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Date: Thu, 26 Mar 2015 11:13:53 +0100
From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
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To: Ingo Molnar <mingo@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>, Michael Ellerman <mpe@ellerman.id.au>,
        Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>,
        Guan Xuetao <gxt@mprc.pku.edu.cn>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Arnd Bergmann <arnd@arndb.de>, linuxppc-dev@lists.ozlabs.org,
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Subject: Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap
References: <20150325121118.GA2542@gmail.com> <cover.1427289960.git.ldufour@linux.vnet.ibm.com> <b6ce07f8e1e0d654371aee70bd8eac310456d0df.1427289960.git.ldufour@linux.vnet.ibm.com> <20150325183316.GA9090@gmail.com> <1427317797.6468.86.camel@kernel.crashing.org> <20150326094844.GB15407@gmail.com>
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On 26/03/2015 10:48, Ingo Molnar wrote:
> 
> * Benjamin Herrenschmidt <benh@kernel.crashing.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.
>>>
>>> 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.