Re: [PATCH 1/2] KVM: Don't fail KVM_GET_SUPPORTED_CPUID if nent is just right

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Nov 17, 2011 at 12:18:44PM +0200, Sasha Levin wrote:
> If we pass just enough entries to KVM_GET_SUPPORTED_CPUID, we would still
> fail with -E2BIG due to wrong comparisons.
> 
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> ---
>  arch/x86/kvm/x86.c |   12 ++++++------
>  1 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9eff4af..460c49b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2664,7 +2664,7 @@ static int kvm_dev_ioctl_get_supported_cpuid(struct kvm_cpuid2 *cpuid,
>  		do_cpuid_ent(&cpuid_entries[nent], func, 0,
>  			     &nent, cpuid->nent);
>  	r = -E2BIG;
> -	if (nent >= cpuid->nent)
> +	if (nent > cpuid->nent)
>  		goto out_free;

"int nent" variable contains the index into the array. 
"__u32 cpuid->nent", from userspace, contains the number
of entries in the array.

So the ">=" comparison is necessary to avoid overwriting past the end of
the array.

The protocol goes like "try size x, if it fails with -E2BIG, increase x,
try again". Its awkward.