Re: [PATCH 1/1] KVM: s390: mark irq_state.flags as non-usable

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 11/21/2017 04:18 PM, David Hildenbrand wrote:
> On 21.11.2017 16:08, Christian Borntraeger wrote:
>> Old kernels did not check for zero in the irq_state.flags field and old
>> QEMUs did not zero the flag field when calling KVM_S390_*_IRQ_STATE.
>> Let's add a comment and dummy code to prevent future usage of flags
>> and pad.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>>  arch/s390/kvm/kvm-s390.c | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 6a5e02f..1baa393 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -3834,6 +3834,16 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>>  			r = -EINVAL;
>>  			break;
>>  		}
>> +		if (irq_state.flags) {
> 
> I don't see the need for if (do I need more coffee?).

It is a dummy construct that matches other checks like "if flags return -EINVAL". But since
it is dummy we can do whatever we want (even just a comment) since nobody reads flags as of
today.

> 
>> +			/*
>> +			 * This is a placeholder to make sure that nobody uses
>> +			 * flags and pad. Old kernels did not check for zero
>> +			 * and old QEMUs did not zero the flag field.
>> +			 * That means that we cannot use the flags field for
>> +			 * any possible extension.
>> +			 */
>> +			irq_state.flags = 0;
>> +		}
>>  		r = kvm_s390_set_irq_state(vcpu,
>>  					   (void __user *) irq_state.buf,
>>  					   irq_state.len);
>> @@ -3849,6 +3859,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>>  			r = -EINVAL;
>>  			break;
>>  		}
>> +		if (irq_state.flags) {
> 
> dito
> 
>> +			/* see above */
> 
> /* same handling as for kvm_s390_set_irq_state() */
> 
>> +			irq_state.flags = 0;
>> +		}
>>  		r = kvm_s390_get_irq_state(vcpu,
>>  					   (__u8 __user *)  irq_state.buf,
>>  					   irq_state.len);
>>
> 
>