localserv/lib/server.c

#include "server.h"

struct {
  int sock_fd;
  char* request;
  uid_t uid;
  char* client_path;
} server_globals = {
  .request = NULL,
  .uid = 0,
  .client_path = NULL,
};

int server_init(char* path) {
  umask(0);
  server_globals.sock_fd = socket(AF_UNIX, SOCK_DGRAM, 0);
  if (server_globals.sock_fd < 0) {
    return errno;
  }

  struct sockaddr_un server_addr;

  memset(&server_addr, 0, sizeof(server_addr));
  server_addr.sun_family = AF_UNIX;
  // copy in the provided directory path for the socket
  memcpy(server_addr.sun_path, path, strlen(path));

  // // is this needed?
  // signal(SIGPIPE, SIG_IGN);

  socklen_t server_addr_len = sizeof(server_addr.sun_family) + strlen(server_addr.sun_path);

  unlink(server_addr.sun_path);
  if (bind(server_globals.sock_fd, (struct sockaddr *)&server_addr, server_addr_len) != 0) {
    return errno;
  }

  int optval = 1;
  if (setsockopt(server_globals.sock_fd, SOL_SOCKET, SO_PASSCRED, &optval, sizeof(optval)) != 0) {
    return errno;
  }

  server_globals.client_path = malloc(CLIENT_PATH_LENGTH);

  return 0;
}

int server_listen() {
  struct sockaddr_un client_addr;
  struct msghdr message_hdr;
  // tells recvmsg where to put the client address name
  message_hdr.msg_name = (struct sockaddr *)&client_addr;
  // only ever expect the abstract address to be 5 chars long (plus one for null at beginning)
  // sockaddr_un is big enough to handle up to 108 characters but we don't want more than 6
  message_hdr.msg_namelen = sizeof(sa_family_t) + CLIENT_PATH_LENGTH;
  
  // this is where the message (the client's request) will be stored, it's weird cause it can support multiple buffers
  struct iovec iovec;
  // the buffer to receive into is just the memory for the request in the return result, this way no copying is required
  iovec.iov_base = NULL;
  iovec.iov_len = 0;
  message_hdr.msg_iov = &iovec;
  // there's only one element in the iovec cause we only use one buffer
  message_hdr.msg_iovlen = 1;

  // set after call, not checked so it shouldn't matter
  message_hdr.msg_flags = 0;

  // this is the buffer for getting ancillary data (namely, the UID of the requesting client)
  char message_control_buffer[CONTROL_BUFFER_SIZE];
  message_hdr.msg_control = &message_control_buffer;
  message_hdr.msg_controllen = CONTROL_BUFFER_SIZE;

  // call gets info about waiting message and sender (address and uid)
  ssize_t msg_size = recvmsg(server_globals.sock_fd, &message_hdr, MSG_PEEK | MSG_TRUNC);

  // for now, just output how big it was, later, we will check to make sure it's the correct size
  if (msg_size < 0) {
    return errno;
  }

  // extract credentials from the message
  struct ucred credentials;
  memcpy(&credentials, CMSG_DATA((struct cmsghdr *)message_hdr.msg_control), sizeof(credentials));

  // recvmsg populated msg_name with the client's actual path info.
  // copy that into the client path
  // client_addr.sun_path should be updated by recvmsg
  memcpy(server_globals.client_path, client_addr.sun_path, CLIENT_PATH_LENGTH);

  server_globals.uid = credentials.uid;
  // username = getpwuid(credentials.uid)->pw_name;

  // allocate buffer to receive the actual message
  free(server_globals.request);
  server_globals.request = malloc(msg_size);

  ssize_t recv_size = recv(server_globals.sock_fd, server_globals.request, msg_size, 0);

  if (recv_size < 0) {
    return errno;
  }

  // request was already added to result, so it's ready to go
  return 0;
}

int server_respond(char* response) {
  // set up client address
  struct sockaddr_un client_addr;
  client_addr.sun_family = AF_UNIX;
  memcpy(client_addr.sun_path, server_globals.client_path, CLIENT_PATH_LENGTH);

  // send stream
  // NOTE: I added a 1 here to include the null byte at the end of the string, because it didn't seem to be included otherwise
  // but it appears to work without that when the client sends the request? Probably needs more investigation
  ssize_t sent = sendto(server_globals.sock_fd, response, strlen(response) + 1, 0, (struct sockaddr *)&client_addr, sizeof(sa_family_t) + CLIENT_PATH_LENGTH);

  if (sent < 0) {
    return errno;
  }

  return 0;
}

char* server_get_request() {
  return server_globals.request;
}

char* server_get_username(uid_t uid) {
  // if this doesn't work sometime you can try storing the uid instead and then calling getpwuid in here
  return getpwuid(uid)->pw_name;
}

uid_t server_get_uid() {
  return server_globals.uid;
}

// void __attribute__ ((constructor)) init(void) {

// }

// TODO: proper signal handling (e.g. this doesn't run when SIGINT stops the program)
void __attribute__ ((destructor)) server_clean_up(void) {
  // TODO: error handling?
  close(server_globals.sock_fd);
  free(server_globals.request);
  free(server_globals.client_path);
  // don't need to free username since it's provided by getpwuid
}