RDMA/core: Check that process is still alive before sending it to the users

The PID information can disappear asynchronously because the task can be
killed and moved to zombie state. In this case, PID will be zero in
similar way to the kernel tasks. Recognize such situation where we are
asking to return orphaned object and simply skip filling PID attribute.

As part of this change, document the same scenario in counter.c code.

Link: https://lore.kernel.org/r/20191010071105.25538-3-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/counters.c

@@ -149,8 +149,18 @@ static bool auto_mode_match(struct ib_qp *qp, struct rdma_counter *counter,
 	struct auto_mode_param *param = &counter->mode.param;
 	bool match = true;
 
-	/* Ensure that counter belongs to the right PID */
-	if (task_pid_nr(counter->res.task) != task_pid_nr(qp->res.task))
+	/*
+	 * Ensure that counter belongs to the right PID.  This operation can
+	 * race with user space which kills the process and leaves QP and
+	 * counters orphans.
+	 *
+	 * It is not a big deal because exitted task will leave both QP and
+	 * counter in the same bucket of zombie process. Just ensure that
+	 * process is still alive before procedding.
+	 *
+	 */
+	if (task_pid_nr(counter->res.task) != task_pid_nr(qp->res.task) ||
+	    !task_pid_nr(qp->res.task))
 		return false;
 
 	if (auto_mask & RDMA_COUNTER_MASK_QP_TYPE)

drivers/infiniband/core/nldev.c

@@ -415,20 +415,34 @@ err:
 static int fill_res_name_pid(struct sk_buff *msg,
 			     struct rdma_restrack_entry *res)
 {
+	int err = 0;
+
 	/*
 	 * For user resources, user is should read /proc/PID/comm to get the
 	 * name of the task file.
 	 */
 	if (rdma_is_kernel_res(res)) {
-		if (nla_put_string(msg, RDMA_NLDEV_ATTR_RES_KERN_NAME,
-		    res->kern_name))
-			return -EMSGSIZE;
+		err = nla_put_string(msg, RDMA_NLDEV_ATTR_RES_KERN_NAME,
+				     res->kern_name);
 	} else {
-		if (nla_put_u32(msg, RDMA_NLDEV_ATTR_RES_PID,
-		    task_pid_vnr(res->task)))
-			return -EMSGSIZE;
+		pid_t pid;
+
+		pid = task_pid_vnr(res->task);
+		/*
+		 * Task is dead and in zombie state.
+		 * There is no need to print PID anymore.
+		 */
+		if (pid)
+			/*
+			 * This part is racy, task can be killed and PID will
+			 * be zero right here but it is ok, next query won't
+			 * return PID. We don't promise real-time reflection
+			 * of SW objects.
+			 */
+			err = nla_put_u32(msg, RDMA_NLDEV_ATTR_RES_PID, pid);
 	}
-	return 0;
+
+	return err ? -EMSGSIZE : 0;
 }
 
 static bool fill_res_entry(struct ib_device *dev, struct sk_buff *msg,