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

From: Mathieu Desnoyers <[email-masked]>
Date: Mon, 18 Aug 2008 22:47:07 -0400

* Steven Rostedt ([email-masked]) wrote:
> 
> 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.
> 

Argh. See text_poke(). It's there exactly for this purpose on x86.

Mathieu

> > 
> > > 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
> 

-- 
Mathieu Desnoyers
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