Messages in this thread | | | Date | Thu, 26 Mar 2015 10:43:30 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap |
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* 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 ...
There's also the question of mprotect(): can users mprotect() the vDSO on PowerPC?
Thanks,
Ingo
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