fuse: Allow exporting BDSs via FUSE

block-export-add type=fuse allows mounting block graph nodes via FUSE on
some existing regular file.  That file should then appears like a raw
disk image, and accesses to it result in accesses to the exported BDS.

Right now, we only implement the necessary block export functions to set
it up and shut it down.  We do not implement any access functions, so
accessing the mount point only results in errors.  This will be
addressed by a followup patch.

We keep a hash table of exported mount points, because we want to be
able to detect when users try to use a mount point twice.  This is
because we invoke stat() to check whether the given mount point is a
regular file, but if that file is served by ourselves (because it is
already used as a mount point), then this stat() would have to be served
by ourselves, too, which is impossible to do while we (as the caller)
are waiting for it to settle.  Therefore, keep track of mount point
paths to at least catch the most obvious instances of that problem.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201027190600.192171-3-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/include/block/fuse.h b/include/block/fuse.h
new file mode 100644
index 0000000000..ffa91fe364
--- /dev/null
+++ b/include/block/fuse.h
@@ -0,0 +1,30 @@
+/*
+ * Present a block device as a raw image through FUSE
+ *
+ * Copyright (c) 2020 Max Reitz <mreitz@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; under version 2 or later of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef BLOCK_FUSE_H
+#define BLOCK_FUSE_H
+
+#ifdef CONFIG_FUSE
+
+#include "block/export.h"
+
+extern const BlockExportDriver blk_exp_fuse;
+
+#endif /* CONFIG_FUSE */
+
+#endif