linux-kernel - Re: [PATCH 2/4] KVM: arm64: Use find_vma

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Date:   Mon, 15 Mar 2021 20:40:16 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        will@...nel.org, alexandru.elisei@....com, shan.gavin@...il.com
Subject: Re: [PATCH 2/4] KVM: arm64: Use find_vma_intersection()

Hi Marc,

On 3/15/21 7:52 PM, Marc Zyngier wrote:
> On Mon, 15 Mar 2021 04:18:42 +0000,
> Gavin Shan <gshan@...hat.com> wrote:
>>
>> find_vma_intersection() has been existing to search the intersected
>> vma. This uses the function where it's applicable, to simplify the
>> code.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 84e70f953de6..286b603ed0d3 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -421,10 +421,11 @@ static void stage2_unmap_memslot(struct kvm *kvm,
>>   	 *     +--------------------------------------------+
>>   	 */
>>   	do {
>> -		struct vm_area_struct *vma = find_vma(current->mm, hva);
>> +		struct vm_area_struct *vma;
>>   		hva_t vm_start, vm_end;
>>   
>> -		if (!vma || vma->vm_start >= reg_end)
>> +		vma = find_vma_intersection(current->mm, hva, reg_end);
> 
> For context, here's the definition of find_vma_intersection():
> 
> <quote>
> static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * mm, unsigned long start_addr, unsigned long end_addr)
> {
> 	struct vm_area_struct * vma = find_vma(mm,start_addr);
> 
> 	if (vma && end_addr <= vma->vm_start)
> 		vma = NULL;
> 	return vma;
> }
> </quote>
> 
> It seems that there is a boundary issue in either the old code or the
> new one in the case where (reg_end == vma->start).
> 
> Which one is which?
> 

The old and new code is interchangeable, meaning "reg_end == vma->start"
is invalid in both cases. So if there is a boundary issue, the old and new
code should have same issue.

According to the code, "reg_end == vma->start" is invalid. So I don't see
there is a boundary issue. Hopefully, I don't miss anything :)

Thanks,
Gavin