Re: [RFC 02/19] xen: guestcopy: Provide an helper to copy string from guest

["Jan Beulich" <JBeulich@suse.com>]

>>> On 18.06.14 at 14:22, <julien.grall@linaro.org> wrote:
> On 06/18/2014 12:59 PM, Jan Beulich wrote:
>>>>> On 18.06.14 at 00:43, <dgdegra@tycho.nsa.gov> wrote:
>>> On 06/17/2014 05:23 AM, Julien Grall wrote:
>>>>
>>>>
>>>> On 17/06/14 10:17, Jan Beulich wrote:
>>>>>>>> On 17.06.14 at 11:09, <julien.grall@linaro.org> wrote:
>>>>>> On 17/06/14 09:01, Jan Beulich wrote:
>>>>>>>>>> On 16.06.14 at 18:17, <julien.grall@linaro.org> wrote:
>>>>>>>> +
>>>>>>>> +    /* Add an extra +1 to append \0. We can't assume the guest will
>>>>>>>> +     * provide a valid string */
>>>>>>>
>>>>>>> Now this is the case for flask, but for a generic string copying
>>>>>>> routine I don't think this is desirable. It seems especially wrong to
>>>>>>> aid the guest with putting a NUL where none was. If you really
>>>>>>> want this, I guess you would be better off adding two variants:
>>>>>>> One which demands the string to be NUL-terminated (in which
>>>>>>> case passing in a size is sort of bogus), and one which takes a
>>>>>>> size and inserts a NUL.
>>>
>>> I'm not sure why you would want a string copy-in function to not
>>> NUL-terminate the strings it copies in.  If you don't want the strings
>>> to be NUL-terminated at all, I would call it buffer copy-in function
>>> (and copy_from_guest seems to cover buffer copy-in better).  If you want
>>> the strings to be NUL-terminated and the guest has passed you a length,
>>> it's simpler to have the hypervisor add the NUL instead of copying it
>>> and then checking that it is there.  The current toolstack code for
>>> XSM/FLASK relies on the hypervisor to add the NUL terminator, since it
>>> often passes in (s, strlen(s)).
>> 
>> I didn't say to just leave such strings unterminated. Instead I said
>> that if there is no zero terminator, rather than putting one there we
>> should just fail the operation if the buffer size limit was exceeded.
> 
> It looks like I use the same trick as for flask, i.e using strlen(s) and
> therefore let the hypervisor set the NUL-terminator.
> 
> I will add a comment on this function to say that we expect the
> hypervisor to set the NUL-terminator.

But just to make sure - the generic helper introduced there shouldn't
behave that way if being given the proposed name.

Jan