[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the macro set_xen_guest_handle_raw
>>> On 04.11.15 at 17:22, <Ian.Jackson@xxxxxxxxxxxxx> wrote: > Julien Grall writes ("Re: [PATCH 4/4] xen/public: arm: rework the macro > set_xen_guest_handle_raw"): >> On 03/11/15 14:18, Stefano Stabellini wrote: >> >> +#define set_xen_guest_handle_raw(hnd, val) \ >> >> + do { \ >> >> + /* Check if the handle is 64-bit (i.e 8-byte) */ \ >> >> + (void) sizeof(struct { int : -!!(sizeof (hnd) != 8); }); \ >> >> + /* Check if the type of val is compatible with the handle */ \ >> >> + (void) sizeof((val) != (hnd).p); \ >> >> + (hnd).q = (uint64_t)(uintptr_t)(val); \ >> >> } while ( 0 ) >> >> #define set_xen_guest_handle(hnd, val) set_xen_guest_handle_raw(hnd, val) > > I hate to throw yet more variables into this, but I had a discussion > with some C experts in the pub about this problem. Further thought > (including a red herring where someone suggested that memcpy would > "launder" the types) results in this proposal: > > > union xen_guest_handle_TYPE { > TYPE *p; > uint64_t align; > }; > > struct xen_guest_handle__pointer { > uint8_t p0,p1,p2,p3; > }; > > /* > * void set_xen_guest_handle_raw(xen_guest_handle_TYPE *hnd, TYPE *val); > */ > #define set_xen_guest_handle_raw(hnd, val) > do { > void *copy = (val); > struct xen_guest_handle__pointer *src = &(hnd); > struct xen_guest_handle__pointer *dst = &(xen_xgh_copy); > dst->p0 = src->p0; > dst->p1 = src->p1; > dst->p2 = src->p2; > dst->p3 = src->p3; > dst[1]->p0 = 0; > dst[1]->p1 = 0; > dst[1]->p2 = 0; > dst[1]->p3 = 0; > sizeof((hnd).p == (val)); /* typecheck */ > } > > #define get_xen_guest_handle(hnd) ((hnd).p) > > > The above is legal for the following reasons (C99 6.5): > > Considering the accesses to src and dst. These are legal according to > (7) point 5. (Accesses via a character type are always legal.) > > After the above macro has been used to set, what is the effective type > of hnd for a subsequent get_xen_guest_handle ? All quite interesting, but pretty moot with there not being any get_xen_guest_handle() anymore. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |