* Benjamin Herrenschmidt <b...@kernel.crashing.org> wrote:
> On Wed, 2015-03-25 at 19:36 +0100, Ingo Molnar wrote:
> > * Ingo Molnar <mi...@kernel.org> 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.
I argue we should use the right condition to clear vdso_base: if the
vDSO gets at least partially unmapped. Otherwise there's little point
in the whole patch: either correctly track whether the vDSO is OK, or
don't ...
There's also the question of mprotect(): can users mprotect() the vDSO
on PowerPC?
Thanks,
Ingo
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