linux-kernel - Re: [PATCH 2/7] KVM: x86: Improve return type handling in kvm_vm_ioctl_get_nr_mmu

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Date:   Tue, 7 Feb 2023 10:26:39 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        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
Subject: Re: [PATCH 2/7] KVM: x86: Improve return type handling in
 kvm_vm_ioctl_get_nr_mmu_pages()

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@...hat.com>
>> ---
>>   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?