ovl: restrict lower null uuid for "xino=auto"

Commit a888db310195 ("ovl: fix regression with re-formatted lower
squashfs") attempted to fix a regression with existing setups that
use a practice that we are trying to discourage.

The discourage part was described this way in the commit message:
"To avoid the reported regression while still allowing the new features
 with single lower squashfs, do not allow decoding origin with lower null
 uuid unless user opted-in to one of the new features that require
 following the lower inode of non-dir upper (index, xino, metacopy)."

The three mentioned features are disabled by default in Kconfig, so
it was assumed that if they are enabled, the user opted-in for them.
Apparently, distros started to configure CONFIG_OVERLAY_FS_XINO_AUTO=y
some time ago, so users upgrading their kernels can still be affected
by said regression even though they never opted-in for any new feature.

To fix this, treat "xino=on" as "user opted-in", but not "xino=auto".
Since we are changing the behavior of "xino=auto" to no longer follow
to lower origin with null uuid, take this one step further and disable
xino in that corner case.  To be consistent, disable xino also in cases
of lower fs without file handle support and upper fs without xattr
support.

Update documentation w.r.t the new "xino=auto" behavior and fix the out
dated bits of documentation regarding "xino" and regarding offline
modifications to lower layers.

Link: https://lore.kernel.org/linux-unionfs/b36a429d7c563730c28d763d4d57a6fc30508a4f.1615216996.git.kevin@kevinlocke.name/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 35 insertions, 6 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 8d8366350093..5fcaf3acc350 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -945,6 +945,16 @@ static int ovl_lower_dir(const char *name, struct path *path,
 		pr_warn("fs on '%s' does not support file handles, falling back to index=off,nfs_export=off.\n",
 			name);
 	}
+	/*
+	 * Decoding origin file handle is required for persistent st_ino.
+	 * Without persistent st_ino, xino=auto falls back to xino=off.
+	 */
+	if (ofs->config.xino == OVL_XINO_AUTO &&
+	    ofs->config.upperdir && !fh_type) {
+		ofs->config.xino = OVL_XINO_OFF;
+		pr_warn("fs on '%s' does not support file handles, falling back to xino=off.\n",
+			name);
+	}
 
 	/* Check if lower fs has 32bit inode numbers */
 	if (fh_type != FILEID_INO32_GEN)
@@ -1401,9 +1411,19 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 	err = ovl_do_setxattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE, "0", 1);
 	if (err) {
 		ofs->noxattr = true;
-		ofs->config.index = false;
-		ofs->config.metacopy = false;
-		pr_warn("upper fs does not support xattr, falling back to index=off and metacopy=off.\n");
+		if (ofs->config.index || ofs->config.metacopy) {
+			ofs->config.index = false;
+			ofs->config.metacopy = false;
+			pr_warn("upper fs does not support xattr, falling back to index=off,metacopy=off.\n");
+		}
+		/*
+		 * xattr support is required for persistent st_ino.
+		 * Without persistent st_ino, xino=auto falls back to xino=off.
+		 */
+		if (ofs->config.xino == OVL_XINO_AUTO) {
+			ofs->config.xino = OVL_XINO_OFF;
+			pr_warn("upper fs does not support xattr, falling back to xino=off.\n");
+		}
 		err = 0;
 	} else {
 		ovl_do_removexattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE);
@@ -1580,7 +1600,8 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
 	 * user opted-in to one of the new features that require following the
 	 * lower inode of non-dir upper.
 	 */
-	if (!ofs->config.index && !ofs->config.metacopy && !ofs->config.xino &&
+	if (!ofs->config.index && !ofs->config.metacopy &&
+	    ofs->config.xino != OVL_XINO_ON &&
 	    uuid_is_null(uuid))
 		return false;
 
@@ -1609,6 +1630,7 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 	dev_t dev;
 	int err;
 	bool bad_uuid = false;
+	bool warn = false;
 
 	for (i = 0; i < ofs->numfs; i++) {
 		if (ofs->fs[i].sb == sb)
@@ -1617,13 +1639,20 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 
 	if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
 		bad_uuid = true;
+		if (ofs->config.xino == OVL_XINO_AUTO) {
+			ofs->config.xino = OVL_XINO_OFF;
+			warn = true;
+		}
 		if (ofs->config.index || ofs->config.nfs_export) {
 			ofs->config.index = false;
 			ofs->config.nfs_export = false;
-			pr_warn("%s uuid detected in lower fs '%pd2', falling back to index=off,nfs_export=off.\n",
+			warn = true;
+		}
+		if (warn) {
+			pr_warn("%s uuid detected in lower fs '%pd2', falling back to xino=%s,index=off,nfs_export=off.\n",
 				uuid_is_null(&sb->s_uuid) ? "null" :
 							    "conflicting",
-				path->dentry);
+				path->dentry, ovl_xino_str[ofs->config.xino]);
 		}
 	}