On Thu, Apr 13, 2023, Takahiro Itazuri wrote: > Modify the KVM_GET_CPUID2 API to return the number of valid entries in > nent field of kvm_cpuid2 even on success. > > Previously, the KVM_GET_CPUID2 API only updated the nent field when an > error was returned. If the API was called with an entry count larger > than necessary (e.g., KVM_MAX_CPUID_ENTRIES), it would succeed, but the > nent field would continue to show a value larger than the actual number > of entries filled by the KVM_GET_CPUID2 API. With this change, users can > rely on the updated nent field and there is no need to traverse > unnecessary entries and check whether an entry is valid or not. > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Takahiro Itazuri <itazur@xxxxxxxxxx> > --- > Changes > v1 -> v2 > * Capitalize "kvm" in the commit title. > * Remove a scratch "nent" variable. > * Link to v1: https://lore.kernel.org/all/20230410141820.57328-1-itazur@xxxxxxxxxx/ > > arch/x86/kvm/cpuid.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 599aebec2d52..20d28ebdc672 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -523,18 +523,18 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu, > struct kvm_cpuid2 *cpuid, > struct kvm_cpuid_entry2 __user *entries) > { > - int r; > + int r = 0; > > - r = -E2BIG; > if (cpuid->nent < vcpu->arch.cpuid_nent) > - goto out; > - r = -EFAULT; > - if (copy_to_user(entries, vcpu->arch.cpuid_entries, > + r = -E2BIG; > + else if (copy_to_user(entries, vcpu->arch.cpuid_entries, > vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2))) > - goto out; > - return 0; > + r = -EFAULT; > > -out: > + /* > + * Update "nent" even on failure, e.g. so that userspace can fix an > + * -E2BIG issue by allocating a larger array. > + */ Gah, this is wrong. The caller only copies @cpuid to userspace on success. My fault for not seeing this in v1, but this is exactly why we write tests. I've written a test and a fix, I'll post 'em shortly.
