Re: [PATCH v10 2/5] KVM: stats: Add fd-based API to read binary stats data

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/06/21 09:03, Greg KH wrote:
>> 3. Fd-based solution provides the possibility that a telemetry can
>>     read KVM stats in a less privileged situation.
> "possiblity"?  Does this work or not?  Have you tested it?
> 

I think this is essentially s/that/for/.  But more precisely:

3. Compared for example to a ioctl, a separate file descriptor makes it 
possible for an external program to read statistics, while maintaining 
privilege separation between VMM and telemetry code.

>>
>> +	snprintf(&kvm_vm_stats_header.id[0], sizeof(kvm_vm_stats_header.id),
>> +			"kvm-%d", task_pid_nr(current));
> 
> Why do you write to this static variable for EVERY read?  Shouldn't you
> just do it once at open?  How can it change?
> 
> Wait, it's a single shared variable, what happens when multiple tasks
> open this thing and read from it?  You race between writing to this
> variable here and then:
> 
>> +	return kvm_stats_read(&kvm_vm_stats_header, &kvm_vm_stats_desc[0],
>> +		&kvm->stat, sizeof(kvm->stat), user_buffer, size, offset);
> 
> Accessing it here.
> 
> So how is this really working?

It's not - Jing, kvm_vm_stats_header is small enough that you can store 
a copy in struct kvm.

Paolo