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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.06.14 at 14:53, <julien.grall@xxxxxxxxxx> wrote:
> On 06/18/2014 01:49 PM, Jan Beulich wrote:
>>>>> On 18.06.14 at 14:22, <julien.grall@xxxxxxxxxx> wrote:
>>> On 06/18/2014 12:59 PM, Jan Beulich wrote:
>>>>>>> On 18.06.14 at 00:43, <dgdegra@xxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>>>>>>> On 17/06/14 09:01, Jan Beulich wrote:
>>>>>>>>>>>> On 16.06.14 at 18:17, <julien.grall@xxxxxxxxxx> 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.
> 
> How will you rename the function?

I don't know. All I know is that the function isn't simply coping in a
string.

Jan


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