Add support for libfscryptctl.so creation.

[DineshkumarJP]

Hi,

This user space tool is now active as a command-line tool. The.so file was required by some embedded devices, however, in order to link their programme.

[ebiggers]

fscryptctl is pretty bare-bones; it doesn't have much functionality beyond the underlying filesystem ioctls, other than handling input from the command line and stdin, and printing output to stdout and stderr. It's hard to see what would be in a libfscryptctl, exactly. What would you expect to be in it, and why would it be useful over just calling the ioctls yourself?

[DineshkumarJP]

If this tool is 'libfscryptctl' compatible, we may access the APIs directly (ex. cmd_add_key) rather than using the command line. No additional system call is required to execute the command line arguments if we are utilising 'libfscryptctl' in any programme. Additionally, to handle the error code in their programme quickly.

[ebiggers]

If that's all you need, you could just rename main() to fscryptctl_main() and build a library that exports that one symbol, right? Note that it would still behave exactly like the command-line program, including using command line arguments as well as stdin / stdout / stderr. I do not know what your use case is, but I still think you should consider just using the ioctls directly.

[josephlr]

I agree with @ebiggers, it sounds like you want to call the ioctls directly. The ioctls are better documented than this CLI tool, and if you run into issues or bugs, you will be in a better position to search/ask for help.

For example, most of the code in cmd_add_key is for formating or reporting output via stdout/stderr, the actual core of the code is just:

  struct fscrypt_add_key_arg *arg = calloc(sizeof(*arg) + FSCRYPT_MAX_KEY_SIZE, 1);
  
  // Read key data in
  memcpy(arg->raw, key_in, FSCRYPT_MAX_KEY_SIZE);
  arg->raw_size = FSCRYPT_MAX_KEY_SIZE;
  arg->key_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
  
  // Should do error checking here
  ioctl(fd, FS_IOC_ADD_ENCRYPTION_KEY, arg);
  
  // Read the identifier from the arg struct
  memcpy(id_out, arg->key_spec.u.identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);