Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX_UNREGISTER

[Thomas Gleixner]

On Fri, 12 Sep 2014, Thomas Gleixner wrote:
> On Thu, 11 Sep 2014, Dave Hansen wrote:
> > Well, we use it to figure out whether we _potentially_ need to tear down
> > an VM_MPX-flagged area.  There's no guarantee that there will be one.
> 
> So what you are saying is, that if user space sets the pointer to NULL
> via the unregister prctl, kernel can safely ignore vmas which have the
> VM_MPX flag set. I really can't follow that logic.
>  
> 	mmap_mpx();
> 	prctl(enable mpx);
> 	do lots of crap which uses mpx;
> 	prctl(disable mpx);
> 
> So after that point the previous use of MPX is irrelevant, just
> because we set a pointer to NULL? Does it just look like crap because
> I do not get the big picture how all of this is supposed to work?

do_bounds() will happily map new BTs no matter whether the prctl was
invoked or not. So what's the value of the prctl at all?

The mapping is flagged VM_MPX. Why is this not sufficient?

Thanks,

	tglx

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