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 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.

Jan


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