[PATCH v4 1/2] exportfs: clarify the documentation of open()/permission() expotrfs ops

[Amir Goldstein <amir73il@gmail.com>]

pidfs and nsfs recently gained support for encode/decode of file handles
via name_to_handle_at(2)/open_by_handle_at(2).

These special kernel filesystems have custom ->open() and ->permission()
export methods, which nfsd does not respect and it was never meant to be
used for exporting those filesystems by nfsd.

Update kernel-doc comments to express the fact the those methods are for
open_by_handle(2) system only and not compatible with nfsd.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 include/linux/exportfs.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 262e24d833134..0660953c3fb76 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -200,6 +200,10 @@ struct handle_to_path_ctx {
  * @get_parent:     find the parent of a given directory
  * @commit_metadata: commit metadata changes to stable storage
  *
+ * Methods for open_by_handle(2) syscall with special kernel file systems:
+ * @permission:     custom permission for opening a file by handle
+ * @open:           custom open routine for opening file by handle
+ *
  * See Documentation/filesystems/nfs/exporting.rst for details on how to use
  * this interface correctly and the definition of the flags.
  *
@@ -244,10 +248,14 @@ struct handle_to_path_ctx {
  *    space cannot be allocated, a %ERR_PTR should be returned.
  *
  * @permission:
- *    Allow filesystems to specify a custom permission function.
+ *    Allow filesystems to specify a custom permission function for the
+ *    open_by_handle_at(2) syscall instead of the default permission check.
+ *    This custom permission function is not respected by nfsd.
  *
  * @open:
- *    Allow filesystems to specify a custom open function.
+ *    Allow filesystems to specify a custom open function for the
+ *    open_by_handle_at(2) syscall instead of the default file_open_root().
+ *    This custom open function is not respected by nfsd.
  *
  * @commit_metadata:
  *    @commit_metadata should commit metadata changes to stable storage.
-- 
2.52.0