68fe2c76 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -427,7 +427,8 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 	 *       either a device number (so FS_REQUIRES_DEV needed)
 	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
 	 * 2:  We must be able to find an inode from a filehandle.
-	 *       This means that s_export_op must be set.
+	 *       This means that s_export_op must be set and comply with
+	 *       the requirements for remote filesystem export.
 	 * 3: We must not currently be on an idmapped mount.
 	 */
 	if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
@@ -437,8 +438,9 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 		return -EINVAL;
 	}
 
-	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
-		dprintk("exp_export: export of invalid fs type.\n");
+	if (!exportfs_may_export(inode->i_sb->s_export_op)) {
+		dprintk("exp_export: export of invalid fs type (%s).\n",
+			inode->i_sb->s_type->name);
 		return -EINVAL;
 	}
 
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index f0cf2714ec52d..9bd93d6bd9a49 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -317,6 +317,15 @@ static inline bool exportfs_can_decode_fh(const struct export_operations *nop)
 	return nop && nop->fh_to_dentry;
 }
 
+static inline bool exportfs_may_export(const struct export_operations *nop)
+{
+	/*
+	 * Do not allow nfs export for filesystems with custom ->open() or
+	 * ->permission() ops, which nfsd does not respect (e.g. pidfs, nsfs).
+	 */
+	return exportfs_can_decode_fh(nop) && !nop->open && !nop->permission;
+}
+
 static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
 					  int fh_flags)
 {
-- 
2.51.0[PATCH 6.19 598/781] nfsd: do not allow exporting of special kernel filesystemsGreg Kroah-Hartman <gregkh@linuxfoundation.org> undefinedstable@vger.kernel.org undefined undefined undefined undefined undefined undefined undefinedÐ