Re: [Xen-devel] [PATCH 1/2] fix locking in offline_page()

[Tim Deegan <tim@xxxxxxx>]

At 10:43 +0000 on 27 Nov (1385545399), Jan Beulich wrote:
> >>> On 27.11.13 at 11:34, Tim Deegan <tim@xxxxxxx> wrote:
> > At 10:05 +0000 on 27 Nov (1385543154), Jan Beulich wrote:
> >> >>> On 27.11.13 at 11:01, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> >> > On 27/11/13 08:25, Jan Beulich wrote:
> >> >>          else
> >> >>          {
> >> >>              *status = PG_OFFLINE_OWNED | PG_OFFLINE_PENDING |
> >> >> -              (owner->domain_id << PG_OFFLINE_OWNER_SHIFT);
> >> >> +                      (owner->domain_id << PG_OFFLINE_OWNER_SHIFT);
> >> > 
> >> > domain_id will be promoted from a uint16_t to an int32_t, then shifted
> >> > left by 16 bits which is undefined if the top of domain_id bit was set.
> >> 
> >> That promotion is done by zero extension, so I don't figure what
> >> you think is undefined here.
> > 
> > If the domid has bit 15 set, it will be shifted into the sign bit of
> > the promoted integer; it should be explicitly cast to an unsigned type
> > before the shift.
> 
> Again - I don't see this. Promotion happens before the shift,
> i.e. 0x8000 -> 0x00008000 -> 0x80000000.

AIUI the default promotion is to a signed integer if the value will
fit, i.e.:
             (unsigned short) 0x8000 
promoted     (signed int) 0x00008000 
shifted left (signed int) 0x80000000 (undefined behaviour)

(but I don't have my copy of the standard here to check).

Tim.

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