Re: [PATCH 1/3] gnttab: adjust pin count overflow checks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/01/2021 14:21, Jan Beulich wrote:
> On 15.01.2021 14:35, Andrew Cooper wrote:
>> On 15/01/2021 13:26, Jan Beulich wrote:
>>> On 15.01.2021 14:09, Andrew Cooper wrote:
>>>> On 14/01/2021 15:23, Jan Beulich wrote:
>>>>> @@ -1052,19 +1063,19 @@ map_grant_ref(
>>>>>      shah = shared_entry_header(rgt, ref);
>>>>>      act = active_entry_acquire(rgt, ref);
>>>>>  
>>>>> -    /* Make sure we do not access memory speculatively */
>>>>> -    status = evaluate_nospec(rgt->gt_version == 1) ? &shah->flags
>>>>> -                                                 : &status_entry(rgt, 
>>>>> ref);
>>>>> -
>>>>>      /* If already pinned, check the active domid and avoid refcnt 
>>>>> overflow. */
>>>>>      if ( act->pin &&
>>>>>           ((act->domid != ld->domain_id) ||
>>>>> -          (act->pin & 0x80808080U) != 0 ||
>>>>> +          (act->pin & (pin_incr << (GNTPIN_cntr_width - 1))) ||
>>>> This, I'm afraid, is not an improvement.  What we want is:
>>>>
>>>> #define GNTPIN_overflow_mask 0x80808080U
>>>>
>>>> The reason for checking all at once is defence in depth (not strictly
>>>> necessary, but also not a problem),
>>> How is this not a problem? There is absolutely no reason to
>>> reject a request just because some unrelated field may be
>>> about to overflow. In fact I now think that I didn't even
>>> leverage the full potential - the "act->pin != old_pin" check
>>> could also be relaxed this way, I think. Just that it sits on
>>> a path we probably don't really care about very much.
>> Hmm - I see your point.  I'd missed the fact that a previous operation
>> could leave this timebomb waiting for us.  (Probably wants a note to
>> this effect in the commit message).
> I've added half a sentence.
>
>> However we go about fixing it, "pin_incr << (GNTPIN_cntr_width - 1)" is
>> too obscure IMO.  Perhaps all we need is a
>>
>> #define GNTPIN_overflow_mask(x) ((x) << (GNTPIN_cntr_width - 1))
>>
>> but it does occur to me that this logic only works to being with when
>> pin_incr is strictly 1 in the relevant bytes.
> Perhaps
>
> #define GNTPIN_overflow_mask(x) ({ \
>     ASSERT(!((x) & ~(GNTPIN_hstw_inc | GNTPIN_hstr_inc | \
>                      GNTPIN_devw_inc | GNTPIN_devr_inc))); \
>     (x) << (GNTPIN_cntr_width - 1); \
> })
>
> ? And maybe name the whole thing e.g. GNTPIN_incr2oflow_mask()
> to clarify the input is an increment (which I personally take
> to mean consisting of only values of 1, but I realize views
> may vary)?

Yeah - lets go with this.  Its a massive improvement on the current code.

~Andrew