, Paul E. McKenney wrote:
> > On Wed, Sep 29, 2021 at 10:01:33PM +0300, Alexander Popov wrote:
> > > On 29.09.2021 21:58, Alexander Popov wrote:
> > > > Currently, the Linux kernel provides two types of reaction to kernel
> > > > warnings:
> > > >  1. Do nothing (by default),
> > > >  2. Call panic() if panic_on_warn is set. That's a very strong reaction,
> > > >     so panic_on_warn is usually disabled on production systems.
> 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.

panic_on_warn() gets used by folks with paranoid security concerns.

> The problem is that many developers do not know about this behavior.
> They use WARN() when they are lazy to write more useful message or when
> they want to see all the provided details: task, registry, backtrace.

The documentation[1] on this hopefully clarifies the situation:

  Note that the WARN()-family should only be used for “expected to be
  unreachable” situations. If you want to warn about “reachable but
  undesirable” situations, please use the pr_warn()-family of functions.
  System owners may have set the panic_on_warn sysctl, to make sure their
  systems do not continue running in the face of “unreachable” conditions.


> Also it is inconsistent with pr_warn() behavior. Why a single line
> warning would be innocent and full info WARN() cause panic/pkill?

Because pr_warn() is intended for system admins. WARN() is for
developers and should not be reachable through any known path.

> What about pr_err(), pr_crit(), pr_alert(), pr_emerg()? They inform
> about even more serious problems. Why a warning should cause panic/pkill
> while an alert message is just printed?

Additionally, pr_*() don't include stack traces, etc. WARN() is for
situations that should never happen. pr_warn() is about undesirable but
reachable states.

For example:

> It somehow reminds me the saga with %pK. We were not able to teach
> developers to use it correctly for years and ended with hashed
> pointers.

And this was pointed out when %pK was introduced, but Linus couldn't be
convinced. He changed his mind, thankfully.

Kees Cook

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