debugfs: inode: debugfs_create_dir uses mode permission from parent

Currently function debugfs_create_dir() creates a new
directory in the debugfs (usually mounted /sys/kernel/debug)
with permission rwxr-xr-x. This is hard coded.

Change this to use the parent directory permission.

Output before the patch:
root@s8360047 ~]# tree -dp -L 1 /sys/kernel/debug/
/sys/kernel/debug/
├── [drwxr-xr-x]  bdi
├── [drwxr-xr-x]  block
├── [drwxr-xr-x]  dasd
├── [drwxr-xr-x]  device_component
├── [drwxr-xr-x]  extfrag
├── [drwxr-xr-x]  hid
├── [drwxr-xr-x]  kprobes
├── [drwxr-xr-x]  kvm
├── [drwxr-xr-x]  memblock
├── [drwxr-xr-x]  pm_qos
├── [drwxr-xr-x]  qdio
├── [drwxr-xr-x]  s390
├── [drwxr-xr-x]  s390dbf
└── [drwx------]  tracing

14 directories
[root@s8360047 linux]#

Output after the patch:
[root@s8360047 ~]# tree -dp -L 1 /sys/kernel/debug/
sys/kernel/debug/
├── [drwx------]  bdi
├── [drwx------]  block
├── [drwx------]  dasd
├── [drwx------]  device_component
├── [drwx------]  extfrag
├── [drwx------]  hid
├── [drwx------]  kprobes
├── [drwx------]  kvm
├── [drwx------]  memblock
├── [drwx------]  pm_qos
├── [drwx------]  qdio
├── [drwx------]  s390
├── [drwx------]  s390dbf
└── [drwx------]  tracing

