binderfs: add support for feature files

Provide userspace with a mechanism to discover features supported by
the binder driver to refrain from using any unsupported ones in the
first place. Starting with "oneway_spam_detection" only new features
are to be listed under binderfs and all previous ones are assumed to
be supported.

Assuming an instance of binderfs has been mounted at /dev/binderfs,
binder feature files can be found under /dev/binderfs/features/.
Usage example:

  $ mkdir /dev/binderfs
  $ mount -t binder binder /dev/binderfs
  $ cat /dev/binderfs/features/oneway_spam_detection
  1

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20210715031805.1725878-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binderfs.c

@@ -58,6 +58,10 @@ enum binderfs_stats_mode {
 	binderfs_stats_mode_global,
 };
 
+struct binder_features {
+	bool oneway_spam_detection;
+};
+
 static const struct constant_table binderfs_param_stats[] = {
 	{ "global", binderfs_stats_mode_global },
 	{}
@@ -69,6 +73,10 @@ static const struct fs_parameter_spec binderfs_fs_parameters[] = {
 	{}
 };
 
+static struct binder_features binder_features = {
+	.oneway_spam_detection = true,
+};
+
 static inline struct binderfs_info *BINDERFS_SB(const struct super_block *sb)
 {
 	return sb->s_fs_info;
@@ -583,6 +591,33 @@ out:
 	return dentry;
 }
 
+static int binder_features_show(struct seq_file *m, void *unused)
+{
+	bool *feature = m->private;
+
+	seq_printf(m, "%d\n", *feature);
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(binder_features);
+
+static int init_binder_features(struct super_block *sb)
+{
+	struct dentry *dentry, *dir;
+
+	dir = binderfs_create_dir(sb->s_root, "features");
+	if (IS_ERR(dir))
+		return PTR_ERR(dir);
+
+	dentry = binderfs_create_file(dir, "oneway_spam_detection",
+				      &binder_features_fops,
+				      &binder_features.oneway_spam_detection);
+	if (IS_ERR(dentry))
+		return PTR_ERR(dentry);
+
+	return 0;
+}
+
 static int init_binder_logs(struct super_block *sb)
 {
 	struct dentry *binder_logs_root_dir, *dentry, *proc_log_dir;
@@ -723,6 +758,10 @@ static int binderfs_fill_super(struct super_block *sb, struct fs_context *fc)
 			name++;
 	}
 
+	ret = init_binder_features(sb);
+	if (ret)
+		return ret;
+
 	if (info->mount_opts.stats_mode == binderfs_stats_mode_global)
 		return init_binder_logs(sb);