ovl: Add an alternative type of whiteout

If a lowerdir contains a fifo with the overlay.whiteout xattr then it
is considered as a whiteout just like a traditional char(0,0)
whiteout.

The advantage of this kind of whiteout is that they can be escaped
using the standard overlay xattrs escaping mechanism. So, a fifo with
a overlay.overlay.whiteout xattr would be unescaped to an
overlay.whiteout fifo, which could then be consumed by another
overlayfs as a whiteout.

Overlayfs itself doesn't create whiteouts like this, but a userspace
mechanism could use this alternative mechanism to convert images
that may contain whiteouts to be used with overlayfs.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

fs/overlayfs/dir.c

@@ -477,7 +477,7 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 		goto out_unlock;
 
 	err = -ESTALE;
-	if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
+	if (d_is_negative(upper) || !ovl_upper_is_whiteout(ofs, upper))
 		goto out_dput;
 
 	newdentry = ovl_create_temp(ofs, workdir, cattr);
@@ -1219,7 +1219,7 @@ static int ovl_rename(struct mnt_idmap *idmap, struct inode *olddir,
 		}
 	} else {
 		if (!d_is_negative(newdentry)) {
-			if (!new_opaque || !ovl_is_whiteout(newdentry))
+			if (!new_opaque || !ovl_upper_is_whiteout(ofs, newdentry))
 				goto out_dput;
 		} else {
 			if (flags & RENAME_EXCHANGE)

fs/overlayfs/namei.c

@@ -251,7 +251,9 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 		err = -EREMOTE;
 		goto out_err;
 	}
-	if (ovl_is_whiteout(this)) {
+	path.dentry = this;
+	path.mnt = d->mnt;
+	if (ovl_path_is_whiteout(OVL_FS(d->sb), &path)) {
 		d->stop = d->opaque = true;
 		goto put_and_out;
 	}
@@ -264,8 +266,6 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 		goto put_and_out;
 	}
 
-	path.dentry = this;
-	path.mnt = d->mnt;
 	if (!d_can_lookup(this)) {
 		if (d->is_dir || !last_element) {
 			d->stop = true;
@@ -438,7 +438,7 @@ int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
 	else if (IS_ERR(origin))
 		return PTR_ERR(origin);
 
-	if (upperdentry && !ovl_is_whiteout(upperdentry) &&
+	if (upperdentry && !ovl_upper_is_whiteout(ofs, upperdentry) &&
 	    inode_wrong_type(d_inode(upperdentry), d_inode(origin)->i_mode))
 		goto invalid;
 
@@ -1383,7 +1383,11 @@ bool ovl_lower_positive(struct dentry *dentry)
 				break;
 			}
 		} else {
-			positive = !ovl_is_whiteout(this);
+			struct path path = {
+				.dentry = this,
+				.mnt = parentpath->layer->mnt,
+			};
+			positive = !ovl_path_is_whiteout(OVL_FS(dentry->d_sb), &path);
 			done = true;
 			dput(this);
 		}

fs/overlayfs/overlayfs.h

@@ -49,6 +49,7 @@ enum ovl_xattr {
 	OVL_XATTR_UUID,
 	OVL_XATTR_METACOPY,
 	OVL_XATTR_PROTATTR,
+        OVL_XATTR_WHITEOUT,
 };
 
 enum ovl_inode_flag {
@@ -469,16 +470,28 @@ void ovl_inode_update(struct inode *inode, struct dentry *upperdentry);
 void ovl_dir_modified(struct dentry *dentry, bool impurity);
 u64 ovl_inode_version_get(struct inode *inode);
 bool ovl_is_whiteout(struct dentry *dentry);
+bool ovl_path_is_whiteout(struct ovl_fs *ofs, const struct path *path);
 struct file *ovl_path_open(const struct path *path, int flags);
 int ovl_copy_up_start(struct dentry *dentry, int flags);
 void ovl_copy_up_end(struct dentry *dentry);
 bool ovl_already_copied_up(struct dentry *dentry, int flags);
 bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
 			      enum ovl_xattr ox);
 bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path);
+bool ovl_path_check_whiteout_xattr(struct ovl_fs *ofs, const struct path *path);
 bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
 			 const struct path *upperpath);
 
