RDMA/ucma: Fix uABI structure layouts for 32/64 compat

The rdma_ucm_event_resp is a different length on 32 and 64 bit compiles.

The kernel requires it to be the expected length or longer so 32 bit
builds running on a 64 bit kernel will not work.

Retain full compat by having all kernels accept a struct with or without
the trailing reserved field.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/ucma.c

@@ -382,7 +382,11 @@ static ssize_t ucma_get_event(struct ucma_file *file, const char __user *inbuf,
 	struct ucma_event *uevent;
 	int ret = 0;
 
-	if (out_len < sizeof uevent->resp)
+	/*
+	 * Old 32 bit user space does not send the 4 byte padding in the
+	 * reserved field. We don't care, allow it to keep working.
+	 */
+	if (out_len < sizeof(uevent->resp) - sizeof(uevent->resp.reserved))
 		return -ENOSPC;
 
 	if (copy_from_user(&cmd, inbuf, sizeof(cmd)))
@@ -417,7 +421,8 @@ static ssize_t ucma_get_event(struct ucma_file *file, const char __user *inbuf,
 	}
 
 	if (copy_to_user((void __user *)(unsigned long)cmd.response,
-			 &uevent->resp, sizeof uevent->resp)) {
+			 &uevent->resp,
+			 min_t(size_t, out_len, sizeof(uevent->resp)))) {
 		ret = -EFAULT;
 		goto done;
 	}

include/uapi/rdma/rdma_user_cm.h

@@ -270,10 +270,15 @@ struct rdma_ucm_event_resp {
 	__u32 id;
 	__u32 event;
 	__u32 status;
+	/*
+	 * NOTE: This union is not aligned to 8 bytes so none of the union
+	 * members may contain a u64 or anything with higher alignment than 4.
+	 */
 	union {
 		struct rdma_ucm_conn_param conn;
 		struct rdma_ucm_ud_param   ud;
 	} param;
+	__u32 reserved;
 };
 
 /* Option levels */