fuse: add support for explicit export disabling

open_by_handle_at(2) can fail with -ESTALE with a valid handle returned
by a previous name_to_handle_at(2) for evicted fuse inodes, which is
especially common when entry_valid_timeout is 0, e.g. when the fuse
daemon is in "cache=none" mode.

The time sequence is like:

	name_to_handle_at(2)	# succeed
	evict fuse inode
	open_by_handle_at(2)	# fail

The root cause is that, with 0 entry_valid_timeout, the dput() called in
name_to_handle_at(2) will trigger iput -> evict(), which will send
FUSE_FORGET to the daemon.  The following open_by_handle_at(2) will send
a new FUSE_LOOKUP request upon inode cache miss since the previous inode
eviction.  Then the fuse daemon may fail the FUSE_LOOKUP request with
-ENOENT as the cached metadata of the requested inode has already been
cleaned up during the previous FUSE_FORGET.  The returned -ENOENT is
treated as -ESTALE when open_by_handle_at(2) returns.

This confuses the application somehow, as open_by_handle_at(2) fails
when the previous name_to_handle_at(2) succeeds.  The returned errno is
also confusing as the requested file is not deleted and already there.
It is reasonable to fail name_to_handle_at(2) early in this case, after
which the application can fallback to open(2) to access files.

Since this issue typically appears when entry_valid_timeout is 0 which
is configured by the fuse daemon, the fuse daemon is the right person to
explicitly disable the export when required.

Also considering FUSE_EXPORT_SUPPORT actually indicates the support for
lookups of "." and "..", and there are existing fuse daemons supporting
export without FUSE_EXPORT_SUPPORT set, for compatibility, we add a new
INIT flag for such purpose.
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/inode.c

@@ -1124,6 +1124,11 @@ static struct dentry *fuse_get_parent(struct dentry *child)
 	return parent;
 }
 
+/* only for fid encoding; no support for file handle */
+static const struct export_operations fuse_export_fid_operations = {
+	.encode_fh	= fuse_encode_fh,
+};
+
 static const struct export_operations fuse_export_operations = {
 	.fh_to_dentry	= fuse_fh_to_dentry,
 	.fh_to_parent	= fuse_fh_to_parent,
@@ -1316,6 +1321,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 				fc->max_stack_depth = arg->max_stack_depth;
 				fm->sb->s_stack_depth = arg->max_stack_depth;
 			}
+			if (flags & FUSE_NO_EXPORT_SUPPORT)
+				fm->sb->s_export_op = &fuse_export_fid_operations;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1362,7 +1369,8 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
 		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
-		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP;
+		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
+		FUSE_NO_EXPORT_SUPPORT;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
@@ -1561,6 +1569,7 @@ static int fuse_fill_super_submount(struct super_block *sb,
 	sb->s_bdi = bdi_get(parent_sb->s_bdi);
 
 	sb->s_xattr = parent_sb->s_xattr;
+	sb->s_export_op = parent_sb->s_export_op;
 	sb->s_time_gran = parent_sb->s_time_gran;
 	sb->s_blocksize = parent_sb->s_blocksize;
 	sb->s_blocksize_bits = parent_sb->s_blocksize_bits;

include/uapi/linux/fuse.h

@@ -215,6 +215,7 @@
  *  7.40
  *  - add max_stack_depth to fuse_init_out, add FUSE_PASSTHROUGH init flag
  *  - add backing_id to fuse_open_out, add FOPEN_PASSTHROUGH open flag
+ *  - add FUSE_NO_EXPORT_SUPPORT init flag
  */
 
 #ifndef _LINUX_FUSE_H
@@ -416,6 +417,7 @@ struct fuse_file_lock {
  *			symlink and mknod (single group that matches parent)
  * FUSE_HAS_EXPIRE_ONLY: kernel supports expiry-only entry invalidation
  * FUSE_DIRECT_IO_ALLOW_MMAP: allow shared mmap in FOPEN_DIRECT_IO mode.
+ * FUSE_NO_EXPORT_SUPPORT: explicitly disable export support
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -456,6 +458,7 @@ struct fuse_file_lock {
 #define FUSE_HAS_EXPIRE_ONLY	(1ULL << 35)
 #define FUSE_DIRECT_IO_ALLOW_MMAP (1ULL << 36)
 #define FUSE_PASSTHROUGH	(1ULL << 37)
+#define FUSE_NO_EXPORT_SUPPORT	(1ULL << 38)
 
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP