[RFC PATCH 17/17] KVM: PPC: Add an ioctl for userspace to select which platform to emulate

[Alexander Graf]

On 03.07.2011, at 10:56, Avi Kivity wrote:

> On 07/03/2011 11:34 AM, Alexander Graf wrote:
>> >>
>> >>  Yup, which requires knowledge in the code on what actually fits :). Logic we don't have today.
>> >
>> >  I don't follow.  What knowledge is required?  Please give an example.
>> 
>> Sure. Let's take an easy example Currently we have for get_pvinfo:
>> 
> 
> <snip>
> 
>> The padding would not be there with your idea. An updated version could look like this:
>> 
>>         /* for KVM_PPC_GET_PVINFO */
>>         struct kvm_ppc_pvinfo {
>>                 /* out */
>>                 __u32 flags;
>>                 __u32 hcall[4];
>>                 __u64 features;  /* only there with PVINFO_FLAGS_FEATURES */
>>         };
>> 
>> Now, your idea was to not use copy_from/to_user directly, but instead some wrapper that could pad with zeros on read or truncate on write. So instead we would essentially get:
>> 
>>         int kvm_vm_ioctl_get_pvinfo(struct kvm_ppc_pvinfo *pvinfo, int *required_size)
>>         {
>>                 [...]
>> 		if (pvinfo_flags&  PVINFO_FLAGS_FEATURES) {
>>                         *required_size = 16;
>>                 } else {
>>                         *required_size = 8;
>>                 }
>>                 [...]
>>         }
> 
> 
> Why?  Kernel code would only consider the full structure.
> 
> 
>>         case KVM_PPC_GET_PVINFO: {
>>                 struct kvm_ppc_pvinfo pvinfo;
>>                 int required_size = 0;
>>                 memset(&pvinfo, 0, sizeof(pvinfo));
>>                 r = kvm_vm_ioctl_get_pvinfo(&pvinfo,&required_size);
>>                 if (copy_to_user(argp,&pvinfo, required_size) {
>>                         r = -EFAULT;
>>                         goto out;
>>                 }
> 
> required_size would come from the size encoded in the ioctl number, no need to calculate it separately.
> 
>>                 break;
>>         }
>> 
>> Otherwise we might write over data the user expected. And that logic that tells to copy_to_user how much data it actually takes to put all the information in is not there today and would have to be added. You can even verify that required_size with the ioctl passed size to make 100% sure user space is sane, but I'd claim that a feature bitmap is plenty of information to ensure that we're not doing something stupid.
> 
> I don't see why we have to caclulate something, then verify it against the correct answer.

Ah, I think I'm grasping your idea. You'd simply truncate the resulting struct according to the size passed by the ioctl and call it a day. Well, that works too. User space simply wouldn't be able to know if all information actually fit into the struct, but I guess that's fine :).


Alex