[PATCH] exportfs: require ->fh_to_parent() to encode connectable file handles

[Amir Goldstein <amir73il@gmail.com>]

When user requests a connectable file handle explicitly with the
AT_HANDLE_CONNECTABLE flag, fail the request if filesystem (e.g. nfs)
does not know how to decode a connected non-dir dentry.

Fixes: c374196b2b9f ("fs: name_to_handle_at() support for "explicit connectable" file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 include/linux/exportfs.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

Christian,

This fixes fstest failures with nfs client (re-export)
reported by Zoro [1].

Thanks,
Amir.

[1] https://lore.kernel.org/fstests/20250525053604.k466kgfcumw2s2qx@dell-per750-06-vm-08.rhts.eng.pek2.redhat.com/

diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index fc93f0abf513..25c4a5afbd44 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -314,6 +314,9 @@ static inline bool exportfs_can_decode_fh(const struct export_operations *nop)
 static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
 					  int fh_flags)
 {
+	if (!nop)
+		return false;
+
 	/*
 	 * If a non-decodeable file handle was requested, we only need to make
 	 * sure that filesystem did not opt-out of encoding fid.
@@ -321,6 +324,13 @@ static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
 	if (fh_flags & EXPORT_FH_FID)
 		return exportfs_can_encode_fid(nop);
 
+	/*
+	 * If a connectable file handle was requested, we need to make sure that
+	 * filesystem can also decode connected file handles.
+	 */
+	if ((fh_flags & EXPORT_FH_CONNECTABLE) && !nop->fh_to_parent)
+		return false;
+
 	/*
 	 * If a decodeable file handle was requested, we need to make sure that
 	 * filesystem can also decode file handles.
-- 
2.34.1