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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2015-03-26 at 10:43 +0100, Ingo Molnar wrote:
> * Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> 
> > On Wed, 2015-03-25 at 19:36 +0100, Ingo Molnar wrote:
> > > * Ingo Molnar <mingo@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 ...

Well, if we are going to clear it at all yes, we should probably be a
bit smarter about it. My point however was we probably don't need to be
super robust about dealing with any crazy scenario userspace might
conceive.

> There's also the question of mprotect(): can users mprotect() the vDSO 
> on PowerPC?

Nothing prevents it. But here too, I wouldn't bother. The user might be
doing on purpose expecting to catch the resulting signal for example
(though arguably a signal from a sigreturn frame is ... odd).

Cheers,
Ben.