binderfs: reserve devices for initial mount

The binderfs instance in the initial ipc namespace will always have a
reserve of 4 binder devices unless explicitly capped by specifying a lower
value via the "max" mount option.
This ensures when binder devices are removed (on accident or on purpose)
they can always be recreated without risking that all minor numbers have
already been used up.

Cc: Todd Kjos <tkjos@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binderfs.c

@@ -40,6 +40,8 @@
 #define INODE_OFFSET 3
 #define INTSTRLEN 21
 #define BINDERFS_MAX_MINOR (1U << MINORBITS)
+/* Ensure that the initial ipc namespace always has devices available. */
+#define BINDERFS_MAX_MINOR_CAPPED (BINDERFS_MAX_MINOR - 4)
 
 static dev_t binderfs_dev;
 static DEFINE_MUTEX(binderfs_minors_mutex);
@@ -127,11 +129,14 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
 	struct inode *inode = NULL;
 	struct super_block *sb = ref_inode->i_sb;
 	struct binderfs_info *info = sb->s_fs_info;
+	bool use_reserve = (info->ipc_ns == &init_ipc_ns);
 
 	/* Reserve new minor number for the new device. */
 	mutex_lock(&binderfs_minors_mutex);
 	if (++info->device_count <= info->mount_opts.max)
-		minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR,
+		minor = ida_alloc_max(&binderfs_minors,
+				      use_reserve ? BINDERFS_MAX_MINOR :
+						    BINDERFS_MAX_MINOR_CAPPED,
 				      GFP_KERNEL);
 	else
 		minor = -ENOSPC;