IB/qib: Fix null pointer subtraction compiler warning

>> drivers/infiniband/hw/qib/qib_sysfs.c:411:1: warning: performing pointer subtraction with a null pointer has undefined behavior
+[-Wnull-pointer-subtraction]
   QIB_DIAGC_ATTR(rc_resends);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
   drivers/infiniband/hw/qib/qib_sysfs.c:408:51: note: expanded from macro 'QIB_DIAGC_ATTR'
                   .counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \

Use offsetof and accomplish the type check using static_assert.

Fixes: 4a7aaf88c89f ("RDMA/qib: Use attributes for the port sysfs")
Link: https://lore.kernel.org/r/0-v1-43ae3c759177+65-qib_type_jgg@nvidia.com
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

diff --git a/drivers/infiniband/hw/qib/qib_sysfs.c b/drivers/infiniband/hw/qib/qib_sysfs.c
index d57e49d..452e235 100644 (file)
--- a/drivers/infiniband/hw/qib/qib_sysfs.c
+++ b/drivers/infiniband/hw/qib/qib_sysfs.c
@@ -403,9 +403,11 @@ static ssize_t diagc_attr_store(struct ib_device *ibdev, u32 port_num,
 }
 
 #define QIB_DIAGC_ATTR(N)                                                      \
 }
 
 #define QIB_DIAGC_ATTR(N)                                                      \

+       static_assert(&((struct qib_ibport *)0)->rvp.n_##N != (u64 *)NULL);    \

        static struct qib_diagc_attr qib_diagc_attr_##N = {                    \
                .attr = __ATTR(N, 0664, diagc_attr_show, diagc_attr_store),    \
        static struct qib_diagc_attr qib_diagc_attr_##N = {                    \
                .attr = __ATTR(N, 0664, diagc_attr_show, diagc_attr_store),    \

-               .counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \
+               .counter =                                                     \
+                       offsetof(struct qib_ibport, rvp.n_##N) / sizeof(u64)   \

        }
 
 QIB_DIAGC_ATTR(rc_resends);
        }
 
 QIB_DIAGC_ATTR(rc_resends);