[PATCH v2 15/16] xen-blkback: Minor cleanups

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

This adds a couple of BUILD_BUG_ON()s and moves some arithmetic after
the validation code that checks the arithmetic’s preconditions.  The
previous code was correct but could potentially trip sanitizers that
check for unsigned integer wraparound.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/block/xen-blkback/blkback.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c 
b/drivers/block/xen-blkback/blkback.c
index 
c362f4ad80ab07bfb58caff0ed7da37dc1484fc5..ac760a08d559085ab875784f1c58cdf2ead95a43
 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -1342,6 +1342,8 @@ static int dispatch_rw_block_io(struct xen_blkif_ring 
*ring,
        nseg = req->operation == BLKIF_OP_INDIRECT ?
               req->u.indirect.nr_segments : req->u.rw.nr_segments;
 
+       BUILD_BUG_ON(offsetof(struct blkif_request, u.rw.id) != 8);
+       BUILD_BUG_ON(offsetof(struct blkif_request, u.indirect.id) != 8);
        if (unlikely(nseg == 0 && operation_flags != REQ_PREFLUSH) ||
            unlikely((req->operation != BLKIF_OP_INDIRECT) &&
                     (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) ||
@@ -1365,13 +1367,13 @@ static int dispatch_rw_block_io(struct xen_blkif_ring 
*ring,
                preq.sector_number     = req->u.rw.sector_number;
                for (i = 0; i < nseg; i++) {
                        pages[i]->gref = req->u.rw.seg[i].gref;
-                       seg[i].nsec = req->u.rw.seg[i].last_sect -
-                               req->u.rw.seg[i].first_sect + 1;
-                       seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
                        if ((req->u.rw.seg[i].last_sect >= (XEN_PAGE_SIZE >> 
9)) ||
                            (req->u.rw.seg[i].last_sect <
                             req->u.rw.seg[i].first_sect))
                                goto fail_response;
+                       seg[i].nsec = req->u.rw.seg[i].last_sect -
+                               req->u.rw.seg[i].first_sect + 1;
+                       seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
                        preq.nr_sects += seg[i].nsec;
                }
        } else {
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab