[PATCH v2 2/2] binderfs: reserve devices for initial mount

[Christian Brauner <christian@brauner.io>]

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>
---
v2:
- patch unchanged
v1:
- patch introduced
v0:
- patch not present
---
 drivers/android/binderfs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
index 386e286f077f..8d54368b3c1a 100644
--- a/drivers/android/binderfs.c
+++ b/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 struct vfsmount *binderfs_mnt;
 
@@ -129,11 +131,14 @@ 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;
-- 
2.19.1