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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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.

Regards,

-- 
Julien Grall

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