ovl: untangle copy up call chain

In an attempt to dedup ~100 LOC, we ended up creating a tangled call chain,
whose branches merge and diverge in several points according to the
immutable c->tmpfile copy up mode.

This call chain was hard to analyse for locking correctness because the
locking requirements for the c->tmpfile flow were very different from the
locking requirements for the !c->tmpfile flow (i.e. directory vs.  regulare
file copy up).

Split the copy up helpers of the c->tmpfile flow from those of the
!c->tmpfile (i.e. workdir) flow and remove the c->tmpfile mode from copy up
context.
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/copy_up.c

@@ -395,7 +395,6 @@ struct ovl_copy_up_ctx {
 	struct dentry *destdir;
 	struct qstr destname;
 	struct dentry *workdir;
-	bool tmpfile;
 	bool origin;
 	bool indexed;
 	bool metacopy;
@@ -440,62 +439,6 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
 	return err;
 }
 
-static int ovl_install_temp(struct ovl_copy_up_ctx *c, struct dentry *temp,
-			    struct dentry **newdentry)
-{
-	int err;
-	struct dentry *upper;
-	struct inode *udir = d_inode(c->destdir);
-
-	upper = lookup_one_len(c->destname.name, c->destdir, c->destname.len);
-	if (IS_ERR(upper))
-		return PTR_ERR(upper);
-
-	if (c->tmpfile)
-		err = ovl_do_link(temp, udir, upper);
-	else
-		err = ovl_do_rename(d_inode(c->workdir), temp, udir, upper, 0);
-
-	if (!err)
-		*newdentry = dget(c->tmpfile ? upper : temp);
-	dput(upper);
-
-	return err;
-}
-
-static struct dentry *ovl_get_tmpfile(struct ovl_copy_up_ctx *c)
-{
-	int err;
-	struct dentry *temp;
-	const struct cred *old_creds = NULL;
-	struct cred *new_creds = NULL;
-	struct ovl_cattr cattr = {
-		/* Can't properly set mode on creation because of the umask */
-		.mode = c->stat.mode & S_IFMT,
-		.rdev = c->stat.rdev,
-		.link = c->link
-	};
-
-	err = security_inode_copy_up(c->dentry, &new_creds);
-	if (err < 0)
-		return ERR_PTR(err);
-
-	if (new_creds)
-		old_creds = override_creds(new_creds);
-
-	if (c->tmpfile)
-		temp = ovl_do_tmpfile(c->workdir, c->stat.mode);
-	else
-		temp = ovl_create_temp(c->workdir, &cattr);
-
-	if (new_creds) {
-		revert_creds(old_creds);
-		put_cred(new_creds);
-	}
-
-	return temp;
-}
-
 static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 {
 	int err;
@@ -547,37 +490,94 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 	return err;
 }
 
-static int ovl_copy_up_locked(struct ovl_copy_up_ctx *c)
+static struct dentry *ovl_get_workdir_temp(struct ovl_copy_up_ctx *c)
+{
+	int err;
+	struct dentry *temp;
+	const struct cred *old_creds = NULL;
+	struct cred *new_creds = NULL;
+	struct ovl_cattr cattr = {
+		/* Can't properly set mode on creation because of the umask */
+		.mode = c->stat.mode & S_IFMT,
+		.rdev = c->stat.rdev,
+		.link = c->link
+	};
+
+	err = security_inode_copy_up(c->dentry, &new_creds);
+	if (err < 0)
+		return ERR_PTR(err);
+
+	if (new_creds)
+		old_creds = override_creds(new_creds);
+
+	temp = ovl_create_temp(c->workdir, &cattr);
+
+	if (new_creds) {
+		revert_creds(old_creds);
+		put_cred(new_creds);
+	}
+
+	return temp;
+}
+
+/*
+ * Move temp file from workdir into place on upper dir.
+ * Used when copying up directories and when upper fs doesn't support O_TMPFILE.
+ *
+ * Caller must hold ovl_lock_rename_workdir().
+ */
+static int ovl_rename_temp(struct ovl_copy_up_ctx *c, struct dentry *temp,
+			   struct dentry **newdentry)
+{
+	int err;
+	struct dentry *upper;
+	struct inode *udir = d_inode(c->destdir);
+
+	upper = lookup_one_len(c->destname.name, c->destdir, c->destname.len);
+	if (IS_ERR(upper))
+		return PTR_ERR(upper);
+
+	err = ovl_do_rename(d_inode(c->workdir), temp, udir, upper, 0);
+	if (!err)
+		*newdentry = dget(temp);
+	dput(upper);
+
+	return err;
+}
+
+/*
+ * Copyup using workdir to prepare temp file.  Used when copying up directories,
+ * special files or when upper fs doesn't support O_TMPFILE.
+ */
+static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
 {
-	struct inode *udir = c->destdir->d_inode;
 	struct inode *inode;
 	struct dentry *newdentry = NULL;
 	struct dentry *temp;
 	int err;
 
