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

From: Laurent Dufour <[email-masked]>
Date: Thu, 26 Mar 2015 11:13:53 +0100

On 26/03/2015 10:48, Ingo Molnar wrote:
> 
> * Benjamin Herrenschmidt <[email-masked]> 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.
>>>
>>> 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 ?
> 
> So we use _install_special_mapping() - maybe PowerPC does that too? 
> That adds VM_DONTEXPAND which ought to prevent some - but not all - of 
> the VM API weirdnesses.

The same is done on PowerPC. So calling mremap() to extend the vDSO is
failing but splitting it or unmapping a part of it is allowed but lead
to an unusable vDSO.

> On x86 we'll just dump core if someone unmaps the vdso.

On PowerPC, you'll get the same result.

Should we prevent the user to break its vDSO ?

Thanks,
Laurent.