Re: [PATCH 2/7] KVM: x86: Improve return type handling in kvm_vm_ioctl_get_nr_mmu_pages()

[Thomas Huth]

On 03/02/2023 18.48, Sean Christopherson wrote:
> On Fri, Feb 03, 2023, Thomas Huth wrote:
> > kvm_vm_ioctl_get_nr_mmu_pages() tries to return a "unsigned long" value,
> > but its caller only stores ther return value in an "int" - which is also
> > what all the other kvm_vm_ioctl_*() functions are returning. So returning
> > values that do not fit into a 32-bit integer anymore does not work here.
> > It's better to adjust the return type, add a sanity check and return an
> > error instead if the value is too big.
> >
> > Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/x86.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index da4bbd043a7b..caa2541833dd 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6007,8 +6007,11 @@ static int kvm_vm_ioctl_set_nr_mmu_pages(struct kvm *kvm,
> >   	return 0;
> >   }
> >   
> > -static unsigned long kvm_vm_ioctl_get_nr_mmu_pages(struct kvm *kvm)
> > +static int kvm_vm_ioctl_get_nr_mmu_pages(struct kvm *kvm)
> >   {
> > +	if (kvm->arch.n_max_mmu_pages > INT_MAX)
> > +		return -EOVERFLOW;
> > +
> >   	return kvm->arch.n_max_mmu_pages;
> >   }
>
> My vote is to skip this patch, skip deprecation, and go straight to deleting
> KVM_GET_NR_MMU_PAGES.  The ioctl() has never worked[*], and none of the VMMs I
> checked use it (QEMU, Google's internal VMM, kvmtool, CrosVM).

I guess I'm living too much in the QEMU world where things need to be 
deprecated first before removing them ;-)
But sure, if everybody agrees that removing this directly is fine, too, I 
can do this in v2.

 Thomas


PS: Has there ever been a discussion about the other deprecated interfaces 
in include/uapi/linux/kvm.h ? Most of the stuff there seems to be from 2009 
... so maybe it's time now to remove that, too?