fuse: don't require /dev/fuse fd to be kept open during mount

With the new mount API the sequence of syscalls would be:

        fs_fd = fsopen("fuse", 0);
	snprintf(opt, sizeof(opt), "%i", devfd);
	fsconfig(fs_fd, FSCONFIG_SET_STRING, "fd", opt, 0);
	/* ... */
	fsconfig(fs_fd, FSCONFIG_CMD_CREATE, 0, 0, 0);

Current mount code just stores the value of devfd in the fs_context and
uses it in during FSCONFIG_CMD_CREATE, which is inelegant.

Instead grab a reference to the underlying fuse_dev, and use that during
the filesystem creation.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/fuse_i.h

@@ -606,13 +606,11 @@ static inline bool fuse_is_inode_dax_mode(enum fuse_dax_mode mode)
 }
 
 struct fuse_fs_context {
-	int fd;
+	struct fuse_dev *fud;
-	struct file *file;
 	unsigned int rootmode;
 	kuid_t user_id;
 	kgid_t group_id;
 	bool is_bdev:1;
-	bool fd_present:1;
 	bool rootmode_present:1;
 	bool user_id_present:1;
 	bool group_id_present:1;

fs/fuse/inode.c

@@ -803,6 +803,29 @@ static const struct fs_parameter_spec fuse_fs_parameters[] = {
 	{}
 };
 
+static int fuse_opt_fd(struct fs_context *fsc, int fd)
+{
+	struct fuse_fs_context *ctx = fsc->fs_private;
+	struct file *file __free(fput) = fget(fd);
+
+	if (!file)
+		return -EBADF;
+
+	if (file->f_op != &fuse_dev_operations)
+		return invalfc(fsc, "fd is not a fuse device");
+	/*
+	 * Require mount to happen from the same user namespace which
+	 * opened /dev/fuse to prevent potential attacks.
+	 */
+	if (file->f_cred->user_ns != fsc->user_ns)
+		return invalfc(fsc, "wrong user namespace for fuse device");
+
+	ctx->fud = file->private_data;
+	refcount_inc(&ctx->fud->ref);
+
+	return 0;
+}
+
 static int fuse_parse_param(struct fs_context *fsc, struct fs_parameter *param)
 {
 	struct fs_parse_result result;
@@ -842,9 +865,7 @@ static int fuse_parse_param(struct fs_context *fsc, struct fs_parameter *param)
 		return 0;
 
 	case OPT_FD:
-		ctx->fd = result.uint_32;
+		return fuse_opt_fd(fsc, result.uint_32);
-		ctx->fd_present = true;
-		break;
 
 	case OPT_ROOTMODE:
 		if (!fuse_valid_type(result.uint_32))
@@ -907,6 +928,8 @@ static void fuse_free_fsc(struct fs_context *fsc)
 	struct fuse_fs_context *ctx = fsc->fs_private;
 
 	if (ctx) {
+		if (ctx->fud)
+			fuse_dev_put(ctx->fud);
 		kfree(ctx->subtype);
 		kfree(ctx);
 	}
@@ -1849,7 +1872,7 @@ EXPORT_SYMBOL_GPL(fuse_init_fs_context_submount);
 
 int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 {
-	struct fuse_dev *fud = ctx->file ? fuse_file_to_fud(ctx->file) : NULL;
+	struct fuse_dev *fud = ctx->fud;
 	struct fuse_mount *fm = get_fuse_mount_super(sb);
 	struct fuse_conn *fc = fm->fc;
 	struct inode *root;
@@ -1950,18 +1973,10 @@ static int fuse_fill_super(struct super_block *sb, struct fs_context *fsc)
 	struct fuse_mount *fm;
 	int err;
 
-	if (!ctx->file || !ctx->rootmode_present ||
+	if (!ctx->fud || !ctx->rootmode_present ||
 	    !ctx->user_id_present || !ctx->group_id_present)
 		return -EINVAL;
 
-	/*
-	 * Require mount to happen from the same user namespace which
-	 * opened /dev/fuse to prevent potential attacks.
-	 */
-	if ((ctx->file->f_op != &fuse_dev_operations) ||
-	    (ctx->file->f_cred->user_ns != sb->s_user_ns))
-		return -EINVAL;
-
 	err = fuse_fill_super_common(sb, ctx);
 	if (err)
 		return err;
@@ -1982,14 +1997,14 @@ static int fuse_set_no_super(struct super_block *sb, struct fs_context *fsc)
 
 static int fuse_test_super(struct super_block *sb, struct fs_context *fsc)
 {
+	struct fuse_dev *fud = fsc->sget_key;
 
-	return fsc->sget_key == get_fuse_conn_super(sb);
+	return fuse_dev_fc_get(fud) == get_fuse_conn_super(sb);
 }
 
 static int fuse_get_tree(struct fs_context *fsc)
 {
 	struct fuse_fs_context *ctx = fsc->fs_private;
-	struct fuse_dev *fud;
 	struct fuse_conn *fc;
 	struct fuse_mount *fm;
 	struct super_block *sb;
@@ -2010,9 +2025,6 @@ static int fuse_get_tree(struct fs_context *fsc)
 
 	fsc->s_fs_info = fm;
 
-	if (ctx->fd_present)
-		ctx->file = fget(ctx->fd);
-
 	if (IS_ENABLED(CONFIG_BLOCK) && ctx->is_bdev) {
 		err = get_tree_bdev(fsc, fuse_fill_super);
 		goto out;
@@ -2022,16 +2034,15 @@ static int fuse_get_tree(struct fs_context *fsc)
 	 * (found by device name), normal fuse mounts can't
 	 */
 	err = -EINVAL;
-	if (!ctx->file)
+	if (!ctx->fud)
 		goto out;
 
 	/*
 	 * Allow creating a fuse mount with an already initialized fuse
 	 * connection
 	 */
-	fud = __fuse_get_dev(ctx->file);
+	if (fuse_dev_fc_get(ctx->fud)) {
-	if (ctx->file->f_op == &fuse_dev_operations && fud) {
+		fsc->sget_key = ctx->fud;
-		fsc->sget_key = fud->fc;
 		sb = sget_fc(fsc, fuse_test_super, fuse_set_no_super);
 		err = PTR_ERR_OR_ZERO(sb);
 		if (!IS_ERR(sb))
@@ -2042,8 +2053,6 @@ static int fuse_get_tree(struct fs_context *fsc)
 out:
 	if (fsc->s_fs_info)
 		fuse_mount_destroy(fm);
-	if (ctx->file)
-		fput(ctx->file);
 	return err;
 }