On 04/30/2018 05:01 PM, Marek Marczykowski-Górecki wrote:
> Using RING_GET_RESPONSE() on a shared ring is easy to use incorrectly
> (i.e., by not considering that the other end may alter the data in the
> shared ring while it is being inspected). Safe usage of a response
> generally requires taking a local copy.
>
> Provide a RING_COPY_RESPONSE() macro to use instead of
> RING_GET_RESPONSE() and an open-coded memcpy(). This takes care of
> ensuring that the copy is done correctly regardless of any possible
> compiler optimizations.
>
> Use a volatile source to prevent the compiler from reordering or
> omitting the copy.
>
> This is complementary to XSA155.
>
> CC: sta...@vger.kernel.org
> Signed-off-by: Marek Marczykowski-Górecki <marma...@invisiblethingslab.com>
> ---
> include/xen/interface/io/ring.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
> index 3f40501..03702f6 100644
> --- a/include/xen/interface/io/ring.h
> +++ b/include/xen/interface/io/ring.h
> @@ -201,6 +201,20 @@ struct __name##_back_ring {
> \
> #define RING_GET_RESPONSE(_r, _idx) \
> (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
>
> +/*
> + * Get a local copy of a response.
> + *
> + * Use this in preference to RING_GET_RESPONSE() so all processing is
> + * done on a local copy that cannot be modified by the other end.
> + *
> + * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause
> this
> + * to be ineffective where _rsp is a struct which consists of only bitfields.
> + */
> +#define RING_COPY_RESPONSE(_r, _idx, _rsp) do {
> \
> + /* Use volatile to force the copy into _rsp. */ \
> + *(_rsp) = *(volatile typeof(_rsp))RING_GET_RESPONSE(_r, _idx); \
> +} while (0)
> +
To avoid repeating essentially the same comment, can you move
RING_COPY_RESPONSE definition next to RING_COPY_REQUEST and adjust the
existing comment? And probably move RING_GET_RESPONSE next to
RING_GET_REQUEST? In other words
#define RING_GET_REQUEST
#define RING_GET_RESPONSE
/* comment */
#define RING_COPY_REQUEST
#define RING_COPY_RESPONSE
Also, perhaps the two can be collapsed together, along the lines of
#define RING_COPY_(action, _r, _idx, _msg) do { \
/* Use volatile to force the copy into _msg. */ \
*(_msg) = *(volatile typeof(_msg))RING_GET_##action(_r, _idx); \
} while (0)
#define RING_COPY_REQUEST(_r, _idx, _req) RING_COPY_(REQUEST, _r, _idx,
_req)
#define RING_COPY_RESPONSE(_r, _idx, _rsp) RING_COPY_(RESPONSE, _r,
_idx, _rsp)
(I have not tried to compile this so it may well be wrong)
-boris
> /* Loop termination condition: Would the specified index overflow the ring?
> */
> #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)
> \
> (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
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