Add more documentation for FUSE_CAP_EXPORT_SUPPORT (#917)

Add more documentation for FUSE_CAP_EXPORT_SUPPORT

Also remove the flag from passthrough_ll.c and passthrough_hp.cc
as these implementations do _not_ handle that flag. They just
cast fuse_ino_t to an inode and cause a heap buffer overflow
for unknown objects (simplest reproducer are the examples
in "man 2 open_by_handle_at", but to unmount/mount the file
system after name_to_handle_at and before open_by_handle_at).

Fixes https://github.com/libfuse/libfuse/issues/838

---------

Co-authored-by: Nikolaus Rath <Nikolaus@rath.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index f052b67..6a060fd 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -183,6 +183,11 @@ struct fuse_loop_config_v1 {
 /**
  * Indicates that the filesystem supports lookups of "." and "..".
  *
+ * When this flag is set, the filesystem must be prepared to receive requests
+ * for invalid inodes (i.e., for which a FORGET request was received or
+ * which have been used in a previous instance of the filesystem daemon) and
+ * must not reuse node-ids (even when setting generation numbers).
+ *
  * This feature is disabled by default.
  */
 #define FUSE_CAP_EXPORT_SUPPORT		(1 << 4)