binder: Add default binder devices through binderfs when configured

Currently, since each binderfs instance needs its own
private binder devices, every time a binderfs instance is
mounted, all the default binder devices need to be created
via the BINDER_CTL_ADD IOCTL. This patch aims to
add a solution to automatically create the default binder
devices for each binderfs instance that gets mounted.
To achieve this goal, when CONFIG_ANDROID_BINDERFS is set,
the default binder devices specified by CONFIG_ANDROID_BINDER_DEVICES
are created in each binderfs instance instead of global devices
being created by the binder driver.

Co-developed-by: Christian Brauner <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Hridya Valsaraju <[email protected]>
Reviewed-by: Joel Fernandes (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Hridya Valsaraju

drivers/android/binder.c

@@ -122,7 +122,7 @@ static uint32_t binder_debug_mask = BINDER_DEBUG_USER_ERROR |
 	BINDER_DEBUG_FAILED_TRANSACTION | BINDER_DEBUG_DEAD_TRANSACTION;
 module_param_named(debug_mask, binder_debug_mask, uint, 0644);
 
-static char *binder_devices_param = CONFIG_ANDROID_BINDER_DEVICES;
+char *binder_devices_param = CONFIG_ANDROID_BINDER_DEVICES;
 module_param_named(devices, binder_devices_param, charp, 0444);
 
 static DECLARE_WAIT_QUEUE_HEAD(binder_user_error_wait);
@@ -6131,7 +6131,8 @@ static int __init binder_init(void)
 				    &transaction_log_fops);
 	}
 
-	if (strcmp(binder_devices_param, "") != 0) {
+	if (!IS_ENABLED(CONFIG_ANDROID_BINDERFS) &&
+	    strcmp(binder_devices_param, "") != 0) {
 		/*
 		* Copy the module_parameter string, because we don't want to
 		* tokenize it in-place.

drivers/android/binder_internal.h

@@ -37,6 +37,8 @@ struct binder_device {
 
 extern const struct file_operations binder_fops;
 
+extern char *binder_devices_param;
+
 #ifdef CONFIG_ANDROID_BINDERFS
 extern bool is_binderfs_device(const struct inode *inode);
 #else

drivers/android/binderfs.c

@@ -186,8 +186,7 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
 	req->major = MAJOR(binderfs_dev);
 	req->minor = minor;
 
-	ret = copy_to_user(userp, req, sizeof(*req));
-	if (ret) {
+	if (userp && copy_to_user(userp, req, sizeof(*req))) {
 		ret = -EFAULT;
 		goto err;
 	}
@@ -467,6 +466,9 @@ static int binderfs_fill_super(struct super_block *sb, void *data, int silent)
 	int ret;
 	struct binderfs_info *info;
 	struct inode *inode = NULL;
+	struct binderfs_device device_info = { 0 };
+	const char *name;
+	size_t len;
 
 	sb->s_blocksize = PAGE_SIZE;
 	sb->s_blocksize_bits = PAGE_SHIFT;
@@ -521,7 +523,22 @@ static int binderfs_fill_super(struct super_block *sb, void *data, int silent)
 	if (!sb->s_root)
 		return -ENOMEM;
 
-	return binderfs_binder_ctl_create(sb);
+	ret = binderfs_binder_ctl_create(sb);
+	if (ret)
+		return ret;
+
+	name = binder_devices_param;
+	for (len = strcspn(name, ","); len > 0; len = strcspn(name, ",")) {
+		strscpy(device_info.name, name, len + 1);
+		ret = binderfs_binder_device_create(inode, NULL, &device_info);
+		if (ret)
+			return ret;
+		name += len;
+		if (*name == ',')
+			name++;
+	}
+
+	return 0;
 }
 
 static struct dentry *binderfs_mount(struct file_system_type *fs_type,