On 10.09.21 12:14, Marek Marczykowski-Górecki wrote:
On Fri, Jul 30, 2021 at 12:38:53PM +0200, Juergen Gross wrote:In order to avoid a malicious backend being able to influence the local copy of a request build the request locally first and then copy it to the ring page instead of doing it the other way round as today. Signed-off-by: Juergen Gross <jgross@xxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> --- V2: - init variable to avoid potential compiler warning (Jan Beulich) --- drivers/block/xen-blkfront.c | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c index 15e840287734..b7301006fb28 100644(...)@@ -827,10 +832,10 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri if (setup.segments) kunmap_atomic(setup.segments);- /* Keep a private copy so we can reissue requests when recovering. */- rinfo->shadow[id].req = *ring_req; + /* Copy request(s) to the ring page. */ + *final_ring_req = *ring_req;Is this guaranteed to not be optimized by the compiler in an unsafe way (like, do the operation the other way around)?
I don't think the C standard allows that. AFAIK reordering writes is allowed only between sequence points. And each external function call is a sequence point, making such an optimization in our case illegal. Juergen
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