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

Fernando Luis Vázquez Cao
2010-04-23T10:17:42Z

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