linux-kernel - Re: [PATCH 1/2] namespaces: give each namespace a serial number

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Date:	Fri, 2 May 2014 10:15:30 -0400
From:	Richard Guy Briggs <rgb@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	linux-audit@...hat.com, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, serge.hallyn@...ntu.com,
	eparis@...hat.com, sgrubb@...hat.com, ebiederm@...ssion.com
Subject: Re: [PATCH 1/2] namespaces: give each namespace a serial number

On 14/05/02, Serge E. Hallyn wrote:
> Quoting Richard Guy Briggs (rgb@...hat.com):
> 
> Most of this looks reasonable, but I'm curious about something,
> 
> > +/**
> > + * ns_serial - compute a serial number for the namespace
> > + *
> > + * Compute a serial number for the namespace to uniquely identify it in
> > + * audit records.
> > + */
> > +unsigned int ns_serial(void)
> > +{
> > +	static DEFINE_SPINLOCK(serial_lock);
> > +	static unsigned int serial = 4; /* reserved for IPC, UTS, user, PID */
> > +
> > +	unsigned long flags;
> > +	unsigned int ret;
> > +
> > +	spin_lock_irqsave(&serial_lock, flags);
> > +	do {
> > +		ret = ++serial;
> > +	} while (unlikely(!ret));
> 
> Why exactly are you doing this?  Surely if serial is going to
> wrap around we've got a bigger problem than just wanting go
> bump one more time?

Thanks for catching this.
The code was templated off audit_serial() which tries to solve a
different problem and rolling it is much more likely.  I hadn't noticed
that rollover protection.  However, I *had* thought of making it a long
(which would be the same size on 32-bit arches, but larger on 64-bit)
since a 64-bit system is more likely to roll it out of sheer speed and
resource availability.  But perhaps a long long would be safer.

> > +	spin_unlock_irqrestore(&serial_lock, flags);
> > +
> > +	return ret;
> > +}

- RGB

--
Richard Guy Briggs <rbriggs@...hat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
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