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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 26 Mar 2015 08:09:57 +1100

On Wed, 2015-03-25 at 19:33 +0100, Ingo Molnar wrote:
> * Laurent Dufour <[email-masked]> wrote:
> 
> > +static inline void arch_unmap(struct mm_struct *mm,
> > +			struct vm_area_struct *vma,
> > +			unsigned long start, unsigned long end)
> > +{
> > +	if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
> > +		mm->context.vdso_base = 0;
> > +}
> 
> So AFAICS PowerPC can have multi-page vDSOs, right?
> 
> So what happens if I munmap() the middle or end of the vDSO? The above 
> condition only seems to cover unmaps that affect the first page. I 
> think 'affects any page' ought to be the right condition? (But I know 
> nothing about PowerPC so I might be wrong.)

You are right, we have at least two pages.
> 
> > +#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.
> 
> In particular, what happens if an mremap() is done with 
> old_start == vdso_base, but a shorter end than the end of the vDSO? 
> (i.e. a partial mremap() with fewer pages than the vDSO size)

Is there a way to forbid splitting ? Does x86 deal with that case at all
or it doesn't have to for some other reason ?

Cheers,
Ben.

> Thanks,
> 
> 	Ingo
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