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

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

FWIW, this is a kernel fallout of the valgrind related QEMU discussion.

On 11/21/2017 04:08 PM, 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) {
> +			/*
> +			 * 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) {
> +			/* see above */
> +			irq_state.flags = 0;
> +		}
>  		r = kvm_s390_get_irq_state(vcpu,
>  					   (__u8 __user *)  irq_state.buf,
>  					   irq_state.len);
>