I am writing a Python script that needs to use the FS_IOC_ADD_ENCRYPTION_KEY ioctl.

This ioctl expects an argument of type fscrypt_add_key_arg*:

struct fscrypt_add_key_arg {
    struct fscrypt_key_specifier key_spec;
    __u32 raw_size;
    __u32 key_id;
    __u32 __reserved[8];
    __u8 raw[];
};

#define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR        1
#define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER        2

struct fscrypt_key_specifier {
    __u32 type;     /* one of FSCRYPT_KEY_SPEC_TYPE_* */
    __u32 __reserved;
    union {
            __u8 __reserved[32]; /* reserve some extra space */
            __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
            __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
    } u;
};

This is my Python code:

import fcntl
import struct

FS_IOC_ADD_ENCRYPTION_KEY = 0xc0506617

policy_data.key_descriptor: str = get_key_descriptor()
policy_key: bytes = get_policy_key(policy_data)

fscrypt_key_specifier = struct.pack(
    'II16s',
    0x2,
    0,
    bytes.fromhex(policy_data.key_descriptor)
)

fscrypt_add_key_arg = struct.pack(
    f'{len(fscrypt_key_specifier)}sII8I{len(policy_key)}s',
    fscrypt_key_specifier,
    len(policy_key),
    0,
    0, 0, 0, 0, 0, 0, 0, 0,
    policy_key
)

fd = os.open('/mnt/external', os.O_RDONLY)
res = fcntl.ioctl(fd, FS_IOC_ADD_ENCRYPTION_KEY, fscrypt_add_key_arg)

print(res)

When I execute this code I get an OSError:

Traceback (most recent call last):
  File "/home/foo/fscryptdump/./main.py", line 101, in <module>
    res = fcntl.ioctl(fd, FS_IOC_ADD_ENCRYPTION_KEY, fscrypt_add_key_arg)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 22] Invalid argument

I have double-checked the documentation of that ioctl and I thunk the values I am passing are correct, but there probably is a problem in the way they are packed.

How can I solve this?