ovl: add support for "nfs_export" configuration

Introduce the "nfs_export" config, module and mount options.

The NFS export feature depends on the "index" feature and enables two
implicit overlayfs features: "index_all" and "verify_lower".
The "index_all" feature creates an index on copy up of every file and
directory. The "verify_lower" feature uses the full index to detect
overlay filesystems inconsistencies on lookup, like redirect from
multiple upper dirs to the same lower dir.

NFS export can be enabled for non-upper mount with no index. However,
because lower layer redirects cannot be verified with the index, enabling
NFS export support on an overlay with no upper layer requires turning off
redirect follow (e.g. "redirect_dir=nofollow").

The full index may incur some overhead on mount time, especially when
verifying that lower directory file handles are not stale.

NFS export support, full index and consistency verification will be
implemented by following patches.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 06119f34a69d..ae81d878248e 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -63,6 +63,22 @@ struct dentry *ovl_indexdir(struct super_block *sb)
 	return ofs->indexdir;
 }
 
+/* Index all files on copy up. For now only enabled for NFS export */
+bool ovl_index_all(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->config.nfs_export && ofs->config.index;
+}
+
+/* Verify lower origin on lookup. For now only enabled for NFS export */
+bool ovl_verify_lower(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->config.nfs_export && ofs->config.index;
+}
+
 struct ovl_entry *ovl_alloc_entry(unsigned int numlower)
 {
 	size_t size = offsetof(struct ovl_entry, lowerstack[numlower]);