-	temp = ovl_get_tmpfile(c);
+	err = ovl_lock_rename_workdir(c->workdir, c->destdir);
+	if (err)
+		return err;
+
+	temp = ovl_get_workdir_temp(c);
+	err = PTR_ERR(temp);
 	if (IS_ERR(temp))
-		return PTR_ERR(temp);
+		goto unlock;
 
 	err = ovl_copy_up_inode(c, temp);
 	if (err)
-		goto out;
+		goto cleanup;
 
 	if (S_ISDIR(c->stat.mode) && c->indexed) {
 		err = ovl_create_index(c->dentry, c->lowerpath.dentry, temp);
 		if (err)
-			goto out;
+			goto cleanup;
 	}
 
-	if (c->tmpfile) {
-		inode_lock_nested(udir, I_MUTEX_PARENT);
-		err = ovl_install_temp(c, temp, &newdentry);
-		inode_unlock(udir);
-	} else {
-		err = ovl_install_temp(c, temp, &newdentry);
-	}
+	err = ovl_rename_temp(c, temp, &newdentry);
 	if (err)
-		goto out;
+		goto cleanup;
 
 	if (!c->metacopy)
 		ovl_set_upperdata(d_inode(c->dentry));
@@ -585,13 +585,94 @@ static int ovl_copy_up_locked(struct ovl_copy_up_ctx *c)
 	ovl_inode_update(inode, newdentry);
 	if (S_ISDIR(inode->i_mode))
 		ovl_set_flag(OVL_WHITEOUTS, inode);
+out_dput:
+	dput(temp);
+unlock:
+	unlock_rename(c->workdir, c->destdir);
+
+	return err;
+
+cleanup:
+	ovl_cleanup(d_inode(c->workdir), temp);
+	goto out_dput;
+}
+
+static struct dentry *ovl_get_tmpfile(struct ovl_copy_up_ctx *c)
+{
+	int err;
+	struct dentry *temp;
+	const struct cred *old_creds = NULL;
+	struct cred *new_creds = NULL;
+
+	err = security_inode_copy_up(c->dentry, &new_creds);
+	if (err < 0)
+		return ERR_PTR(err);
+
+	if (new_creds)
+		old_creds = override_creds(new_creds);
+
+	temp = ovl_do_tmpfile(c->workdir, c->stat.mode);
+
+	if (new_creds) {
+		revert_creds(old_creds);
+		put_cred(new_creds);
+	}
+
+	return temp;
+}
+
+/* Link O_TMPFILE into place on upper dir */
+static int ovl_link_tmpfile(struct ovl_copy_up_ctx *c, struct dentry *temp,
+			    struct dentry **newdentry)
+{
+	int err;
+	struct dentry *upper;
+	struct inode *udir = d_inode(c->destdir);
+
+	inode_lock_nested(udir, I_MUTEX_PARENT);
+
+	upper = lookup_one_len(c->destname.name, c->destdir, c->destname.len);
+	err = PTR_ERR(upper);
+	if (IS_ERR(upper))
+		goto unlock;
+
+	err = ovl_do_link(temp, udir, upper);
+	if (!err)
+		*newdentry = dget(upper);
+	dput(upper);
+
+unlock:
+	inode_unlock(udir);
+
+	return err;
+}
+
+/* Copyup using O_TMPFILE which does not require cross dir locking */
+static int ovl_copy_up_tmpfile(struct ovl_copy_up_ctx *c)
+{
+	struct dentry *newdentry = NULL;
+	struct dentry *temp;
+	int err;
+
+	temp = ovl_get_tmpfile(c);
+	if (IS_ERR(temp))
+		return PTR_ERR(temp);
+
+	err = ovl_copy_up_inode(c, temp);
+	if (err)
+		goto out;
+
+	err = ovl_link_tmpfile(c, temp, &newdentry);
+	if (err)
+		goto out;
+
+	if (!c->metacopy)
+		ovl_set_upperdata(d_inode(c->dentry));
+	ovl_inode_update(d_inode(c->dentry), newdentry);
 
 out:
-	if (err && !c->tmpfile)
-		ovl_cleanup(d_inode(c->workdir), temp);
 	dput(temp);
 	return err;
-
 }
 
 /*
@@ -645,18 +726,10 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 	}
 
 	/* Should we copyup with O_TMPFILE or with workdir? */
-	if (S_ISREG(c->stat.mode) && ofs->tmpfile) {
-		c->tmpfile = true;
-		err = ovl_copy_up_locked(c);
-	} else {
-		err = ovl_lock_rename_workdir(c->workdir, c->destdir);
-		if (!err) {
-			err = ovl_copy_up_locked(c);
-			unlock_rename(c->workdir, c->destdir);
-		}
-	}
-
-
+	if (S_ISREG(c->stat.mode) && ofs->tmpfile)
+		err = ovl_copy_up_tmpfile(c);
+	else
+		err = ovl_copy_up_workdir(c);
 	if (err)
 		goto out;