linux-kernel - Re: [PATCH 6/7] KVM: arm64: Change return type of kvm_vm_ioctl_mte_copy

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Date:   Tue, 7 Feb 2023 11:09:00 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Gavin Shan <gshan@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Steven Price <steven.price@....com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        linuxppc-dev@...ts.ozlabs.org, Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH 6/7] KVM: arm64: Change return type of
 kvm_vm_ioctl_mte_copy_tags() to "int"

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@...hat.com>
>> ---
>>   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...

  Thomas