Re: [PATCH 6/7] KVM: arm64: Change return type of kvm_vm_ioctl_mte_copy_tags() to "int"

[Gavin Shan]

On 2/7/23 9:09 PM, Thomas Huth wrote:
> On 07/02/2023 01.09, Gavin Shan wrote:
> > Hi Thomas,
> >
> > On 2/3/23 8:42 PM, Thomas Huth wrote:
> > > This function only returns normal integer values, so there is
> > > no need to declare its return value as "long".
> > >
> > > Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
> > > ---
> > >   arch/arm64/include/asm/kvm_host.h | 4 ++--
> > >   arch/arm64/kvm/guest.c            | 4 ++--
> > >   2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > index 35a159d131b5..b1a16343767f 100644
> > > --- a/arch/arm64/include/asm/kvm_host.h
> > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > @@ -963,8 +963,8 @@ int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
> > >   int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
> > >                      struct kvm_device_attr *attr);
> > > -long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> > > -                struct kvm_arm_copy_mte_tags *copy_tags);
> > > +int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> > > +                   struct kvm_arm_copy_mte_tags *copy_tags);
> > >   /* Guest/host FPSIMD coordination helpers */
> > >   int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
> > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > > index cf4c495a4321..80e530549c34 100644
> > > --- a/arch/arm64/kvm/guest.c
> > > +++ b/arch/arm64/kvm/guest.c
> > > @@ -1013,8 +1013,8 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
> > >       return ret;
> > >   }
> > > -long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> > > -                struct kvm_arm_copy_mte_tags *copy_tags)
> > > +int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> > > +                   struct kvm_arm_copy_mte_tags *copy_tags)
> > >   {
> > >       gpa_t guest_ipa = copy_tags->guest_ipa;
> > >       size_t length = copy_tags->length;
> >
> > It's possible for the function to return number of bytes have been copied.
> > Its type is 'size_t', same to 'unsigned long'. So 'int' doesn't have sufficient
> > space for it if I'm correct.
> >
> > long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> >                                  struct kvm_arm_copy_mte_tags *copy_tags)
> > {
> >          gpa_t guest_ipa = copy_tags->guest_ipa;
> >          size_t length = copy_tags->length;
> >          :
> >          :
> > out:
> >          mutex_unlock(&kvm->slots_lock);
> >          /* If some data has been copied report the number of bytes copied */
> >          if (length != copy_tags->length)
> >                  return copy_tags->length - length;
> >          return ret;
> > }
>
> Oh, drat, I thought I had checked all return statements ... this must have fallen through the cracks, sorry!
>
> Anyway, this is already a problem now: The function is called from kvm_arch_vm_ioctl() (which still returns a long), which in turn is called from kvm_vm_ioctl() in virt/kvm/kvm_main.c. And that functions stores the return value in an "int r" variable. So the upper bits are already lost there.
>
> Also, how is this supposed to work from user space? The normal "ioctl()" libc function just returns an "int" ? Is this ioctl already used in a userspace application somewhere? ... at least in QEMU, I didn't spot it yet...

The ioctl command KVM_ARM_MTE_COPY_TAGS was merged recently and not used
by QEMU yet. I think struct kvm_arm_copy_mte_tags::length needs to be
'__u32' instead of '__u64' in order to standardize the return value.
Something like below. Documentation/virt/kvm/api.rst::section-4.130
needs update accordingly.

   struct kvm_arm_copy_mte_tags {
        __u64 guest_ipa;
        __u32 pad;
        __u32 length;
        void __user *addr;
        __u64 flags;
        __u64 reserved[2];
  };

Thanks,
Gavin