fuse_session_custom_io: Add a struct size field

example/hello_ll_uds.c

@@ -341,7 +341,7 @@ int main(int argc, char *argv[])
 	if (cfd == -1)
 		goto err_out3;
 
-	if (fuse_session_custom_io(se, &io, cfd) != 0)
+	if (fuse_session_custom_io(se, &io, sizeof(io), cfd) != 0)
 		goto err_out3;
 
 	/* Block until ctrl+c */

include/fuse_lowlevel.h

@@ -2043,15 +2043,20 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
  * @param se session object
  * @param io Custom io to use when retrieving/sending requests/responses
  * @param fd file descriptor for the session
- *
+ * @param custom_io_size the size of the struct the callers knows about.
+ *        The struct might be extended in higher libfuse versions and in order
+ *        to stay ABI compatible extended fields are zeroed - libuse needs to know
+ *        the size the caller knows about.
  * @return 0  on success
  * @return -EINVAL if `io`, `io->read` or `ìo->writev` are NULL
  * @return -EBADF  if `fd` was smaller than 0
  * @return -errno  if failed to allocate memory to store `io`
  *
  **/
 int fuse_session_custom_io(struct fuse_session *se,
-				   const struct fuse_custom_io *io, int fd);
+			   const struct fuse_custom_io *io,
+			   size_t custom_io_size,
+			   int fd);
 
 /**
  * Mount a FUSE file system.

lib/fuse_lowlevel.c

@@ -3117,8 +3117,17 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
 }
 
 int fuse_session_custom_io(struct fuse_session *se, const struct fuse_custom_io *io,
-			   int fd)
+			   size_t prog_custom_io_sz, int fd)
 {
+	size_t lib_custom_io_sz = sizeof(*se->io);
+
+	if (prog_custom_io_sz > lib_custom_io_sz) {
+		fuse_log(FUSE_LOG_ERR,
+			 "Provided 'fuse_custom_io_sz' (%zu) is larger than struct (%zu)\n",
+			 prog_custom_io_sz, lib_custom_io_sz);
+		return -EINVAL;
+	}
+
 	if (fd < 0) {
 		fuse_log(FUSE_LOG_ERR, "Invalid file descriptor value %d passed to "
 			"fuse_session_custom_io()\n", fd);
@@ -3138,15 +3147,20 @@ int fuse_session_custom_io(struct fuse_session *se, const struct fuse_custom_io
 		return -EINVAL;
 	}
 
-	se->io = malloc(sizeof(struct fuse_custom_io));
+	se->io = calloc(1, lib_custom_io_sz);
 	if (se->io == NULL) {
 		fuse_log(FUSE_LOG_ERR, "Failed to allocate memory for custom io. "
 			"Error: %s\n", strerror(errno));
 		return -errno;
 	}
 
 	se->fd = fd;
-	*se->io = *io;
+
+	/* Above the struct was zeroed. For ABI compatibility we memcopy in
+	 * what the caller knows about.
+	 */
+	memcpy(se->io, io, prog_custom_io_sz);
+
 	return 0;
 }