Use NK_get_status to implement Device::get_config

libnitrokey’s NK_read_config function returns a pointer to an array that
has been allocated using new[].  We would have to delete this pointer
using delete[], but we only have access to free.  Therefore this patch
modifies the Device::get_config function to call NK_get_status instead
of NK_read_config.

This also makes the code more safe as we get the data as a struct
instead of an array.  It does not add much overhead as NK_read_config
also executes the GET_STATUS command on the Nitrokey device.

3 files changed, 14 insertions, 22 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f5bb7cb..c94e170 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ SPDX-License-Identifier: CC0-1.0
     _>` to `Result<u32, _>`.
   - Change the type of the field `DeviceInfo.serial_number` from
     `Option<String>` to `Option<u32>`.
+- Use `NK_get_status` instead of `NK_read_config` to implement the
+  `Device::get_config` function.
 
 # v0.5.2 (2020-01-28)
 - Use `CString` to store the temporary password instead of `Vec<u8>`.
diff --git a/src/config.rs b/src/config.rs
index 9b9de3c..120a51b 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -94,17 +94,6 @@ impl From<&nitrokey_sys::NK_status> for RawConfig {
     }
 }
 
-impl From<[u8; 5]> for RawConfig {
-    fn from(data: [u8; 5]) -> Self {
-        RawConfig {
-            numlock: data[0],
-            capslock: data[1],
-            scrollock: data[2],
-            user_password: data[3] != 0,
-        }
-    }
-}
-
 impl Into<Config> for RawConfig {
     fn into(self) -> Config {
         Config {
diff --git a/src/device/mod.rs b/src/device/mod.rs
index 403bb1f..e9ca0dc 100644
--- a/src/device/mod.rs
+++ b/src/device/mod.rs
@@ -9,7 +9,6 @@ use std::convert::{TryFrom, TryInto};
 use std::ffi;
 use std::fmt;
 
-use libc;
 use nitrokey_sys;
 
 use crate::auth::Authenticate;
@@ -18,8 +17,7 @@ use crate::error::{CommunicationError, Error};
 use crate::otp::GenerateOtp;
 use crate::pws::GetPasswordSafe;
 use crate::util::{
-    get_command_result, get_cstring, get_last_error, owned_str_from_ptr, result_from_string,
-    result_or_error,
+    get_command_result, get_cstring, owned_str_from_ptr, result_from_string, result_or_error,
 };
 
 pub use pro::Pro;
@@ -386,14 +384,17 @@ pub trait Device<'a>: Authenticate<'a> + GetPasswordSafe<'a> + GenerateOtp + fmt
     /// # }
     /// ```
     fn get_config(&self) -> Result<Config, Error> {
-        let config_ptr = unsafe { nitrokey_sys::NK_read_config() };
-        if config_ptr.is_null() {
-            return Err(get_last_error());
-        }
-        let config_array_ptr = config_ptr as *const [u8; 5];
-        let raw_config = unsafe { RawConfig::from(*config_array_ptr) };
-        unsafe { libc::free(config_ptr as *mut libc::c_void) };
-        Ok(raw_config.into())
+        let mut raw_status = nitrokey_sys::NK_status {
+            firmware_version_major: 0,
+            firmware_version_minor: 0,
+            serial_number_smart_card: 0,
+            config_numlock: 0,
+            config_capslock: 0,
+            config_scrolllock: 0,
+            otp_user_password: false,
+        };
+        get_command_result(unsafe { nitrokey_sys::NK_get_status(&mut raw_status) })?;
+        Ok(RawConfig::from(&raw_status).into())
     }
 
     /// Changes the administrator PIN.