Re: [PATCH RFC v2 6/6] KVM: introduce a new API for getting dirty bitmaps

["Fernando Luis Vázquez Cao" <fernando@oss.ntt.co.jp>]

On 04/23/2010 08:29 AM, Alexander Graf wrote:
> 
> On 22.04.2010, at 08:09, Fernando Luis Vázquez Cao wrote:
> 
>> On 04/22/2010 11:45 AM, Fernando Luis Vázquez Cao wrote:
>>> On 04/21/2010 06:41 PM, Alexander Graf wrote:
>>>> On 21.04.2010, at 10:29, Fernando Luis Vázquez Cao wrote:
>>>>
>>>>> On 04/20/2010 08:03 PM, Takuya Yoshikawa wrote:
>>>>>> @@ -318,7 +318,7 @@ struct kvm_dirty_log {
>>>>>> 	__u32 padding1;
>>>>>> 	union {
>>>>>> 		void __user *dirty_bitmap; /* one bit per page */
>>>>>> -		__u64 padding2;
>>>>>> +		__u64 addr;
>>>>>
>>>>> This can break on x86_32 and x86_64-compat. addr is a long not a __u64.
>>>>
>>>> So the high 32 bits are zero. Where's the problem?
>>>
>>> If we are careful enough to cast the addr appropriately we should be fine,
>>> even if we keep the padding field in the union. I am not saying that it
>>> breaks 32 architectures but that it can potentially be problematic.
>>>
>>>>>> +	case KVM_SWITCH_DIRTY_LOG: {
>>>>>> +		struct kvm_dirty_log log;
>>>>>> +
>>>>>> +		r = -EFAULT;
>>>>>> +		if (copy_from_user(&log, argp, sizeof log))
>>>>>> +			goto out;
>>>>>> +		r = kvm_vm_ioctl_switch_dirty_log(kvm, &log);
>>>>>> +		if (r)
>>>>>> +			goto out;
>>>>>> +		r = -EFAULT;
>>>>>> +		if (copy_to_user(argp, &log, sizeof log))
>>>>>> +			goto out;
>>>>>> +		r = 0;
>>>>>> +		break;
>>>>>> +	}
>>>>>
>>>>> In x86_64-compat mode we are handling 32bit user-space addresses
>>>>> so we need the compat counterpart of KVM_SWITCH_DIRTY_LOG too.
>>>>
>>>> The compat code just forwards everything to the generic ioctls.
>>>
>>> The compat code uses struct compat_kvm_dirty_log instead of
>>> struct kvm_dirty_log to communicate with user space so
>>> the necessary conversions needs to be done before invoking
>>> the generic ioctl (see KVM_GET_DIRTY_LOG in kvm_vm_compat_ioctl).
>>>
>>> By the way we probable should move the definition of struct
>>> compat_kvm_dirty_log to a header file.
>>
>> It seems that it was you and Arnd who added the kvm_vm compat ioctl :-).
>> Are you considering a different approach to tackle the issues that we
>> have with a big-endian userspace?
> 
> IIRC the issue was a pointer inside of a nested structure, no?

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

Am I missing something?

Thanks,

Fernando