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

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Message-ID: <Y91JAb0kKBYQjO8a@google.com>
Date:   Fri, 3 Feb 2023 17:48:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Huth <thuth@...hat.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 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).

[*] https://lore.kernel.org/all/YpZu6%2Fk+8EydfBKf@google.com