From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Date: Fri, 23 Apr 2010 11:57:17 +0000
Subject: Re: [PATCH RFC v2 6/6] KVM: introduce a new API for getting dirty
Message-Id: <4BD18B1D.1080604@redhat.com>
List-Id: <kvm-ia64.vger.kernel.org>
References: <20100420200353.2d2a6dec.yoshikawa.takuya@oss.ntt.co.jp>
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To: kvm-ia64@vger.kernel.org

On 04/23/2010 01:20 PM, Alexander Graf wrote:
>
>> I would say the reason is that if we did not convert the user-space pointer to
>> a "void *" kvm_get_dirty_log() would end up copying the dirty log to
>>
>> (log->dirty_bitmap<<  32) | 0x00000000
>>      
> Well yes, that was the problem. If we always set the __u64 value to the pointer we're safe though.
>
> union {
>    void *p;
>    __u64 q;
> }
>
> void x(void *r)
> {
>    // breaks:
>    p = r;
>
>    // works:
>    q = (ulong)r;
> }
>    

In that case it's better to avoid p altogether, since users will 
naturally assign to the pointer.

Using a 64-bit integer avoids the problem (though perhaps not sufficient 
for s390, Arnd?)

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.