Re: [patch RFC] kvm, cpuid: silence a buffer overflow warning

[Paolo Bonzini <pbonzini@redhat.com>]

Il 20/02/2014 14:18, walter harms ha scritto:
>
>
> Am 20.02.2014 14:07, schrieb Paolo Bonzini:
>> Il 20/02/2014 13:34, Dan Carpenter ha scritto:
>>> This seems like a harmless off by one overflow if "i" is the last
>>> element in the vcpu->arch.cpuid_entries[] array.
>>>
>>> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
>>> ---
>>> Not tested.  I always wonder if it's worth fixing these or if it's worth
>>> reporting them?  Either of those seem like a lot of work for something
>>> harmless.
>>
>> Could it oops if cpuid_nent is INT_MAX?  If so, it's not entirely harmless.
>> In this case I'd rather take the occasion to cleanup the code like this
>> (compile-tested):
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 9fed5bedaad6..2fd6e7169936 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -656,18 +656,19 @@ out:
>>  static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
>>  {
>>  	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
>> -	int j, nent = vcpu->arch.cpuid_nent;
>> +	struct kvm_cpuid_entry2 *ej;
>> +	int j = i, nent = vcpu->arch.cpuid_nent;
>>
>>  	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
>> +
>>  	/* when no next entry is found, the current entry[i] is reselected */
>> -	for (j = i + 1; ; j = (j + 1) % nent) {
>> -		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
>> -		if (ej->function = e->function) {
>> -			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
>> -			return j;
>> -		}
>> -	}
>> -	return 0; /* silence gcc, even though control never reaches here */
>> +	do {
>> +		j = (j + 1) % nent;
>> +		ej = &vcpu->arch.cpuid_entries[j];
>> +	} while (ej->function != e->function);
>> +
>> +	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
>> +	return j;
>>  }
>>
>>  /* find an entry with matching function, matching index (if needed), and that
>>
>> What do you think?
>>
>
> Is there any guaranty that this will not loop forever ?
>
> an if (i=j) return 0; would be on the save side. (I guess that
> these was the idea behind the for).

Once i=j you'll get ej->function = e->function and exit.

Paolo