+static inline bool ovl_upper_is_whiteout(struct ovl_fs *ofs,
+					 struct dentry *upperdentry)
+{
+	struct path upperpath = {
+		.dentry = upperdentry,
+		.mnt = ovl_upper_mnt(ofs),
+	};
+	return ovl_path_is_whiteout(ofs, &upperpath);
+}
+
 static inline bool ovl_check_origin_xattr(struct ovl_fs *ofs,
 					  struct dentry *upperdentry)
 {

fs/overlayfs/readdir.c

@@ -163,7 +163,7 @@ static struct ovl_cache_entry *ovl_cache_entry_new(struct ovl_readdir_data *rdd,
 	p->is_upper = rdd->is_upper;
 	p->is_whiteout = false;
 
-	if (d_type == DT_CHR) {
+	if (d_type == DT_CHR || d_type == DT_FIFO) {
 		p->next_maybe_whiteout = rdd->first_maybe_whiteout;
 		rdd->first_maybe_whiteout = p;
 	}
@@ -280,7 +280,12 @@ static int ovl_check_whiteouts(const struct path *path, struct ovl_readdir_data
 			rdd->first_maybe_whiteout = p->next_maybe_whiteout;
 			dentry = lookup_one(mnt_idmap(path->mnt), p->name, dir, p->len);
 			if (!IS_ERR(dentry)) {
-				p->is_whiteout = ovl_is_whiteout(dentry);
+				struct path childpath = {
+					.dentry = dentry,
+					.mnt = path->mnt,
+				};
+				p->is_whiteout = ovl_path_is_whiteout(OVL_FS(rdd->dentry->d_sb),
+								      &childpath);
 				dput(dentry);
 			}
 		}

fs/overlayfs/super.c

@@ -579,7 +579,7 @@ static int ovl_check_rename_whiteout(struct ovl_fs *ofs)
 	if (IS_ERR(whiteout))
 		goto cleanup_temp;
 
-	err = ovl_is_whiteout(whiteout);
+	err = ovl_upper_is_whiteout(ofs, whiteout);
 
 	/* Best effort cleanup of whiteout and temp file */
 	if (err)

fs/overlayfs/util.c

@@ -430,6 +430,16 @@ bool ovl_dentry_is_whiteout(struct dentry *dentry)
 	return !dentry->d_inode && ovl_dentry_is_opaque(dentry);
 }
 
+/*
+ * Use this over ovl_is_whiteout for upper and lower files, as it also
+ * handles OVL_XATTR_WHITEOUT based whiteout files.
+ */
+bool ovl_path_is_whiteout(struct ovl_fs *ofs, const struct path *path)
+{
+	return ovl_is_whiteout(path->dentry) ||
+		ovl_path_check_whiteout_xattr(ofs, path);
+}
+
 void ovl_dentry_set_opaque(struct dentry *dentry)
 {
 	ovl_dentry_set_flag(OVL_E_OPAQUE, dentry);
@@ -676,6 +686,19 @@ bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path)
 	return false;
 }
 
+bool ovl_path_check_whiteout_xattr(struct ovl_fs *ofs, const struct path *path)
+{
+	struct dentry *dentry = path->dentry;
+	int res;
+
+	/* xattr based whiteouts must be fifo */
+	if (!d_is_special(dentry))
+		return false;
+
+	res = ovl_path_getxattr(ofs, path, OVL_XATTR_WHITEOUT, NULL, 0);
+	return res >= 0;
+}
+
 /*
  * Load persistent uuid from xattr into s_uuid if found, or store a new
  * random generated value in s_uuid and in xattr.
@@ -760,6 +783,7 @@ bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
 #define OVL_XATTR_UUID_POSTFIX		"uuid"
 #define OVL_XATTR_METACOPY_POSTFIX	"metacopy"
 #define OVL_XATTR_PROTATTR_POSTFIX	"protattr"
+#define OVL_XATTR_WHITEOUT_POSTFIX	"whiteout"
 
 #define OVL_XATTR_TAB_ENTRY(x) \
 	[x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
@@ -775,6 +799,7 @@ const char *const ovl_xattr_table[][2] = {
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_UUID),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_METACOPY),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
+	OVL_XATTR_TAB_ENTRY(OVL_XATTR_WHITEOUT),
 };
 
 int ovl_check_setxattr(struct ovl_fs *ofs, struct dentry *upperdentry,