ovl: refactor ovl_fill_super()

Split the core into a separate helper in preparation of converting the
caller to the scoped ovl cred guard.

Link: https://patch.msgid.link/20251117-work-ovl-cred-guard-v4-40-b31603935724@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/overlayfs/super.c

@@ -1364,53 +1364,35 @@ static void ovl_set_d_op(struct super_block *sb)
 	set_default_d_op(sb, &ovl_dentry_operations);
 }
 
-int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
+static int ovl_fill_super_creds(struct fs_context *fc, struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
+	struct cred *creator_cred = (struct cred *)ofs->creator_cred;
 	struct ovl_fs_context *ctx = fc->fs_private;
-	const struct cred *old_cred = NULL;
-	struct dentry *root_dentry;
-	struct ovl_entry *oe;
 	struct ovl_layer *layers;
-	struct cred *cred;
+	struct ovl_entry *oe = NULL;
 	int err;
 
-	err = -EIO;
-	if (WARN_ON(fc->user_ns != current_user_ns()))
-		goto out_err;
-
-	ovl_set_d_op(sb);
-
-	err = -ENOMEM;
-	if (!ofs->creator_cred)
-		ofs->creator_cred = cred = prepare_creds();
-	else
-		cred = (struct cred *)ofs->creator_cred;
-	if (!cred)
-		goto out_err;
-
-	old_cred = ovl_override_creds(sb);
-
 	err = ovl_fs_params_verify(ctx, &ofs->config);
 	if (err)
-		goto out_err;
+		return err;
 
 	err = -EINVAL;
 	if (ctx->nr == 0) {
 		if (!(fc->sb_flags & SB_SILENT))
 			pr_err("missing 'lowerdir'\n");
-		goto out_err;
+		return err;
 	}
 
 	err = -ENOMEM;
 	layers = kcalloc(ctx->nr + 1, sizeof(struct ovl_layer), GFP_KERNEL);
 	if (!layers)
-		goto out_err;
+		return err;
 
 	ofs->config.lowerdirs = kcalloc(ctx->nr + 1, sizeof(char *), GFP_KERNEL);
 	if (!ofs->config.lowerdirs) {
 		kfree(layers);
-		goto out_err;
+		return err;
 	}
 	ofs->layers = layers;
 	/*
@@ -1443,26 +1425,26 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 		err = -EINVAL;
 		if (!ofs->config.workdir) {
 			pr_err("missing 'workdir'\n");
-			goto out_err;
+			return err;
 		}
 
 		err = ovl_get_upper(sb, ofs, &layers[0], &ctx->upper);
 		if (err)
-			goto out_err;
+			return err;
 
 		upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
 		if (!ovl_should_sync(ofs)) {
 			ofs->errseq = errseq_sample(&upper_sb->s_wb_err);
 			if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) {
 				err = -EIO;
 				pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n");
-				goto out_err;
+				return err;
 			}
 		}
 
 		err = ovl_get_workdir(sb, ofs, &ctx->upper, &ctx->work);
 		if (err)
-			goto out_err;
+			return err;
 
 		if (!ofs->workdir)
 			sb->s_flags |= SB_RDONLY;
@@ -1473,7 +1455,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 	oe = ovl_get_lowerstack(sb, ctx, ofs, layers);
 	err = PTR_ERR(oe);
 	if (IS_ERR(oe))
-		goto out_err;
+		return err;
 
 	/* If the upper fs is nonexistent, we mark overlayfs r/o too */
 	if (!ovl_upper_mnt(ofs))
@@ -1526,7 +1508,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 		sb->s_export_op = &ovl_export_fid_operations;
 
 	/* Never override disk quota limits or use reserved space */
-	cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
+	cap_lower(creator_cred->cap_effective, CAP_SYS_RESOURCE);
 
 	sb->s_magic = OVERLAYFS_SUPER_MAGIC;
 	sb->s_xattr = ovl_xattr_handlers(ofs);
@@ -1544,27 +1526,50 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_iflags |= SB_I_EVM_HMAC_UNSUPPORTED;
 
 	err = -ENOMEM;
-	root_dentry = ovl_get_root(sb, ctx->upper.dentry, oe);
-	if (!root_dentry)
+	sb->s_root = ovl_get_root(sb, ctx->upper.dentry, oe);
+	if (!sb->s_root)
 		goto out_free_oe;
 
-	sb->s_root = root_dentry;
-
-	ovl_revert_creds(old_cred);
 	return 0;
 
 out_free_oe:
 	ovl_free_entry(oe);
+	return err;
+}
+
+int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+	const struct cred *old_cred = NULL;
+	struct cred *cred;
+	int err;
+
+	err = -EIO;
+	if (WARN_ON(fc->user_ns != current_user_ns()))
+		goto out_err;
+
+	ovl_set_d_op(sb);
+
+	err = -ENOMEM;
+	if (!ofs->creator_cred)
+		ofs->creator_cred = cred = prepare_creds();
+	else
+		cred = (struct cred *)ofs->creator_cred;
+	if (!cred)
+		goto out_err;
+
+	old_cred = ovl_override_creds(sb);
+
+	err = ovl_fill_super_creds(fc, sb);
+
+	ovl_revert_creds(old_cred);
+
 out_err:
-	/*
-	 * Revert creds before calling ovl_free_fs() which will call
-	 * put_cred() and put_cred() requires that the cred's that are
-	 * put are not the caller's creds, i.e., current->cred.
-	 */
-	if (old_cred)
-		ovl_revert_creds(old_cred);
-	ovl_free_fs(ofs);
-	sb->s_fs_info = NULL;
+	if (err) {
+		ovl_free_fs(ofs);
+		sb->s_fs_info = NULL;
+	}
+
 	return err;
 }