test user auth with pam on linux

pam_authenticate.cpp

#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <string>

int pam_conversation(int num_msg, const struct pam_message **msg, struct pam_response **resp, void *appdata_ptr) {
		auto pass = static_cast<std::string *>(appdata_ptr);
		auto reply = (struct pam_response *) malloc(sizeof(struct pam_response));
		reply->resp = strdup(pass->c_str());
		reply->resp_retcode = 0;
		*resp = reply;
		return PAM_SUCCESS;
	}

	bool checkLogin(std::string user, std::string pass) {
		pam_conv local_conversation = {pam_conversation, &pass};

		pam_handle_t *handle = nullptr;

		auto checkError = [&](int retval) -> bool{
			if (retval != PAM_SUCCESS) {
				LOG(ERROR) << pam_strerror(handle, retval) << " (" << user << ")";
				pam_end(handle, retval);
				return false;
			}
			return true;
		};

		if(!checkError(pam_start("system-auth", user.c_str(), &local_conversation, &handle))) return false;
		if(!checkError(pam_authenticate(handle, 0))) return false;
		pam_end(handle, 0);
		return true;
	}