ovl: Add versioned header for overlay.metacopy xattr

Historically overlay.metacopy was a zero-size xattr, and it's
existence marked a metacopy file. This change adds a versioned header
with a flag field, a length and a digest. The initial use-case of this
will be for validating a fs-verity digest, but the flags field could
also be used later for other new features.

ovl_check_metacopy_xattr() now returns the size of the xattr,
emulating a size of OVL_METACOPY_MIN_SIZE for empty xattrs to
distinguish it from the no-xattr case.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 34cc72f8fb6a..2e659b355f61 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -7,6 +7,7 @@
 #include <linux/kernel.h>
 #include <linux/uuid.h>
 #include <linux/fs.h>
+#include <linux/fsverity.h>
 #include <linux/namei.h>
 #include <linux/posix_acl.h>
 #include <linux/posix_acl_xattr.h>
@@ -132,6 +133,26 @@ struct ovl_fh {
 #define OVL_FH_FID_OFFSET	(OVL_FH_WIRE_OFFSET + \
 				 offsetof(struct ovl_fb, fid))
 
+/* On-disk format for "metacopy" xattr (if non-zero size) */
+struct ovl_metacopy {
+	u8 version;	/* 0 */
+	u8 len;         /* size of this header + used digest bytes */
+	u8 flags;
+	u8 digest_algo;	/* FS_VERITY_HASH_ALG_* constant, 0 for no digest */
+	u8 digest[FS_VERITY_MAX_DIGEST_SIZE];  /* Only the used part on disk */
+} __packed;
+
+#define OVL_METACOPY_MAX_SIZE (sizeof(struct ovl_metacopy))
+#define OVL_METACOPY_MIN_SIZE (OVL_METACOPY_MAX_SIZE - FS_VERITY_MAX_DIGEST_SIZE)
+#define OVL_METACOPY_INIT { 0, OVL_METACOPY_MIN_SIZE }
+
+static inline int ovl_metadata_digest_size(const struct ovl_metacopy *metacopy)
+{
+	if (metacopy->len < OVL_METACOPY_MIN_SIZE)
+		return 0;
+	return (int)metacopy->len - OVL_METACOPY_MIN_SIZE;
+}
+
 extern const char *const ovl_xattr_table[][2];
 static inline const char *ovl_xattr(struct ovl_fs *ofs, enum ovl_xattr ox)
 {
@@ -458,7 +479,8 @@ bool ovl_need_index(struct dentry *dentry);
 int ovl_nlink_start(struct dentry *dentry);
 void ovl_nlink_end(struct dentry *dentry);
 int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir);
-int ovl_check_metacopy_xattr(struct ovl_fs *ofs, const struct path *path);
+int ovl_check_metacopy_xattr(struct ovl_fs *ofs, const struct path *path,
+			     struct ovl_metacopy *data);
 bool ovl_is_metacopy_dentry(struct dentry *dentry);
 char *ovl_get_redirect_xattr(struct ovl_fs *ofs, const struct path *path, int padding);
 int ovl_sync_status(struct ovl_fs *ofs);