chore: modularize PAM

also helps with sanitizing the code flow when spawning the child, more
code though

src/pam.c

@@ -0,0 +1,167 @@
+#include <pwd.h>
+#include <security/_pam_types.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "macros.h"
+#include "pam.h"
+#include "sessions.h"
+
+struct envpair {
+  const char* NNULLABLE name;
+  char* NULLABLE value;
+};
+
+char* NULLABLE make_env_kv(const char* NNULLABLE key, char* NNULLABLE value) {
+  char* buf = NULL;
+  asprintf(&buf, "%s=%s", key, value);
+  return buf;
+}
+
+void free_envlist(char** NNULLABLE envlist) {
+  for (char** ptr = envlist; *ptr; ptr++)
+    free(*ptr);
+  free((void*)envlist);
+}
+
+// NULL when allocation failure
+// in any case, envlist would be freed after this function
+char** NULLABLE merge_envlist(char** NNULLABLE envlist, struct envpair extra[],
+                              size_t extra_len) {
+  size_t envlist_len = 0;
+  while (envlist[envlist_len])
+    envlist_len++;
+
+  size_t nonnullelems = 0;
+  for (size_t i = 0; i < extra_len; i++) {
+    if (extra[i].value) nonnullelems++;
+  }
+
+  size_t new_envlist_len = envlist_len + nonnullelems + 1;
+  char** new_envlist =
+      (char**)realloc((void*)envlist, sizeof(char*) * new_envlist_len);
+  if (!new_envlist) {
+    free_envlist(envlist);
+    return NULL;
+  }
+
+  // NOLINTNEXTLINE(readability-identifier-length)
+  size_t k = 0;
+  for (size_t i = 0; i < extra_len; i++) {
+    if (!extra[i].value) continue;
+    char* env_kv = make_env_kv(extra[i].name, extra[i].value);
+    if (!env_kv) goto free_new_envlist_extra;
+    new_envlist[envlist_len + k++] = env_kv;
+  }
+
+  new_envlist[envlist_len + nonnullelems] = NULL;
+  return new_envlist;
+
+free_new_envlist_extra:
+  for (size_t j = 0; j < envlist_len + k; j++) {
+    free(new_envlist[envlist_len + j]);
+  }
+  free((void*)new_envlist);
+  return NULL;
+}
+
+char* NULLABLE xdg_ssession_type_str(enum session_type typ) {
+  char* xdg_session_type = NULL;
+  if (typ == SHELL) xdg_session_type = "tty";
+  if (typ == XORG) xdg_session_type = "x11";
+  if (typ == WAYLAND) xdg_session_type = "wayland";
+  return xdg_session_type;
+}
+
+#define FAIL_ALLOC(status)                \
+  {                                       \
+    (status).error_flag = PAMH_ERR_ALLOC; \
+    return (status);                      \
+  }
+#define FAIL(status, ERR, ERRFN) \
+  {                              \
+    (status).error_flag = (ERR); \
+    (status).errfn = (ERRFN);    \
+    return (status);             \
+  }
+
+struct pamh_getenv_status pamh_get_complete_env(pam_handle_t* handle,
+                                                char* NNULLABLE user,
+                                                struct passwd* NNULLABLE pw,
+                                                enum session_type session_typ) {
+  struct pamh_getenv_status status;
+  char** envlist = pam_getenvlist(handle);
+  if (!envlist) FAIL(status, PAMH_ERR_ERRNO, "pam_getenvlist");
+
+  struct envpair extra_env[] = {
+      {"TERM", getenv("TERM")},
+      {"PATH", getenv("PATH")},
+      {"HOME", pw->pw_dir},
+      {"USER", pw->pw_name},
+      {"SHELL", pw->pw_shell},
+      {"LOGNAME", pw->pw_name},
+      {"XDG_SESSION_TYPE", xdg_ssession_type_str(session_typ)}};
+
+  status.error_flag = PAMH_ERR_NOERR;
+  status.envlist = merge_envlist(envlist, extra_env, LEN(extra_env));
+  if (!status.envlist) FAIL_ALLOC(status);
+
+  return status;
+}
+
+#undef FAIL
+#undef FAIL_ALLOC
+
+///////////////
+
+int pam_conversation(int num_msg, const struct pam_message** msg,
+                     struct pam_response** resp, void* appdata_ptr) {
+  struct pam_response* reply = malloc(sizeof(struct pam_response) * num_msg);
+  if (!reply) {
+    return PAM_BUF_ERR;
+  }
+  for (size_t i = 0; i < num_msg; i++) {
+    reply[i].resp = NULL;
+    reply[i].resp_retcode = 0;
+    if (msg[i]->msg_style == PAM_PROMPT_ECHO_OFF ||
+        msg[i]->msg_style == PAM_PROMPT_ECHO_ON) {
+      char* input = (char*)appdata_ptr;
+      reply[i].resp = strdup(input);
+    }
+  }
+  *resp = reply;
+  return PAM_SUCCESS;
+}
+
+#ifndef PAM_SERVICE_FALLBACK
+  #define PAM_SERVICE_FALLBACK "login"
+#endif
+
+#define CHECK_PAM_RET(call) \
+  ret = (call);             \
+  if (ret != PAM_SUCCESS) { \
+    pam_end(pamh, ret);     \
+    return NULL;            \
+  }
+pam_handle_t* get_pamh(char* user, char* passwd) {
+  pam_handle_t* pamh = NULL;
+  struct pam_conv pamc = {pam_conversation, (void*)passwd};
+  int ret;
+
+  char* pam_service_override = getenv("LIDM_PAM_SERVICE");
+  char* pam_service_name =
+      pam_service_override ? pam_service_override : PAM_SERVICE_FALLBACK;
+
+  CHECK_PAM_RET(pam_start(pam_service_name, user, &pamc, &pamh))
+  CHECK_PAM_RET(pam_authenticate(pamh, 0))
+  CHECK_PAM_RET(pam_acct_mgmt(pamh, 0))
+  CHECK_PAM_RET(pam_setcred(pamh, PAM_ESTABLISH_CRED))
+  CHECK_PAM_RET(pam_open_session(pamh, 0))
+  CHECK_PAM_RET(pam_setcred(pamh, PAM_REINITIALIZE_CRED))
+
+  return pamh;
+}
+#undef CHECK_PAM_RET