linux-kernel - Re: [PATCH] printk: fix discarding message when recursion

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Message-ID: <20081219223558.GC13409@elte.hu>
Date:	Fri, 19 Dec 2008 23:35:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc:	Toshikazu Nakayama <nakayama.ts@...s.nec.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: fix discarding message when recursion_bug


* Hiroshi Shimamoto <h-shimamoto@...jp.nec.com> wrote:

> Ingo Molnar wrote:
> > * Hiroshi Shimamoto <h-shimamoto@...jp.nec.com> wrote:
> > 
> >> From: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
> >>
> >> Impact: fix
> >>
> >> When recursion_bug is true, kernel discards original message because printk_buf
> >> contains recursion_bug_msg with NULL terminator. The sizeof(recursion_bug_msg)
> >> makes this, use strlen() to get correct length without NULL terminator.
> >>
> >> Reported-by: Toshikazu Nakayama <nakayama.ts@...s.nec.co.jp>
> >> Signed-off-by: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
> >> ---
> >>  kernel/printk.c |    2 +-
> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > Good one - applied to tip/core/printk, thanks!
> > 
> > Btw., i'm curious: in what situation was such recursion observed, and how 
> > did the kernel behave? You saw a truncated recursion message in dmesg - or 
> > was it worse?
> 
> I'm not sure about the real situation. I've heard this issue was found 
> in printing messages in NMI handler, maybe with artificial kernel stuck.

yeah, accidental NMI recursion back into printk could indeed cause the 
recursion message to pop up.

	Ingo
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