linux-kernel - Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

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Message-ID: <alpine.DEB.1.10.0808182240330.13401@gandalf.stny.rr.com>
Date:	Mon, 18 Aug 2008 22:41:00 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Scott Wood <scottwood@...escale.com>,
	Eran Liberty <liberty@...ricom.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Steven Rostedt <srostedt@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)


On Tue, 19 Aug 2008, Benjamin Herrenschmidt wrote:

> 
> > Hmm, this was originally copied from x86, where we did a cmpxchg, but that 
> > is probably not needed since all of this is done in kstop_machine. Also, 
> > only the "get" is needed. If we don't fault there, we wont fault on the 
> > put (unless we have permissions wrong, and that would be a bug).
> 
> Would it ? How do we make sure the kernel text is mapped writeable ?

We map it writeable if FTRACE is enabled.

> 
> > So are you recommending something like
> > 
> >         int cmd;
> > 
> > 	if (__get_user(cmd, ip))
> > 		goto fault;
> > 
> > 	if (cmd != old)
> > 		goto not_same;
> > 
> > 	WARN_ON_ONCE(__put_user(cmd, ip));
> > 
> > If we did this, we could probably put this into the generic code:
> 
> That would work I suppose, I'll give it a try.
> 
> > 	if (copy_from_user(cmd, ip, ARCH_CALL_SIZE))
> > 		goto fault;
> > 
> > 	if (memcmp(cmd, old, ARCH_CALL_SIZE) != 0)
> > 		goto not_same;
> > 
> > 	WARN_ON_ONCE(copy_to_user(cmd, ip, ARCH_CALL_SIZE));
> 
> You need the __ variants or the access_ok() checks will bite
> you bad.

Ah, good point ;-)

-- Steve

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