tools/blktap2: fix possible '\0' truncation

gcc-8 complains:

    tapdisk-vbd.c: In function 'tapdisk_vbd_resume_ring':
    tapdisk-vbd.c:1671:53: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
       snprintf(params.name, sizeof(params.name) - 1, "%s", message);
                                                         ^
    tapdisk-vbd.c:1671:3: note: 'snprintf' output between 1 and 256 bytes into a destination of size 255
       snprintf(params.name, sizeof(params.name) - 1, "%s", message);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The "- 1" in buffer size should be actually applied to message, to leave
place for terminating '\0', not the other way around (truncate '\0' even
if it would fit).

    In function 'tapdisk_control_open_image',
        inlined from 'tapdisk_control_handle_request' at tapdisk-control.c:660:10:
    tapdisk-control.c:465:2: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
      strncpy(params.name, vbd->name, BLKTAP2_MAX_MESSAGE_LEN);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    In function 'tapdisk_control_create_socket',
        inlined from 'tapdisk_control_open' at tapdisk-control.c:836:9:
    tapdisk-control.c:793:2: error: 'strncpy' specified bound 108 equals destination size [-Werror=stringop-truncation]
      strncpy(saddr.sun_path, td_control.path, sizeof(saddr.sun_path));
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    block-qcow.c: In function 'qcow_create':
    block-qcow.c:1216:5: error: 'strncpy' specified bound 4096 equals destination size [-Werror=stringop-truncation]
         strncpy(backing_filename, backing_file,
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          sizeof(backing_filename));
          ~~~~~~~~~~~~~~~~~~~~~~~~~

I those cases, reduce size of copied string and make sure final '\0' is
added.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>

tools/blktap2/drivers/block-qcow.c

@@ -1214,7 +1214,8 @@ int qcow_create(const char *filename, uint64_t total_size,
 			if (p && (p - backing_file) >= 2) {
 				/* URL like but exclude "c:" like filenames */
 				strncpy(backing_filename, backing_file,
-					sizeof(backing_filename));
+					sizeof(backing_filename) - 1);
+				backing_filename[sizeof(backing_filename) - 1] = '\0';
 			} else {
 				if (realpath(backing_file, backing_filename) == NULL ||
 				    stat(backing_filename, &st) != 0) {

tools/blktap2/drivers/tapdisk-control.c

@@ -462,7 +462,8 @@ tapdisk_control_open_image(struct tapdisk_control_connection *connection,
 
 	params.capacity = image.size;
 	params.sector_size = image.secsize;
-	strncpy(params.name, vbd->name, BLKTAP2_MAX_MESSAGE_LEN);
+	strncpy(params.name, vbd->name, BLKTAP2_MAX_MESSAGE_LEN - 1);
+	params.name[BLKTAP2_MAX_MESSAGE_LEN - 1] = '\0';
 
 	err = ioctl(vbd->ring.fd, BLKTAP2_IOCTL_CREATE_DEVICE, &params);
 	if (err && errno != EEXIST) {
@@ -790,7 +791,7 @@ tapdisk_control_create_socket(char **socket_path)
 	}
 
 	memset(&saddr, 0, sizeof(saddr));
-	strncpy(saddr.sun_path, td_control.path, sizeof(saddr.sun_path));
+	strncpy(saddr.sun_path, td_control.path, sizeof(saddr.sun_path) - 1);
 	saddr.sun_family = AF_UNIX;
 
 	err = bind(td_control.socket,

tools/blktap2/drivers/tapdisk-vbd.c

@@ -1668,7 +1668,8 @@ out:
 
 		params.sector_size = image.secsize;
 		params.capacity    = image.size;
-		snprintf(params.name, sizeof(params.name) - 1, "%s", message);
+		snprintf(params.name, sizeof(params.name),
+			 "%.*s", (int)sizeof(params.name) - 1, message);
 
 		ioctl(vbd->ring.fd, BLKTAP2_IOCTL_SET_PARAMS, &params);
 		td_flag_clear(vbd->state, TD_VBD_PAUSED);