linux-kernel - Re: [PATCH 2/6] KVM MMU: fix kvm_mmu_zap

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Date:	Mon, 12 Apr 2010 13:25:58 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] KVM MMU: fix kvm_mmu_zap_page() and its calling path

On 04/12/2010 12:22 PM, Xiao Guangrong wrote:
> Hi Avi,
>
> Avi Kivity wrote:
>
>    
>> hlist_for_each_entry_safe() is supposed to be be safe against removal of
>> the element that is pointed to by the iteration cursor.
>>      
> If we destroyed the next point, hlist_for_each_entry_safe() is unsafe.
>
> List hlist_for_each_entry_safe()'s code:
>
> |#define hlist_for_each_entry_safe(tpos, pos, n, head, member) 		 \
> |	for (pos = (head)->first;					 \
> |	     pos&&  ({ n = pos->next; 1; })&&  				\
> |		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
> |	     pos = n)
>
> if n is destroyed:
> 'pos = n, n = pos->next'
> then it access n again, it's unsafe/illegal for us.
>    

But kvm_mmu_zap_page() will only destroy sp == tpos == pos; n points at 
pos->next already, so it's safe.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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