14 directories
[root@s8360047 linux]#

Here is the full diff output done with:
[root@s8360047 ~]# diff -u treefull.before treefull.after |
	sed 's-^- # -' > treefull.diff
 # --- treefull.before	2018-04-27 13:22:04.532824564 +0200
 # +++ treefull.after	2018-04-27 13:24:12.106182062 +0200
 # @@ -1,55 +1,55 @@
 #  /sys/kernel/debug/
 # -├── [drwxr-xr-x]  bdi
 # -│   ├── [drwxr-xr-x]  1:0
 # -│   ├── [drwxr-xr-x]  1:1
 # -│   ├── [drwxr-xr-x]  1:10
 # -│   ├── [drwxr-xr-x]  1:11
 # -│   ├── [drwxr-xr-x]  1:12
 # -│   ├── [drwxr-xr-x]  1:13
 # -│   ├── [drwxr-xr-x]  1:14
 # -│   ├── [drwxr-xr-x]  1:15
 # -│   ├── [drwxr-xr-x]  1:2
 # -│   ├── [drwxr-xr-x]  1:3
 # -│   ├── [drwxr-xr-x]  1:4
 # -│   ├── [drwxr-xr-x]  1:5
 # -│   ├── [drwxr-xr-x]  1:6
 # -│   ├── [drwxr-xr-x]  1:7
 # -│   ├── [drwxr-xr-x]  1:8
 # -│   ├── [drwxr-xr-x]  1:9
 # -│   └── [drwxr-xr-x]  94:0
 # -├── [drwxr-xr-x]  block
 # -├── [drwxr-xr-x]  dasd
 # -│   ├── [drwxr-xr-x]  0.0.e18a
 # -│   ├── [drwxr-xr-x]  dasda
 # -│   └── [drwxr-xr-x]  global
 # -├── [drwxr-xr-x]  device_component
 # -├── [drwxr-xr-x]  extfrag
 # -├── [drwxr-xr-x]  hid
 # -├── [drwxr-xr-x]  kprobes
 # -├── [drwxr-xr-x]  kvm
 # -├── [drwxr-xr-x]  memblock
 # -├── [drwxr-xr-x]  pm_qos
 # -├── [drwxr-xr-x]  qdio
 # -│   └── [drwxr-xr-x]  0.0.f5f2
 # -├── [drwxr-xr-x]  s390
 # -│   └── [drwxr-xr-x]  stsi
 # -├── [drwxr-xr-x]  s390dbf
 # -│   ├── [drwxr-xr-x]  0.0.e18a
 # -│   ├── [drwxr-xr-x]  cio_crw
 # -│   ├── [drwxr-xr-x]  cio_msg
 # -│   ├── [drwxr-xr-x]  cio_trace
 # -│   ├── [drwxr-xr-x]  dasd
 # -│   ├── [drwxr-xr-x]  kvm-trace
 # -│   ├── [drwxr-xr-x]  lgr
 # -│   ├── [drwxr-xr-x]  qdio_0.0.f5f2
 # -│   ├── [drwxr-xr-x]  qdio_error
 # -│   ├── [drwxr-xr-x]  qdio_setup
 # -│   ├── [drwxr-xr-x]  qeth_card_0.0.f5f0
 # -│   ├── [drwxr-xr-x]  qeth_control
 # -│   ├── [drwxr-xr-x]  qeth_msg
 # -│   ├── [drwxr-xr-x]  qeth_setup
 # -│   ├── [drwxr-xr-x]  vmcp
 # -│   └── [drwxr-xr-x]  vmur
 # +├── [drwx------]  bdi
 # +│   ├── [drwx------]  1:0
 # +│   ├── [drwx------]  1:1
 # +│   ├── [drwx------]  1:10
 # +│   ├── [drwx------]  1:11
 # +│   ├── [drwx------]  1:12
 # +│   ├── [drwx------]  1:13
 # +│   ├── [drwx------]  1:14
 # +│   ├── [drwx------]  1:15
 # +│   ├── [drwx------]  1:2
 # +│   ├── [drwx------]  1:3
 # +│   ├── [drwx------]  1:4
 # +│   ├── [drwx------]  1:5
 # +│   ├── [drwx------]  1:6
 # +│   ├── [drwx------]  1:7
 # +│   ├── [drwx------]  1:8
 # +│   ├── [drwx------]  1:9
 # +│   └── [drwx------]  94:0
 # +├── [drwx------]  block
 # +├── [drwx------]  dasd
 # +│   ├── [drwx------]  0.0.e18a
 # +│   ├── [drwx------]  dasda
 # +│   └── [drwx------]  global
 # +├── [drwx------]  device_component
 # +├── [drwx------]  extfrag
 # +├── [drwx------]  hid
 # +├── [drwx------]  kprobes
 # +├── [drwx------]  kvm
 # +├── [drwx------]  memblock
 # +├── [drwx------]  pm_qos
 # +├── [drwx------]  qdio
 # +│   └── [drwx------]  0.0.f5f2
 # +├── [drwx------]  s390
 # +│   └── [drwx------]  stsi
 # +├── [drwx------]  s390dbf
 # +│   ├── [drwx------]  0.0.e18a
 # +│   ├── [drwx------]  cio_crw
 # +│   ├── [drwx------]  cio_msg
 # +│   ├── [drwx------]  cio_trace
 # +│   ├── [drwx------]  dasd
 # +│   ├── [drwx------]  kvm-trace
 # +│   ├── [drwx------]  lgr
 # +│   ├── [drwx------]  qdio_0.0.f5f2
 # +│   ├── [drwx------]  qdio_error
 # +│   ├── [drwx------]  qdio_setup
 # +│   ├── [drwx------]  qeth_card_0.0.f5f0
 # +│   ├── [drwx------]  qeth_control
 # +│   ├── [drwx------]  qeth_msg
 # +│   ├── [drwx------]  qeth_setup
 # +│   ├── [drwx------]  vmcp
 # +│   └── [drwx------]  vmur
 #  └── [drwx------]  tracing
 #      ├── [drwxr-xr-x]  events
 #      │   ├── [drwxr-xr-x]  alarmtimer

Fixes: edac65ea ("debugfs: take mode-dependent parts of debugfs_get_inode() into callers")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/inode.c

@@ -512,7 +512,9 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
 	if (unlikely(!inode))
 		return failed_creating(dentry);
 
-	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
+	if (!parent)
+		parent = debugfs_mount->mnt_root;
+	inode->i_mode = S_IFDIR | ((d_inode(parent)->i_mode & 0770));
 	inode->i_op = &simple_dir_inode_operations;
 	inode->i_fop = &simple_dir_operations;