[PATCH 1/4] exportfs: add flag to indicate local file handles

[Christian Brauner <brauner@kernel.org>]

Some filesystems like kernfs and pidfs support file handles as a
convenience to use name_to_handle_at(2) and open_by_handle_at(2) but
don't want to and cannot be reliably exported. Add a flag that allows
them to mark their export operations accordingly.

Fixes: aa8188253474 ("kernfs: add exportfs operations")
Cc: stable <stable@kernel.org> # >= 4.14
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nfsd/export.c         | 8 +++++++-
 include/linux/exportfs.h | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index eacafe46e3b673cb306bd3c7caabd3283a1e54b1..786551595cc1c2043e8c195c00ca72ef93c769d6 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -417,6 +417,7 @@ static struct svc_export *svc_export_lookup(struct svc_export *);
 static int check_export(struct path *path, int *flags, unsigned char *uuid)
 {
 	struct inode *inode = d_inode(path->dentry);
+	const struct export_operations *nop;
 
 	/*
 	 * We currently export only dirs, regular files, and (for v4
@@ -449,11 +450,16 @@ static int check_export(struct path *path, int *flags, unsigned char *uuid)
 		return -EINVAL;
 	}
 
-	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
+	if (!exportfs_can_decode_fh(nop)) {
 		dprintk("exp_export: export of invalid fs type.\n");
 		return -EINVAL;
 	}
 
+	if (nop && nop->flags & EXPORT_OP_LOCAL_FILE_HANDLE) {
+		dprintk("exp_export: filesystem only supports non-exportable file handles.\n");
+		return -EINVAL;
+	}
+
 	if (is_idmapped_mnt(path->mnt)) {
 		dprintk("exp_export: export of idmapped mounts not yet supported.\n");
 		return -EINVAL;
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index a087606ace194ccc9d1250f990ce55627aaf8dc5..40f78c8e4f31b97b2101b66634e8d1807c1bcc14 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -280,6 +280,7 @@ struct export_operations {
 						*/
 #define EXPORT_OP_FLUSH_ON_CLOSE	(0x20) /* fs flushes file data on close */
 #define EXPORT_OP_ASYNC_LOCK		(0x40) /* fs can do async lock request */
+#define EXPORT_OP_LOCAL_FILE_HANDLE		(0x80) /* fs only supports file handles, no proper export */
 	unsigned long	flags;
 };
 

-- 
2.45.2