Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

[Benjamin Herrenschmidt]

On Wed, 2015-03-25 at 19:36 +0100, Ingo Molnar wrote:
> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> > > +#define __HAVE_ARCH_REMAP
> > > +static inline void arch_remap(struct mm_struct *mm,
> > > +			      unsigned long old_start, unsigned long old_end,
> > > +			      unsigned long new_start, unsigned long new_end)
> > > +{
> > > +	/*
> > > +	 * mremap() doesn't allow moving multiple vmas so we can limit the
> > > +	 * check to old_start == vdso_base.
> > > +	 */
> > > +	if (old_start == mm->context.vdso_base)
> > > +		mm->context.vdso_base = new_start;
> > > +}
> > 
> > mremap() doesn't allow moving multiple vmas, but it allows the 
> > movement of multi-page vmas and it also allows partial mremap()s, 
> > where it will split up a vma.
> 
> I.e. mremap() supports the shrinking (and growing) of vmas. In that 
> case mremap() will unmap the end of the vma and will shrink the 
> remaining vDSO vma.
> 
> Doesn't that result in a non-working vDSO that should zero out 
> vdso_base?

Right. Now we can't completely prevent the user from shooting itself in
the foot I suppose, though there is a legit usage scenario which is to
move the vDSO around which it would be nice to support. I think it's
reasonable to put the onus on the user here to do the right thing.

Cheers,
Ben.

> Thanks,
> 
> 	Ingo
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