linux-kernel - Re: [PATCH] kmod: prevent kmod_loop_msg overflow in __request

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Date:	Fri, 30 Sep 2011 00:18:17 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andrew Morton <akpm00@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kmod: prevent kmod_loop_msg overflow in
 __request_module()

On Thu, 29 Sep 2011, Andrew Morton wrote:

> > Due to post-decrement
> 
> (post-increment)

Indeed, thanks for noticing.

> > in condition of kmod_loop_msg in __request_module(),
> > the system log can be spammed by much more than 5 instances of the 'runaway
> > loop' message if the number of events triggering it makes the kmod_loop_msg
> > to overflow.
> > 
> > Fix that by making sure we never increment it past the threshold.
> > 
> > Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> > ---
> >  kernel/kmod.c |    4 +++-
> >  1 files changed, 3 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/kmod.c b/kernel/kmod.c
> > index ddc7644..a4bea97 100644
> > --- a/kernel/kmod.c
> > +++ b/kernel/kmod.c
> > @@ -114,10 +114,12 @@ int __request_module(bool wait, const char *fmt, ...)
> >  	atomic_inc(&kmod_concurrent);
> >  	if (atomic_read(&kmod_concurrent) > max_modprobes) {
> >  		/* We may be blaming an innocent here, but unlikely */
> > -		if (kmod_loop_msg++ < 5)
> > +		if (kmod_loop_msg < 5) {
> >  			printk(KERN_ERR
> >  			       "request_module: runaway loop modprobe %s\n",
> >  			       module_name);
> > +			kmod_loop_msg++;
> > +		}
> >  		atomic_dec(&kmod_concurrent);
> >  		return -ENOMEM;
> >  	}
> 
> Well, it will take two billion passes through here to cause the
> overflow.  I really hope you found this problem by inspection!

Unfortunately not, it was more painful experience than code reading :)

Thanks,

-- 
Jiri Kosina
SUSE Labs
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