Re: [PATCH for-5.18] KVM: fix bad user ABI for KVM_EXIT_SYSTEM_EVENT

[Paolo Bonzini]

On 4/29/22 16:03, Sean Christopherson wrote:
> On Fri, Apr 22, 2022, Paolo Bonzini wrote:
> > For compatibility with userspace that was using the flags field,
> > a union overlaps flags with data[0].
>
> I think "compatibility" is slightly misleading, e.g. the offset of the field is
> changing for 32-bit userspace.

Well, the only such userspace AFAIK is crosvm on ARM and there's no 
compat ABI for ARM.  But yeah, your wording below sounds good.

>    To avoid breaking compilation of userspace that was using the flags
>    field, provide a userspace-only union to overlap flags with data[0].
>
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 91a6fe4e02c0..f903ab0c8d7a 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -445,7 +445,11 @@ struct kvm_run {
> >   #define KVM_SYSTEM_EVENT_RESET          2
> >   #define KVM_SYSTEM_EVENT_CRASH          3
> >   			__u32 type;
> > -			__u64 flags;
> > +			__u32 ndata;
> > +			union {
> > +				__u64 flags;
>
> As alluded to above, what about wrapping flags in
>
> #ifndef __KERNEL__
> 				__u64 flags;
> #endif
>
> so that KVM doesn't try to use flags?

Interesting idea.  I'll apply it and push the patch.

Thanks for the review!

Paolo

> > +				__u64 data[16];
> > +			};
> >   		} system_event;
> >   		/* KVM_EXIT_S390_STSI */
> >   		struct {